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4.2.2: General Morphology - Biology


Viruses have a variety of shapes and structures.

Learning Objectives

  • Distinguish between the 5 main morphological virus types

Key Points

  • Viruses are very small and to reliably visualize them, stains and electron microscopy are needed.
  • Each virus is a nucleic acid (RNA or DNA) surrounded by a coating, referred to as an envelope or capsid.
  • Viruses encode capsid proteins which encase the nucleic acid. Sometimes, viral proteins combine with host proteins to make the envelope.
  • The shape of a viral coat has implications on how a virus infects a host.

Key Terms

  • capsomere: Any of the individual protein subunits of a viral capsid
  • capsid: The outer protein shell of a virus.
  • icosahedral: of, relating to, or having the shape of an icosahedron

Viruses display a wide diversity of shapes and sizes, called morphologies. In general, there are five main morphological virus types:

  1. Helical – These viruses are composed of a single type of capsomer stacked around a central axis to form a helical structure, which may have a central cavity, or hollow tube.
  2. Icosahedral – Most animal viruses are icosahedral or near-spherical with icosahedral symmetry.
  3. Prolate – This is an isosahedron elongated along one axis and is a common arrangement of the heads of bacteriophages.
  4. Envelope – Some species of virus envelop themselves in a modified form of one of the cell membranes, either the outer membrane surrounding an infected host cell or internal membranes such as nuclear membrane or endoplasmic reticulum, thus gaining an outer lipid bilayer known as a viral envelope.
  5. Complex – These viruses possess a capsid that is neither purely helical nor purely icosahedral, and that may possess extra structures such as protein tails or a complex outer wall.

A complete virus particle, known as a virion, consists of nucleic acid surrounded by a protective coat of protein called a capsid. These are formed from identical protein subunits called capsomeres. Viruses can have a lipid “envelope” derived from the host cell membrane. The capsid is made from proteins encoded by the viral genome and its shape serves as the basis for morphological distinction. Virally coded protein subunits will self-assemble to form a capsid, in general requiring the presence of the virus genome. Complex viruses code for proteins that assist in the construction of their capsid. Proteins associated with nucleic acid are known as nucleoproteins, and the association of viral capsid proteins with viral nucleic acid is called a nucleocapsid. The capsid and entire virus structure can be mechanically (physically) probed through atomic force microscopy.

viruses are much smaller than bacteria. Most viruses that have been studied have a diameter between 20 and 300 nanometers. Some filoviruses have a total length of up to 1400 nm; their diameters are only about 80 nm. Most viruses, such as virions, cannot be seen with an optical microscope, so scanning and transmission electron microscopes are used to visualize them.

To increase the contrast between viruses and the background, electron-dense “stains” are used. These are solutions of salts of heavy metals, such as tungsten, that scatter the electrons from regions covered with the stain. When virions are coated with stain (positive staining), fine detail is obscured. Negative staining overcomes this problem by staining the background only.


Pteridophytes: Meaning, General Characters and Affinities

In this article we will discuss about:- 1. Meaning of Pteridophytes 2. General Characters of Pteridophytes 3. Reproduction 4. Affinities 5. Development of Sporangia 6. Life Cycle Patterns 7. Economic Importance.

  1. Meaning of Pteridophytes
  2. General Characters of Pteridophytes
  3. Reproduction in Pteridophytes
  4. Affinities of Pteridophytes
  5. Development of Sporangia in Pteridophytes
  6. Life Cycle Patterns of Pteridophytes
  7. Economic Importance of Pteridophytes

1. Meaning of Pteridophytes:

Pteridophyta (Gr, Pteron = feather, phyton = plant), the name was originally given to those groups of plants which have well developed pinnate or frond like leaves. Pteridophytes are cryptogams (Gr. kruptos = hidden, and Gamos = wedded) which have well developed vascular tissue.

Therefore, these plants are also known as vascular cryptogams or snakes of plant kingdom. They are represented by about 400 living and fossil genera and some 10,500 species. Palaeobotanical studies reveal that these plants were dominant on the earth during the Devonian period and they were originated about 400 million years ago in the Silurian period of the Palaeozoic era.

2. General Characters of Pteridophytes:

(i) Majority of the living Pteridophytes are terrestrial and prefer to grow in cool, moist and shady places e.g., ferns. Some members are aquatic (e.g., Marsileci, Azolla), xerophytic (e.g., Selaginella rupestris, Equisetum) or epiphytic (e.g., Lycopodium squarrosum) (Fig. 1).

(ii) Majority of the Pteridophytes are herbaceous but a few are perennial and tree like (e.g., Angiopteris). Smallest Pteridophyte is Azolla (an aquatic fern) and largest is Cyathea (tree fern).

(iii) Plant body is sporophytic and can be differentiated into root, stem and leaves.

(iv) Roots are adventitious in nature with monopodial or dichotomous branching. Internally usually they are diarch.

(v) Stem is usually branched. Branching is monopodial or dichotomous. Branches do not arise in the axil of the leaves. In many Pteridophytes stem is represented by rhizome.

(vi) Leaves may be small, thin, scaly (microphyllous e.g., Equisetum), simple and sessile (e.g., Selaginella) or large and pinnately compound (megaphyllous e.g., Dryopteris, Adiantum).

(vii) Vascular tissue is present in stem and root. It consists of xylem and phloem. Xylem consists of tracheids only and phloem has only sieve tubes.

(viii) The steel is protostele (e.g., Rhynia, Lycopodium), siphonostele (e.g., Equisetum), dictyostele Adiantum) or polycyclic (e.g., Angiopteris).

(ix) Cambium is absent hence, they do not show secondary growth.

3. Reproduction in Pteridophytes:

(i) Reproduction takes place by means of spores which are produced inside sporangia.

(ii) The development of the sporangium may be leptosporangiate (sporangium originates from a single cell) or eusporangiate (sporangium develops from a group of cells).

(iii) Sporangia may be borne either on stem or leaves. On the stem they may be terminal (e.g., Rhynia) or lateral (e.g., Lycopodium). On the leaves (sporophylls) they may be ventral, marginal (Pteris, Adiantum) or dorsal (e.g., Polypodiceae). In Equisetum the sporangia are borne on special structures called sporangiophores which constitute a cone. In Marsilea, Azolla, Salvinia sporangia are produced in sporocarps.

(iv) Spores on germination give rise to multicellular gametophytic bodies called prothalli (sing. prothallus).

(v) In homosporous Pteridophytes prothalli are monoecious (antheridia and archegonia develop on the same prothallus). In heterosporous species prothalli are always dioecious. Microspores on germination give rise to male prothalli and megaspores to the female prothalli.

(vi) Antheridia and archegonia are developed on prothalli.

(vii) Antheridium is surrounded by a single layered sterile jacket.

(viii) Archegonium consists of four vertical rows of neck cells, 1-2 neck canal cells, ventral canal cell and egg.

(ix) Antherozoids are unicellular, biflagellate (e.g., Selaginella) or multiflagellate (e.g., Equisetum and ferns) and motile.

(x) Antherozoids are attracted towards the neck of the archegonium chemotactically by certain substances like malic acid) present in the mucilaginous substance formed by the degeneration of neck canal cells and venter canal cell.

(xi) Water is essential for fertilization (zooidogamous). Therefore, Pteridophytes are also known as amphibians of the plant kingdom.

(xii) Fertilization results in the formation of zygote or oospore, which ultimately develops into well-developed sporophyte.

(xiii) The fertilized egg divides transversely or vertically. Another cross wall forms a quadrant stage producing stem, leaf, foot and root.

(xiv) Plants show heteromorphic alternation of generation. The main plant body is sporophytic and forms a dominant phase in the life cycle.

4. Affinities of Pteridophytes:

Similarities with Gymnosperms:

(i) Plant body is sporophytic, dominant and can be differentiated into root, stem and leaves in both the groups.

(ii) Gametophytic phase is of short duration.

(iii) Young leaves show circinate vernation.

(iv) Vascular tissue is well developed. Xylem lacks vessels (except in order Gnetales of Gymnosperms) and companion cells are absent in phloem).

(v) Like Gymnosperms many Pteridophytes are heterosporous (e.g., Marsilea, Selaginella).

(vi) Like Pteridophytes many Gymnosperms show ciliate antherozoids (e.g., Cycas, Ginkgo).

(vii) Like Gymnosperms, in some Pteridophytes megaspore is retained within the megasporangium (e.g., Selaginella).

(viii) Regular alternation of sporophytic and gametophytic phase is present.

5. Development of Sporangia in Pteridophytes:

On the basis of development, the sporangia in Pteridophytes are divided into two types:

Sporangium develops from group of superficial cells. These cells divide periclinally into primary wall layers and inner primary sporogenous cells (Fig. 2A, B). The outer wall layers form the wall of the sporangium while inner sporogenous cells divide meiotically and form spores (Fig. 2 A-F).

(ii) Leptosporangiate Type:

This type of sporangium arises from a single superficial cell. It divides transversely to form an outer and an inner cell (Fig. 3A, B). While the inner cell forms the stalk, the entire sporangium develops from the outer cell. The outer cell divides by three successive periclinal divisions and in this way a tetrahedral apical cell is formed (Fig. 3C).

It divides by periclinal division to form the outer jacket cell and inner primary sporogenous cell (Fig. 3D). Jacket cell forms the single layered sporangial wall while primary sporogenous cell divides into tapetal initial and sporogenous tissue (Fig. 3E). Sporogenous tissue divides meiotically to give rise to haploid spores while tapetal initial forms two layered tapetum (Fig. 3 F, G).

6. Life Cycle Patterns in Pteridophytes:

Pteridophytes show heteromorphic alternation of generation. The main plant body is sporophytic and forms a dominant phase in the life cycle. Sporophytic plant body develops sporangia in which sporogenous tissue is formed. Sporogenous tissue divides meiotically to form haploid spores.

Majority of the Pteridophytes are homosporous e.g., Lycopodium, Pteris etc. Spores on germination produce monoecious gametophyte. Some Pteridophytes are heterosporous and produce two types of spores: microspores and megaspores.

Microspores on germination produce male gametophyte (prothallus) while megaspores on germination produce female gametophyte (prothallus). So, the prothalli are dioecious.

Antheridia and archegonia develop on the same prothallus (monoecious) or on different prothalli (dioecious). The male and female gametes fuse to form zygote which develops into sporophyte. Thus, the life cycle of a Pteridophyte consists of an alternate succession of sporophytic and gametophytic generations.

Apogamy, Apospory and Parthenogenesis:

Pteridophytes show heteromorphic alternation of generation. However, there are certain other modifications where the essential stages of life cycle are eliminated. These modifications are called apogamy, apospory and parthenogenesis.

Apogamy:

The formation of a sporophyte directly from the vegetative cells of the gametophyte without the act of syngamy or gametic union is called apogamy (Winkler, 1908). The term apogamy was first used by De Bary (1878). It was first reported in Pteris cretica by Farlow (1874).

Later, apogamy has been described in many Pteridophytes e.g., Selaginella (Hieronymus, 1911, 1913), Marsilea (strasburger, 1907) etc. The apogamous embryo may develop from one or more cells of the gametophyte. The sporophytes, produced as a result of apogamy, possess the same number of chromosomes as the gametophyte.

Apospory:

The formation of gametophyte from a sporophytic cell without meiosis is known as apospory. It was first discovered by Druery (1884) as a natural phenomenon in Athyrium filix-femina var. clarissima. Since then it has been reported in many Pteridophytes e.g., Tricohmanes (Bower, 1888), Pteris aquilina (Farlow, 889) Asplenium dimorphum (Goebel, 1905), Osmunda Javanica (Sarbadhikari, 1936), Tectaria trifoliata Steil. 1944) etc.

In apospory, a filamentous or heart shaped gametophyte may be formed from one or more cells of any vegetative portion of a young or mature sporophyte. Due to apospory polyploidy is common in ferns. It has been observed that there is no change in the chromosome number (from the parent plant) when the aposporous gametophytes originate.

Parthenogenesis:

Formation of sporophyte from egg without fertilization is called parthenogenesis. In homosporous leptosporangiate ferns, it was observed that apospory was followed by neither apogamy nor fertilization but by parthenogenesis (Farmer and Digby, 1907). Strasburger (1907) reported parthenogenesis in Marsilea drummondii.

In many species of Selaginella (e.g., S. spinulosa, S. rubricaulis etc.), archegonia failed to open and the egg developed into sporophyte parthenogenetically. Parthenogenesis in S. intermedia and S. langere was first reported by Hieronymus in 1911.

7. Economic Importance of Pteridophytes:

The Pteridophytes are of little economic value.

Some of the major uses of Pteridophytes are:

Many species of Pteridophytes are grown as ornamental plants e.g., Selaginella, (species of Selaginella for e.g., S. willedenovii, S. caesia have metallic tints and therefore used as ornamentals, me other species of Selaginella e.g., S. lepidophylla and S. pilifera are sold as curiosities under the name of resurrection plants.

Plant is a ball like structure under dry conditions and on availability of water, it becomes green and flat on soil Lycopodium, Osmunda, Polypodium, Pteridium, Nephrolepis etc.

(ii) In Soil conservation:

Many species of Pteridophytes are used in soil conservation e.g., Lycopodium, Selaginella etc.

(a) Extracts of Lycopodium plants are used as kidney stimulant.

(b) Lycopodium clavatum is used in skin diseases.

(c) Equisetum arvense is used as diuretic (promoting urine discharge).

(d) Rhizome and frond bases of Dryopteris filix-mas are used as taenifuge.

(a) Several species of Lycopodium e.g., L. obscurum are used in Christmas wreaths and other decorations. It is commonly called as ‘Christmas green’.

(b) Equisetum arvense is biological indicator (for the presence of gold in the soil).

(c) Equisetum deposit large amount of silica in their cell walls. So formerly, it was used in cleaning and polishing the metal pots. Therefore, the plant has been given the name ‘scouring rushes’.

(d) Young shoots of Dryopteris filix-mas are used as vegetables.

(e) Starchy paste of sporocarps of Marsilea drummondii is used in making cakes and is called ‘nardoo’

(f) Azolla is grown in the rice fields to maintain its fertility because it has the symbiotic association with the cyanobacteria (blue green algae) Nostoc.


Cycas: Distribution, Morphology and Reproduction| Cycadales

In this article we will discuss about Cycas. After reading this article you will learn about: 1. Distribution of Cycas 2. General Morphology of Cycas 3.Anatomy of Vegetative Parts 4. Reproduction 5. Economic Importance.

  1. Distribution of Cycas
  2. General Morphology of Cycas
  3. Anatomy of Vegetative Parts of Cycas
  4. Reproduction of Cycas
  5. Economic Importance of Cycas

1. Distribution of Cycas:

Cycas, the largest genus among the Old World Cycads, is the most widely distributed genus of order Cycadales. It is distributed in Japan, Australia, India, Indochina, China, Mauritius, Africa, Nepal, Bangladesh, Sri Lanka and Myanmar. In India, Cycas grows naturally in Orissa, Assam, Meghalaya, Tamil Nadu, Karnataka and Andaman and Nicobar Islands (Fig. 8.7).

Cycas is represented by 15 species but according to Willis (1966) there are 20 species of the genus. Schuster (1932), however, recognizes only 8 species, mentioning for the rest as the forms, varieties or sub-species of the other species.

Besides Cycas circinalis, C pectinata, C. rumphii and C. beddomei, which occur in the wild state in India, C. revoluta and C. siamensis are such species which are cultivated commonly in the Indian gardens. Cycas revoluta is the most commonly cultivated species of the Indian gardens.

Some Indian Species:

1. Cycas beddomei Dyre:

A small shrub with a trunk of about 40 cm long. It is distributed in Andhra Pradesh, Madras, Calicut, etc. Leaves are large and reach up to 1 metre in length with quadrangular rachis. Leaflets are narrow and linear. Male cones are oblong to ovoid, bearing a short peduncle. Megasporophylls are ovate, lanceolate with dentate margins. They are produced in November-December.

2. Cycas circinalis Linn:

Commonly called ‘Jangli-madan-mast-ka-Phul’ (Hindi) or ‘Kamakshi’ (Telugu), C. circinalis is commonly distributed in western part of Peninsular India, Western Ghat and Orissa Hills in India. It is often cultivated in Indian gardens.

It is an evergreen tree bearing leaves of 1.5 to 3 metres in length with about 100 pairs of leaflets. Leaflets are linear-lanceolate with flat margin and acuminate apex. Upper sterile part of megasporophyll is longer than broad with dentate margins. Male cones are cylindrical to ovoid with a short peduncle. Megasporophylls contain brown tomentose hairs.

3. Cycas pectinata Griff:

It is distributed in Sikkim, Assam, Manipur and Someshwar Hills of Bihar in India along with some other countries including Nepal and Bangladesh. Its trunk ranges from 1.5 to 2.5 metres in length.

Leaves attain a length of about 1.5 to 2 metres. Leaflets are narrow, linear, tapering into a minute spine and measure from 14 to 25 cm. in length. Male cone is cylindrical-ovoid. The upper part of the megasporophyll is as broad as long.

4. Cycas revoluta Thunb:

It grows in wild state in Japan, China and Taiwan and is widely cultivated in several parts of the world, including India. It is so named because of the revoluted margins of its leaflets It is a palm-like tree, the trunk of which reaches up to 2 metres in length. Male cones are cylindrical or ovoid-oblong. Megasporophylls are 10-25 cm in length and densely tomentose

5. Cycas Rumphii Miq:

It is an evergreen palm-like tree distributed in Andaman and Nicobar Islands of India along with Sri Lanka, Malaysia and Australia. Its trunk reaches up to 4 metres while the leaves attain a length of 1-2 metres with 50 to 100 or more pairs of leaflets. Male cone is shortly stalked and ellipsoidal to oblong in shape. Megasporophylls are ovate-lanceolate with many small teeth.

6. Cycas siamensis Miq:

It is found in Myanmar, Thailand, China and Laos. It is a palm­ like tree. The leaves reach about 1 metre in length. Leaflets are narrow, linear with mucronate or acuminate apex. Male cone is ovoid oblong.

Megasporophyll’s sterile blade is as broad as long with usually only 2 ovules. Burkill (1933) considered Cycas siamensis as a geographical form of C. pectinata. Pant and Nautial (1963) also consider the two species similar, mainly because of their epidermal and anatomical studies.

2. General Morphology of Cycas:

Cycas is a palm-like, evergreen plant (Fig. 8.8). Prior to the anatomical studies of the stem of Cycas revoluta by Brongniart (1829), the Cycas was actually considered a palm. The plant body consists of a columnar aerial trunk with a crown of pinnately compound leaves as its top.

According of Eichler (1889), Coulter and Chamberlain (1910), Schuster (1932) and others, a tap root system persists in the adult plant, but according to Worsdell (1906) the tap roots are soon replaced by adventitious roots.

Roots in Cycas are of two types, i.e., normal tap roots forming a tap root system, and coralloid roots. Normal tap-roots are positively geotropic, grow deep into the soil and generally possess no root hairs. Their function is to fix the plant in the soil and to absorb water and other minerals.

From the normal roots develop some lateral branches near the ground surface. These lateral roots get infected with some bacteria, fungi and algae, and are called coralloid roots (Fig. 8.9). They grow- first horizontally in the soil and become swollen at their tips.

They divide repeatedly to form big bunches of greenish or brownish structures, which are coral like in appearance. They divide dichotomously, come out of the soil on the ground surface and are phototrophic in nature. Young plants bear more coralloid roots than the older ones.

Recently, Pant and Das (1990) reported non-coralloid aerial roots in Cycas circinalis, C. revoluta and C. rumphii. The charactenstic algal zone of coralloid roots is absent in these roots. These are positively geotropic, adventitious and develop from the lower sides of leaf bases or bulbils when they are still attached to the plant.

The stem is thick, woody and usually un-branched. It is tuberous when young but columnar, erect and stout at maturity. Branching in stem (Fig. 8.10) is also not rare after the plants have reached a certain age. The aerial part of the trunk remains covered by a thick armour of large and small rhomboidai leaf bases.

These occur regularly in alternate bands (Fig. 8 .11). The larger leaf bases represent the bases of foliage leaves, while the smaller ones are the bases of scaly leaves in male plants and scales and megasporophylls in female plants. The age of the plant can be calculated by counting the number of crowns of leaves and megasporophylls which are produced every year.

Among all Cycas species, C. media is tallest, attaining a height up to 20 metres. Regarding the age of Cycas, the plants can survive for a long period. C. circinalis, if allowed to grow undisturbed, may attain an age of 100 years or even more.

Two types of leaves are present in Cycas. These are green, assimilatory ox foliage leaves, and scaly leaves or cataphylls.

1. Foliage Leaves or Assimilatory Fronds:

These are green, large, pinnately compound and stout leaves with a spiny petiole and large, strong rachis. They are produced at the apex of the stem in the form of crown. The rachis bears many leaflets.

With the help of a transversely expanded rhomboidai leaf base, a leaf remains attached with the stem Two rows of strong and stiff spines are present on the petiole. These spines gradually transform into two rows of pinnae towards the upper side of the leaf (Fig. 8.12).

Pant (1953) reported many abnormalities in Cycas leaves. Author, along with two of his colleagues, also reported many abnormalities in the vegetative parts of an year-old plant of Cycas circinalis growing in the Botanical Garden of Meerut College, Meerut.

Cycas leaf is very large and may reach up to 3 metres in length in some species such as C. thouarsii. Two rows of pinnae on the leaves may be alternate or opposite. The number of pinnae varies in different species. As many as hundred pairs of pinnae may be present in a mature leaf.

Each pinna is sessile, elongated, ovate or lanceolate in shape with a spiny or acute apex. Pinnae are repeatedly and deeply dichotomized in C. micholitzii (Fig. 8.13). Each pinna or of leaflet contains a midrib without any lateral branching.

Forking of the midvein of the leaflet has been reported in C. circinalis by author in 1976. Margins of the leaflets are revolute in C. revoluta and C. beddomei, while in C. rumphii and C. circinalis they are flat.

According to Chamberlain (1935) the “vernation is circinate in the midrib and pinnules of Cycas”. Leaves, when young, have circinately coiled pinnae like those of ferns (Fig. 8.14). Very young parts of Cycas are also covered by fern-like hairs or ramenta.

2. Scaly Leaves or Cataphylls:

These are dry, brown-coloured, somewhat triangular leaves with their one end pointed. They are present at the apex of the stem and remain covered with several ramental hairs (Fig. 8.15).

3. Anatomy of Vegetative Parts of Cycas:

(i) Normal Root (Young):

It is circular in outline and resembles structurally with dicotyledons (Fig. 8.16). Outermost layer is epiblema or exodermis, which surrounds the large parenchymatous cortex. Epiblema consists of tangentially elongated cells. From some of its cells arise root hairs.

In the wide zone of parenchymatous cortex there are present many intercellular spaces. Cells of the cortex remain filled with starch. Some tannin-filled cells, mucilage cells and sometimes sphaeraphides are also present in the cortex. The cortex is delimited by a single- layered endodermis. Casparian steps are present in the barrel-shaped cells of the endodermis.

Endodermis is followed by multilayered pericycle. Xylem and phloem bundles in the roots are radially arranged, i.e. present on different radii. The roots are usually diarch but sometimes the number of protoxylem strands range between 3 to 8.

The protoxylem consists of spiral tracheids while the metaxylem consists of scalariform tracheids. Vessels are absent. Phloem is present alternately with xylem groups and consists of sieve tubes and phloem parenchyma. Pith is generally absent.

(ii) Normal Root (old) Showing Secondary Growth:

The older roots (Fig. 8.17) undergo secondary growth. The cambium cuts secondary phloem towards the outer side and secondary xylem towards the inner side. After sometime the pericycle cells also become meristematic and form a complete cambial ring.

The secondary xylem consists of radial rows of tracheids separated by parenchymatous cells. The crushed primary phloem is present in the form of dark streaks outside the secondary phloem. The secondary xylem is manoxvlic and contains many multiseriate rays.

Periderm starts to develop in the cortex of old roots. Some of the cells of the outermost cortical region start to become meristematic and function as cork cambium. It cuts cork towards outer side and secondary cortex towards inner side. Cork cells are dead and remain filled with subenn. Cycas roots often show two layers of periderm (Fig. 8.17).

Epiblema is ruptured and there are no root hairs in the older roots.

(iii) Coralloid Root:

Anatomically, the coralloid roots (Fig. 8.18) resemble normal roots except some under mentioned differences:

1. The secondary vascular tissue in coralloid roots is either totally absent or poorly-developed.

2. The cortex is wider in comparison with the normal root.

3. Presence of a greenish algal-zone in the middle of the cortex. But according to Chaudhary and Akhtar (1931) the algal-zone is not of universal occurrence in the coralloid roots of Cycas. It may be absent in such coralloid roots which go very deep in the soil. According to these workers only those coralloid roots are negatively geotropic which are infected by algal members.

Algal-zone consists of radially elongated, large, thin-walled cells having large intercellular spaces occupied by algae. Life (1901) opined that these spaces are formed because of the retardation of growth of such cells which are already infected by fungi and bacteria.

Such infected cells cannot keep pace with the neighbouring cells, and a tension is produced which results in the formation of air spaces by breaking of certain cells. These spaces are further widened by the algal infection. But according to Chaudhary and Akhtar (1931) the alga is mainly responsible for the formation of these large intercellular spaces.

Following members have been reported from the algal zone of coralloid roots:

Anabaena cycadae, Nostoc punctiforme, Oscillatoria, Azotobacter, Pseudomonas radicicola and even a few fungi. According to Kubitzki (1990) blue green algae or Cyanoba cteria (Anabaena, Nostoc and Calothrix) may rarely be present intracellularly (i.e. inside the cell) in the coralloid roots of Cycas. He opined that these algae fix nitrogen and promote the growth of host plant.

Due to the presence of blue-green algal members and some nitrogen-fixing bacteria, the function assigned to the coralloid roots is chiefly the nitrogen fixation. The presence and structure of endodermis, pericycle and vascular bundles in the coralloid roots are similar to that of normal roots. The xylem is exarch and triarch.

(iv) Stem:

Similar to root, the stem of Cycas also resembles internally with a dicotyledonous stem.

It shows the following anatomical features:

Epidermis is the outermost layer consisting of compactly arranged thick- walled cells. Presence of several persistent leaf bases makes the epidermis a discontinuous and ruptured layer. Cortex is large and consists of thin- walled, parenchymatous cells, filled densely with starch grains. It contains numerous mucilaginous canals and girdle traces.

Each mucilage canal is lined with many radially elongated epithelial or secretory cells (Fig. 8.19). Medullary rays connect the mucilage canals of the cortex with that of the pith Starch in the parenchymatous cells of the cortex is the source of ‘sago’. Endodermis and pericycle are not clearly demarcated.

Numerous vascular bundles remain arranged in a ring. The stele is ectophloic siphonostele. Each vascular bundle is conjoint, collateral, open and endarch (Fig. 8.20). The xylem consists of tracheids and xylem parenchyma (Fig. 8.21).

Protoxylem contains tracheids with spiral thickenings while the metaxylem has scalariform thickenings with bordered pits. Vessels are absent. The phloem is located outside the xylem and consists of sieve tubes and phloem parenchyma. Companion cells are absent.

Between the xylem and phloem lies the primary cambium, which remains active only for a short period. It is soon replaced by another ring of secondary cambium somewhere in the cortex. These successive cambial rings form 2-14 different vascular rings showing polyxylic condition in the old stem (Fig. 8.22).

Several broad and well-developed medullary rays are present between the vascular bundles. Pith is large, well-developed and parenchymatous. It contains many mucilaginous canals.

(v) Leaf Traces:

The leaf traces remain scattered in the cortical region of the stem and constitute the vascular supply to the leaves from the main vascular cylinder. Normally, there are four leaf traces which form the vascular supply to the leaf. Two of these are direct traces, while the remaining two axe girdle traces (Fig. 8.23).

The direct traces originate from the vascular cylinder lying in front of the leaf base while the girdle traces develop from the vascular cylinder lying opposite to that of direct traces. They proceed together and curve soon in opposite directions, and by girdling round the vascular cylinder they enter in the leaf base.

In the cortical region the girdle traces also remain connected with other leaf traces. At the time of their entrance in the petiole, the leaf trace bundles subdivide and form many petiole bundles. Such type of unique girdle traces of Cycas, which also occur in Magnoliaceae. show a close relationship of Cycadales of Gymnosperms and Magnoliaceae of dicotyledons.

(vi) Secondary Growth:

It is similar to that of dicotyledons. In the beginning, Cycas is monoxylic, i.e. contains a single ring of vascular bundles. But one or more concentric rings of vascular bundles appear outside the primary ring of bundles in the older stems showing polyxylic condition (Fig. 8.24)

By the activity of inter-fascicular and intra-fascicular cambia, which unite to form a cambium ring, the secondary growth is initiated. This cambium ring cuts secondary phloem towards outer side and secondary xylem towards inner side. Well-developed medullary rays traverse through the so-formed secondary vascular tissue.

After a short while this cambium ring stops functioning and a second cambium ring develops either in the parenchymatous cortex or in the region of pericycle This cambium ring also behaves in the similar fashion.

In this fashion, as many as 14 rings of vascular tissue may develop in the stem of Cycas pectinata of about 20 cm diameter showing polyxylic condition. Seward (1917) reported 12 such rings in the stem of C. media of about 30 cm diameter, and Schuster (1932) reported 22 such rings in the stem of C. rumphii having a diameter of about 85 cm.

Cambial rings towards the periphery of the stem form lesser number of vascular bundles. The cork cambium develops on the outer region of the cortex and cuts cork towards outer side and secondary cortex towards inner side.

(vii) Rachis:

The outline of transverse section is rhomboidal in the basal region of the rachis, biconvex in the middle cambium and roughly cylindrical at the tip region or at the apex of the rachis. Two arms of the bases of leaflets are present on the rachis, one on each side (Fig. 8.25).

In T.S. the rachis reveals the following structures from outside within:

Epidermis is the outermost layer of the rachis consisting of thick-walled cells. It is heavily circularized. On its upper as well as lower sides are present irregularly distributed sunken stomata. Hypodermis is present below the epidermis.

It is differentiated into outer 2-3 layers of chlorophyll-containing thin-walled cells of chlorenchyma and inner 4-6 layers of thick- walled lignified cells of sclerenchyma. Sclerenchyma is poorly-developed on the lateral sides. It is also seen intermixed with chlorenchyma.

Ground tissue is a large region consisting of thin- walled, parenchymatous cells. Many mucilaginous canals and vascular bundles are present in this region. The number and arrangement of mucilage canals have no definite relation with that of vascular bundles. Each mucilage canal is a double-layered structure consisting of an inner layer of epithelium cells surrounded by an outer layer.

Vascular bundles are arranged in the shape of an inverted Greek letter Omega (Ω) (Fig. 8.25). Towards the tip of the rachis the bundles are arranged in C-shaped manner and their number is comparatively less. Each vascular bundle remains surrounded by a bundle sheath (Fig. 8.26). It is conjoint, collateral and open.

The xylem in each vascular bundle is present towards inner side. It consists of tracheids and xylem parenchyma. Cambium separates the xylem from the phloem. Vessels are absent.

The vascular bundles are diploxylic, i.e. consists of two types of xylem viz. centripetal xylem and centrifugal xylem. Phloem, present towards the outer side of the vascular bundle, consists of sieve tubes and phloem parenchyma. Companion cells are absent.

The vascular bundles show different structure at different levels of rachis starting from the base up to the apex, especially with regard to their diploxylic nature.

Their brief description is under mentioned:

(a) Vascular Bundles At the Base of Rachis:

Only the centrifugal xylem is well-developed in the vascular bundles (Fig. 8.27A). Its protoxylem faces towards the centre showing endarch condition. Centripetal xylem is not developed.

(b) Vascular Bundles In the Middle of Rachis:

Both centripetal as well as centrifugal xylem are present showing diploxylic condition (Fig. 8.27B). Centripetal xylem is present just opposite to the protoxylem of the centrifugal xylem.

(c) Vascular Bundles At the Apex of Rachis:

Centripetal xylem is well-developed, triangular and exarch (Fig. 8.27C). Centrifugal xylem is much reduced and present in the form of two patches lying one on each side of the protoxylem elements of centripetal xylem. Centrifugal xylem is totally absent at the extreme tip of the rachis.

(viii) Leaflet:

Cycas leaflets are large, tough, thick and leathery. In a vertical section the leaflet is differentiated into a swollen midrib portion and two lateral wings (Figs. 8.28, 8.29). In C. revoluta and C. beddomei the wings are curved downward or revoluted at the margins but in C. circinalis, C. rumphii, C. pectinata and C. siamensis the margins are flat.

Epidermis is the outermost layer consisting of thick-walled cells. It is surrounded by a thick layer of cuticle. Upper epidermis is a continuous layer while the continuity of the lower epidermis is broken by many sunken stomata. On all the sides of the epidermal cells occur simple pits almost in regular series.

According to Pant and Mehra (1964), the stomata are of haplocheilic type (perigenous) in Cycas circinalis, C. revoluta and C rumphii. Hypodermis is sclerenchymatous and present below the epidermis. It is absent below the lower epidermis but in the midrib region it is several-celled thick.

Mesophyll is well-developed and remains differentiated into palisade and spongy parenchyma. A continuous layer of palisade is present below the sclerenchymatous hypodermis. Its cells are radially elongated and filled with chloroplasts. The palisade may be a continuous layer over the midrib as in Cycas beddomei, C. media, C. pectinata and C. revoluta, or it may be a discontinuous layer as in C. circinalis and C. rumphii.

Spongy parenchyma is present only in the wings, directly above the lower epidermis. Its cells are oval, filled with chloroplasts, and loosely arranged having many air-filled intercellular spaces. Transfusion tissue consists of two small groups of short and wide tracheid-like cells with reticulate thickenings or bordered pits on their walls.

These cells have been named as transfusion tissue by Von Mohl (1871), and were first described by Frank (1864). Few layers of transversely elongated cells are present in both the wings just in between the palisade and spongy parenchyma.

This represents the accessory transfusion tissue or secondary transfusion tissue. The secondary’ transfusion tissue has also been named as hydrostereom by Bernard (1904) or radial parenchyma by Pilger (1926). A great phylogenetic significance has been attributed to the transfusion tissue by Worsdell (1897).

Vascular bundle is one, and present in the midrib region of the leaflet. It is conjoint, collateral, open and diploxylic. The triangular centrifugal xylem is well-developed with endarch protoxylem. It is represented by two or sometimes more small groups on either side of the protoxylem.

Phloem is arc-shaped and remains separated by cambium. Phloem consists of sieve tubes and phloem parenchyma. Companion cells are absent. The portion of the midrib in between the palisade layer and lower hypodermal region is filled with parenchymatous cells. Some of these cells contain calcium oxalate crystals.

4. Reproduction in Cycas:

(i) Vegetative Reproduction:

The most common method of vegetative propagation in Cycas is by bulbils. The bulbils develop from the axil of the scaly leaves. They are more or less oval structures with a broad base narrowing towards the apex. Several scaly leaves are arranged spirally and compactly over a dormant stem in a bulbil (Fig. 8.30).

On detachment from the stem, a bulbil starts germination by producing many roots towards the lower side and a leaf towards the upper side. A bulbil from male plant will develop only into the male plant, while from the female plant will form only the female plant because Cycas is a strictly dioecious plant.

(ii) Sexual Reproduction:

Cycas is strictly dioecious, i.e. male and female sex organs are borne on separate plants. After several years of vegetative growth the plants start to form sex organs. Generally, Cycads of more than 10 years of age produce the sex organs.

The male plants develop male cones or male strobili bearing microsporophyll’s, while the female plants produce a loose collection of megasporophylls. The male cone is terminal while the megasporophylls are produced in succession with the leaves at the top of the stem.

Male Reproductive Structures:

The male cone (Fig. 8.31) or male strobilus is a large, conical or ovoid, compact, solitary and shortly-stalked structure, which is generally terminal in position. It sometimes attains a length of as much as 1.5 metre. In the centre of the cone is present a cone axis (Fig. 8.32).

Several perpendicularly attached microsporophyll’s are arranged around the cone axis in closely set spirals. At the base of male cone are present many young leaves. All the microsporophyll’s in a male cone are fertile except a few at its basal and apical parts.

The terminal growth of the stem is checked for sometime when a male cone appears at its apex. It is because of the fact that the apical meristem is used up during the development of the male cone. Cones of some species of Cycas are amongst the largest cones in the plant kingdom.

2. Microsporophyll’s, Microsporangia and Microspores:

Microsporophyll’s (Fig. 8.33) are flat, leaf-like, woody and brown-coloured structures with narrow base and expanded upper portion. The upper expanded portion becomes pointed and is called apophysis. Narrow base is attached to the cone axis with a short stalk.

Each microsporophyll contains two surfaces, i.e. an adaxial or upper surface and an abaxial or lower surface. On the adaxial surface is present a ridge-like projection in the middle and an apophysis at the apex (Fig. 8.33).

On the abaxial surface (Fig. 8.34A) are present thousands of microsporangia in the middle region in the groups of 3-5. Each such group is called a sorus. In between these groups are present many hair-like structures, which are very soft and one or two- celled structures (Fig. 8.34B).

In T.S. of a microsporophyll, there are present many microsporangia on the abaxial side (Fig. 8.35). Each shortly-stalked, oval or sac-like microsporangium is surrounded by 5-6 layers. The wall layers of each sporangium include an outer thick epidermis or exothecium, middle zone of thin-walled cells and an innermost layer of tapetum (Fig. 8.36).

Many pollen grains or microspores are present in each sporangium. In the expanded region of microsporophyll are present many mucilaginous canals and vascular bundles. Each sporangium is provided with a radial line of dehiscence, which helps in the dispersal of spores.

Microsporophyll’s are un-branched but Kashyap (1930) reported some abnormal branching of microsporophyll’s. On an average 700 (Cycas circinalis) to 1160 (C. media) sporangia per sporophyll have been reported. More than 7,00,00,00,000 microspores per cone may be present.

Each microspore or pollen grain is a rounded, unicellular and uninucleate structure surrounded by an outer thick exine and inner thin intine. Cytoplasm surrounds the centrally located nucleus. A large vacuole is also present (Fig. 8.37).

Scanning electron microscopic studies of Sahashi and Ueno (1986) on the pollen grains of Cycas revoluta suggest that they are oblong with 1-sulcate shrunken aperture. Reticulum-like sculpting’s are present on the inner layer of exine, and in this character Cycas resembles with Ginkgo biloba.

3. Development of Microsporangium:

It is of eusporangiate type (Fig. 8 .38). Few hypodermal sporangial initials divide penclinally to form outer primary wall cells and inner primary sporogenous cells. Primary wall cells divide and re-divide periclinally as well as anticlinally to form 5-7 cells thick wall of the sporangium while the primary sporogenous cells divide to form many sporogenous cells.

By further divisions the sporogenous cells develop into microspore mother cells. The latter divide reductionally to form haploid microspores or pollen grains arranged tetrahedrally.

The tapetum, which is utilized for the spore formation, develops either from the outermost layer of the sporogenous tissue or from the innermost layer of the wall tissue. Microspore is the first cell of the male gametophyte having haploid number of chromosomes.

The haploid chromosome number in Cycas is 11. But in C. revoluta it is also sometimes 12. The female plants are homogametic with XX-type of chromosomes while the male plants are heterogametic having chromosomes of XY-type.

Female Reproductive Organs:

True female cone or strobilus is absent Cycas. Female reproductive organs are present in the form of megasporophylls. Many megasporophylls are present around the apex of the monopodial trunk of the female plant above each crown of foliage and scaly leaves (Fig. 8.39).

Similar to foliage leaves, megasporophylls also remain spirally arranged at the apex of the stem but their number is very large and thus they appear like a rosette. Vegetative leaves and fertile megasporophylls are produced in an alternate succession without showing any effect on apical men stem.

Pant (1953) observed that usually the megasporophylls in Cycas are produced only once in a year. From the apex of the main stem the megasporophylls arise in an acropetal succession.

Each megasporophyll is considered a modification of foliage leaf. It reaches up to 30 cm or more in length in different species. It is a flat body consisting of an upper dissected or pinnate leafy portion, middle ovule-bearing portion and proximal petiole. Petiole varies in length in different species.

The middle part is comparatively wider than petiole and bears ovules arranged in two pinnate rows. The number of ovules varies between 2-12 in different species. The ovules are green when young but at maturity they are fleshy and bright orange or red-coloured structures.

The upper, conical sterile part of the megasporophyll is pinnately divided in Cycas revoluta (Fig. 8.40), C. pectinata (Fig. 8.41 B) and C. siamensis (Fig. 8.41 A). But the margin of the upper part is variously serrate with a tapering acute apex in C. beddomei (Fig. 8.42C), C. circinalis (Fig. 8.42A) and C. rumphii (Fig. 8.42B).

Cycas thouarsi contains the largest ovule amongst the living gymnosperms measuring about 7 cm in length. The megasporophylls remain covered by many yellow or brown-coloured hairs.

Cycas ovules are orthotropous, unitegmic and shortly-stalked. Generally, one or sometimes a few more ovules develop fully on a megasporophyll. Many un-pollinated ones remain small and ultimately abort.

Outer surface of the ovule may be smooth as in C. circinalis or covered with orange-yellow hairs as in C. revoluta. After fertilization these hairs are lost, the ovule changes into seed and its colour changes from orange-yellow to bright red.

The single integument is very thick and covers the ovule from all sides except a mouth-like opening called micropyle.

The integument consists of three layers:

(i) Outer, green or orange, fleshy layer called sarcotesta,

(ii) Middle, yellow, stony layer called sclerotesta, and

Several tannin cells and mucilage canals are present in the parenchymatous region of sarcotesta. Some pigments are also present in sarcotesta and epidermis. The sclerotesta consists of lignified thick-walled cells. The inner fleshy layer consists of parenchymatous cells, and it remains in close association with the nucellus.

The nucellus grows out into a beak-like portion called nucellar beak. The latter protrudes into the micropylar canal. Certain cells at the top of the nucellus dissolve and form a cavity like structure called pollen chamber (Fig. 8.43). Pollen grains are received in the pollen chamber after pollination.

The nucellus gets reduced in the form of a thin papery layer in mature seeds and encloses the massive female gametophyte (endosperm). An enlarged megaspore or the embryo-sac is present within the nucellus. The endosperm is formed by the repeated divisions of the megaspore nucleus followed by free cell formation.

Just below the pollen chamber is present an archegonial chamber. 3-6 archegonia are present in the female gametophyte near the archegonial chamber. The latter remains filled with a fluid.

3. Vascular Supply of the Ovule:

Stopes (1904) has worked on the vascular supply of Cycas seed. Out of several bundles of the megasporophyll only three enter the base of the ovule (Fig. 8.43). Out of these three bundles, the central one entefs into the base of the inner fleshy layer of the integument. After its entrance it divides into number of branches, all of which reach up to chalazal end of the nucellus. But none of them penetrates the nucellus.

Each of the remaining two lateral bundles enters the outer fleshy layer and bifurcates into a large outer branch and a small inner branch. The collateral and mesarch outer branch runs all through the outer fleshy layer up to the apex of the ovule. The remaining inner branch penetrates the strong middle stony layer and enters the inner fleshy layer, to which it supplies up to the micropylar end of the ovule.

4. Formation of Megaspores:

In the central region of the nucellus, the nucleus of one of the cell enlarges. Its cytoplasmic contents become dense and it also increases in its size. This cell represents the megaspore mother cell, which divides reductionally to form four haploid megaspores arranged in a linear tetrad (Fig. 8.44).

Out of these four megaspores, the upper three present towards the micropylar end degenerate, leaving only the lowermost functional megaspore or embryo sac cell. This is the fist cell of the female gametophyte.

5. Economic Importance of Cycas:

1. Cycas is used as a source of food in Japan, Australia, South East Asia, southern and eastern parts of India and some other countries. It is used in the preparation of starch and alcoholic drinks. The starch, extracted from its stem, is called ‘sago’.

‘Sago’ is prepared in the following way:

The bark of the trunk is removed, and the trunk is cut into thin discs. These are dried, ground and a paste is prepared by adding water Excess of water is added, and the paste is left for some time in a standstill position.

The starch settles down, and the clear upper liquid is drained off. Between the boards, the starch is rolled. This gives the starch a characteristic round shape. It is finally dried and sold as ‘sago’ in the market.

2. In Japan, seeds and stem of Cycas revoluta are used for preparing wine.

3. The juice obtained from young leaves of Cycas circinalis is used in skin diseases, vomiting of blood and stomach disorders.

4. The decoction of young red seeds of C. circinalis is used as a purgative and emetic.

5. To relieve the headache, giddiness and sore throat, the seeds of Cycas revoluta are prepared in the form of a tincture and used.

6. Cycas revoluta and C. circinalis plants are grown for ornamental purposes in various parts of the world.

7. The wood of Cycas revoluta is used for preparing small boxes and dishes.

8. Cycas leaves, being very large, are used for preparing baskets, mats, etc.

9. Cycas circinalis seeds are used in Democratic Kampuchea as a fish-poison.


Colony Shape

It includes form, elevation, and margin of the bacterial colony.

Form of the bacterial colony: – The form refers to the shape of the colony. These forms represent the most common colony shapes you are likely to encounter. e.g. circular, irregular, filamentous, rhizoid, etc.

Elevation of the bacterial colony: It gives information about, how much does the colony rise above the agar. This describes the “side view” of a colony. These are the most common elevations e.g. flat, raised, umbonate (having a knobby protuberance), crateriform, convex, pulvinate (cushion-shaped).

Margin of bacterial colony: The margin or edge of a colony may be an important characteristic in identifying organisms. Common examples are entire (smooth), irregular, undulate (wavy), lobate, curled, filiform, etc.

Colonies that are irregular in shape and/or have irregular margins are likely to be motile organisms. Highly motile organisms swarmed over the culture media, such as Proteus spp.


105 Viral Evolution, Morphology, and Classification

By the end of this section, you will be able to do the following:

  • Describe how viruses were first discovered and how they are detected
  • Discuss three hypotheses about how viruses evolved
  • Describe the general structure of a virus
  • Recognize the basic shapes of viruses
  • Understand past and emerging classification systems for viruses
  • Describe the basis for the Baltimore classification system

Viruses are diverse entities: They vary in structure, methods of replication, and the hosts they infect. Nearly all forms of life—from prokaryotic bacteria and archaeans, to eukaryotes such as plants, animals, and fungi—have viruses that infect them. While most biological diversity can be understood through evolutionary history (such as how species have adapted to changing environmental conditions and how different species are related to one another through common descent), much about virus origins and evolution remains unknown.

Discovery and Detection

Viruses were first discovered after the development of a porcelain filter—the Chamberland-Pasteur filter—that could remove all bacteria visible in the microscope from any liquid sample. In 1886, Adolph Meyer demonstrated that a disease of tobacco plants— tobacco mosaic disease —could be transferred from a diseased plant to a healthy one via liquid plant extracts. In 1892, Dmitri Ivanowski showed that this disease could be transmitted in this way even after the Chamberland-Pasteur filter had removed all viable bacteria from the extract. Still, it was many years before it was proved that these “filterable” infectious agents were not simply very small bacteria but were a new type of very small, disease-causing particle.

Most virions , or single virus particles, are very small, about 20 to 250 nanometers in diameter. However, some recently discovered viruses from amoebae range up to 1000 nm in diameter. With the exception of large virions, like the poxvirus and other large DNA viruses, viruses cannot be seen with a light microscope. It was not until the development of the electron microscope in the late 1930s that scientists got their first good view of the structure of the tobacco mosaic virus (TMV) ((Figure)), discussed above, and other viruses ((Figure)). The surface structure of virions can be observed by both scanning and transmission electron microscopy, whereas the internal structures of the virus can only be observed in images from a transmission electron microscope. The use of electron microscopy and other technologies has allowed for the discovery of many viruses of all types of living organisms.


Evolution of Viruses

Although biologists have a significant amount of knowledge about how present-day viruses mutate and adapt, much less is known about how viruses originated in the first place. When exploring the evolutionary history of most organisms, scientists can look at fossil records and similar historic evidence. However, viruses do not fossilize, as far as we know, so researchers must extrapolate from investigations of how today’s viruses evolve and by using biochemical and genetic information to create speculative virus histories.

Most scholars agree that viruses don’t have a single common ancestor, nor is there a single reasonable hypothesis about virus origins. There are current evolutionary scenarios that may explain the origin of viruses. One such hypothesis, the “devolution” or the regressive hypothesis, suggests that viruses evolved from free-living cells, or from intracellular prokaryotic parasites. However, many components of how this process might have occurred remain a mystery. A second hypothesis, the escapist or the progressive hypothesis, suggests that viruses originated from RNA and DNA molecules that escaped from a host cell. A third hypothesis, the self-replicating hypothesis, suggests that viruses may have originated from self-replicating entities similar to transposons or other mobile genetic elements. In all cases, viruses are probably continuing to evolve along with the cells on which they rely on as hosts.

As technology advances, scientists may develop and refine additional hypotheses to explain the origins of viruses. The emerging field called virus molecular systematics attempts to do just that through comparisons of sequenced genetic material. These researchers hope one day to better understand the origin of viruses—a discovery that could lead to advances in the treatments for the ailments they produce.

Viral Morphology

Viruses are noncellular , meaning they are biological entities that do not have a cellular structure. They therefore lack most of the components of cells, such as organelles, ribosomes, and the plasma membrane. A virion consists of a nucleic acid core, an outer protein coating or capsid , and sometimes an outer envelope made of protein and phospholipid membranes derived from the host cell. Viruses may also contain additional proteins, such as enzymes, within the capsid or attached to the viral genome. The most obvious difference between members of different viral families is the variation in their morphology, which is quite diverse. An interesting feature of viral complexity is that the complexity of the host does not necessarily correlate with the complexity of the virion. In fact, some of the most complex virion structures are found in the bacteriophages —viruses that infect the simplest living organisms, bacteria.

Morphology

Viruses come in many shapes and sizes, but these features are consistent for each viral family. As we have seen, all virions have a nucleic acid genome covered by a protective capsid. The proteins of the capsid are encoded in the viral genome, and are called capsomeres . Some viral capsids are simple helices or polyhedral “spheres,” whereas others are quite complex in structure ((Figure)).


In general, the capsids of viruses are classified into four groups: helical, icosahedral, enveloped, and head-and-tail. Helical capsids are long and cylindrical. Many plant viruses are helical, including TMV. Icosahedral viruses have shapes that are roughly spherical, such as those of poliovirus or herpesviruses. Enveloped viruses have membranes derived from the host cell that surrounds the capsids. Animal viruses, such as HIV, are frequently enveloped. Head-and-tail viruses infect bacteria and have a head that is similar to icosahedral viruses and a tail shaped like helical viruses.

Many viruses use some sort of glycoprotein to attach to their host cells via molecules on the cell called viral receptors . For these viruses, attachment is required for later penetration of the cell membrane only after penetration takes place can the virus complete its replication inside the cell. The receptors that viruses use are molecules that are normally found on cell surfaces and have their own physiological functions. It appears that viruses have simply evolved to make use of these molecules for their own replication. For example, HIV uses the CD4 molecule on T lymphocytes as one of its receptors ((Figure)). CD4 is a type of molecule called a cell adhesion molecule, which functions to keep different types of immune cells in close proximity to each other during the generation of a T lymphocyte immune response.


One of the most complex virions known, the T4 bacteriophage (which infects the Escherichia coli) bacterium, has a tail structure that the virus uses to attach to host cells and a head structure that houses its DNA.

Adenovirus, a non-enveloped animal virus that causes respiratory illnesses in humans, uses glycoprotein spikes protruding from its capsomeres to attach to host cells. Non-enveloped viruses also include those that cause polio (poliovirus), plantar warts (papillomavirus), and hepatitis A (hepatitis A virus).

Enveloped virions, such as the influenza virus, consist of nucleic acid (RNA in the case of influenza) and capsid proteins surrounded by a phospholipid bilayer envelope that contains virus-encoded proteins. Glycoproteins embedded in the viral envelope are used to attach to host cells. Other envelope proteins are the matrix proteins that stabilize the envelope and often play a role in the assembly of progeny virions. Chicken pox, HIV, and mumps are other examples of diseases caused by viruses with envelopes. Because of the fragility of the envelope, non-enveloped viruses are more resistant to changes in temperature, pH, and some disinfectants than enveloped viruses.

Overall, the shape of the virion and the presence or absence of an envelope tell us little about what disease the virus may cause or what species it might infect, but they are still useful means to begin viral classification ((Figure)).


Which of the following statements about virus structure is true?

  1. All viruses are encased in a viral membrane.
  2. The capsomere is made up of small protein subunits called capsids.
  3. DNA is the genetic material in all viruses.
  4. Glycoproteins help the virus attach to the host cell.

Types of Nucleic Acid

Unlike nearly all living organisms that use DNA as their genetic material, viruses may use either DNA or RNA. The virus core contains the genome—the total genetic content of the virus. Viral genomes tend to be small, containing only those genes that encode proteins which the virus cannot get from the host cell. This genetic material may be single- or double-stranded. It may also be linear or circular. While most viruses contain a single nucleic acid, others have genomes divided into several segments. The RNA genome of the influenza virus is segmented, which contributes to its variability and continuous evolution, and explains why it is difficult to develop a vaccine against it.

In DNA viruses, the viral DNA directs the host cell’s replication proteins to synthesize new copies of the viral genome and to transcribe and translate that genome into viral proteins. Human diseases caused by DNA viruses include chickenpox, hepatitis B, and adenoviruses. Sexually transmitted DNA viruses include the herpes virus and the human papilloma virus (HPV), which has been associated with cervical cancer and genital warts.

RNA viruses contain only RNA as their genetic material. To replicate their genomes in the host cell, the RNA viruses must encode their own enzymes that can replicate RNA into RNA or, in the retroviruses, into DNA. These RNA polymerase enzymes are more likely to make copying errors than DNA polymerases, and therefore often make mistakes during transcription. For this reason, mutations in RNA viruses occur more frequently than in DNA viruses. This causes them to change and adapt more rapidly to their host. Human diseases caused by RNA viruses include influenza, hepatitis C, measles, and rabies. The HIV virus, which is sexually transmitted, is an RNA retrovirus.

The Challenge of Virus Classification

Because most viruses probably evolved from different ancestors, the systematic methods that scientists have used to classify prokaryotic and eukaryotic cells are not very useful. If viruses represent “remnants” of different organisms, then even genomic or protein analysis is not useful. Why?, Because viruses have no common genomic sequence that they all share. For example, the 16S rRNA sequence so useful for constructing prokaryote phylogenies is of no use for a creature with no ribosomes! Biologists have used several classification systems in the past. Viruses were initially grouped by shared morphology. Later, groups of viruses were classified by the type of nucleic acid they contained, DNA or RNA, and whether their nucleic acid was single- or double-stranded. However, these earlier classification methods grouped viruses differently, because they were based on different sets of characters of the virus. The most commonly used classification method today is called the Baltimore classification scheme, and is based on how messenger RNA (mRNA) is generated in each particular type of virus.

Past Systems of Classification

Viruses contain only a few elements by which they can be classified: the viral genome, the type of capsid, and the envelope structure for the enveloped viruses. All of these elements have been used in the past for viral classification ((Figure) and (Figure)). Viral genomes may vary in the type of genetic material (DNA or RNA) and its organization (single- or double-stranded, linear or circular, and segmented or non-segmented). In some viruses, additional proteins needed for replication are associated directly with the genome or contained within the viral capsid.

  • RNA
  • DNA
  • Rabies virus, retroviruses
  • Herpesviruses, smallpox virus
  • Single-stranded
  • Double-stranded
  • Rabies virus, retroviruses
  • Herpesviruses, smallpox virus
  • Linear
  • Circular
  • Rabies virus, retroviruses, herpesviruses, smallpox virus
  • Papillomaviruses, many bacteriophages
  • Non-segmented: genome consists of a single segment of genetic material
  • Segmented: genome is divided into multiple segments
  • Parainfluenza viruses
  • Influenza viruses


Viruses can also be classified by the design of their capsids ((Figure) and (Figure)). Capsids are classified as naked icosahedral, enveloped icosahedral, enveloped helical, naked helical, and complex. The type of genetic material (DNA or RNA) and its structure (single- or double-stranded, linear or circular, and segmented or non-segmented) are used to classify the virus core structures ((Figure)).

Virus Classification by Capsid Structure
Capsid Classification Examples
Naked icosahedral Hepatitis A virus, polioviruses
Enveloped icosahedral Epstein-Barr virus, herpes simplex virus, rubella virus, yellow fever virus, HIV-1
Enveloped helical Influenza viruses, mumps virus, measles virus, rabies virus
Naked helical Tobacco mosaic virus
Complex with many proteins some have combinations of icosahedral and helical capsid structures Herpesviruses, smallpox virus, hepatitis B virus, T4 bacteriophage


Baltimore Classification

The most commonly and currently used system of virus classification was first developed by Nobel Prize-winning biologist David Baltimore in the early 1970s. In addition to the differences in morphology and genetics mentioned above, the Baltimore classification scheme groups viruses according to how the mRNA is produced during the replicative cycle of the virus.

Group I viruses contain double-stranded DNA (dsDNA) as their genome. Their mRNA is produced by transcription in much the same way as with cellular DNA, using the enzymes of the host cell.

Group II viruses have single-stranded DNA (ssDNA) as their genome. They convert their single-stranded genomes into a dsDNA intermediate before transcription to mRNA can occur.

Group III viruses use dsRNA as their genome. The strands separate, and one of them is used as a template for the generation of mRNA using the RNA-dependent RNA polymerase encoded by the virus.

Group IV viruses have ssRNA as their genome with a positive polarity, which means that the genomic RNA can serve directly as mRNA. Intermediates of dsRNA, called replicative intermediates , are made in the process of copying the genomic RNA. Multiple, full-length RNA strands of negative polarity (complementary to the positive-stranded genomic RNA) are formed from these intermediates, which may then serve as templates for the production of RNA with positive polarity, including both full-length genomic RNA and shorter viral mRNAs.

Group V viruses contain ssRNA genomes with a negative polarity , meaning that their sequence is complementary to the mRNA. As with Group IV viruses, dsRNA intermediates are used to make copies of the genome and produce mRNA. In this case, the negative-stranded genome can be converted directly to mRNA. Additionally, full-length positive RNA strands are made to serve as templates for the production of the negative-stranded genome.

Group VI viruses have diploid (two copies) ssRNA genomes that must be converted, using the enzyme reverse transcriptase , to dsDNA the dsDNA is then transported to the nucleus of the host cell and inserted into the host genome. Then, mRNA can be produced by transcription of the viral DNA that was integrated into the host genome.

Group VII viruses have partial dsDNA genomes and make ssRNA intermediates that act as mRNA, but are also converted back into dsDNA genomes by reverse transcriptase, necessary for genome replication.

The characteristics of each group in the Baltimore classification are summarized in (Figure) with examples of each group.

Baltimore Classification
Group Characteristics Mode of mRNA Production Example
I Double-stranded DNA mRNA is transcribed directly from the DNA template Herpes simplex (herpesvirus)
II Single-stranded DNA DNA is converted to double-stranded form before RNA is transcribed Canine parvovirus (parvovirus)
III Double-stranded RNA mRNA is transcribed from the RNA genome Childhood gastroenteritis (rotavirus)
IV Single stranded RNA (+) Genome functions as mRNA Common cold (picornavirus)
V Single stranded RNA (-) mRNA is transcribed from the RNA genome Rabies (rhabdovirus)
VI Single stranded RNA viruses with reverse transcriptase Reverse transcriptase makes DNA from the RNA genome DNA is then incorporated in the host genome mRNA is transcribed from the incorporated DNA Human immunodeficiency virus (HIV)
VII Double stranded DNA viruses with reverse transcriptase The viral genome is double-stranded DNA, but viral DNA is replicated through an RNA intermediate the RNA may serve directly as mRNA or as a template to make mRNA Hepatitis B virus (hepadnavirus)

Section Summary

Viruses are tiny, noncellular entities that usually can be seen only with an electron microscope. Their genomes contain either DNA or RNA—never both—and they replicate either by using the replication proteins of a host cell or by using proteins encoded in the viral genome. Viruses are diverse, infecting archaea, bacteria, fungi, plants, and animals. Viruses consist of a nucleic acid core surrounded by a protein capsid with or without an outer lipid envelope. The capsid shape, presence of an envelope, and core composition dictate some elements of the classification of viruses. The most commonly used classification method, the Baltimore classification, categorizes viruses based on how they produce their mRNA.

Visual Connection Questions

(Figure) Which of the following statements about virus structure is true?


1 - Morphology and classification of the Marchantiophyta

Liverworts are a diverse phylum of small, herbaceous, terrestrial plants, estimated to comprise about 5000 species in 391 genera. They occupy an assortment of habitats, including disturbed soil along stream banks, road cuts and trails, as well as rocks, logs and trees in natural landscapes. They occur on all continents, including Antarctica, but are most diversified in the montane rain forests of the southern hemisphere. Many species are quite tolerant of repeated cycles of drying and wetting (Clausen 1964, Wood 2007), a feature that has allowed them also to exploit epiphytic substrates, including leaves and branches of the forest canopy. Like mosses and hornworts, they have a heteromorphic life cycle with a sporophyte that is comparatively short-lived and nutritionally dependent on the free-living, usually perennial gametophyte. However, they differ from both of these groups in numerous cytological, biochemical, and anatomical features as detailed by Crandall-Stotler (1984). Significant diagnostic characters of the phylum include the following: they tend to have a flattened appearance, even when leafy, because their leaves are always arranged in rows, never in spiral phyllotaxis rhizoids are unicellular, thin-walled, and usually hyaline both leafy and thalloid forms frequently develop endosymbiotic associations with fungi sporophytes mature completely enclosed by gametophytic tissue and are incapable of self-sustaining photosynthesis sporophyte setae are parenchymatous and elongate by cell expansion, rather than cell division and capsules lack the stomates, cuticle, and columella that are common in mosses and hornworts.

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