(a) How are viruses classified based on their symmetry? With a suitable diagram, explain the structure of T4 phage. 3+7=10 (b)…

(a) Classification of viruses by symmetry; structure of T4 phage

The protein coat (capsid), made of subunits called capsomeres, has one of three symmetries:

• Helical: capsomeres are arranged in a spiral around the nucleic acid, giving a rod or filament. Examples are tobacco mosaic virus (a rigid rod about 300 × 18 nm) and the nucleocapsid of influenza.
• Icosahedral (cubic): a closed shell with 20 triangular faces and 12 vertices, the most economical way to enclose a genome. Examples are adenovirus (252 capsomeres), poliovirus and cauliflower mosaic virus.
• Complex (binal): no single symmetry. Poxviruses are brick-shaped, and tailed phages combine an icosahedral head with a helical tail.

Structure of bacteriophage T4 (a tailed phage that infects E. coli):

• Head: a prolate (elongated) icosahedron about 111 × 78 nm. It holds about 169 kb of linear double-stranded DNA, in which cytosine is replaced by glucosylated 5-hydroxymethylcytosine. A terminase motor packs the DNA into the head until it is full ("headful" packaging).
• Collar and whiskers: a thin collar joins the head to the tail, and fine whisker fibres help assembly.
• Tail: about 113 nm long, made of a hollow central tube surrounded by a contractile sheath of 144 subunits arranged in 24 rings.
• Base plate: hexagonal, carrying six short tail fibres (pins) and six long, kinked tail fibres.
• Infection: the long fibres recognise receptors on the host surface (lipopolysaccharide or OmpC), and the base plate changes shape. The sheath contracts from about 98 to 42 nm, driving the tube through the cell wall, and the DNA is injected while the empty coat remains outside.

(b) Modes of entry of plant pathogens into their hosts

• Direct penetration of intact surfaces: many fungi attach by an adhesive germ tube and form an appressorium. A narrow infection peg then pierces the cuticle and cell wall by turgor pressure and enzymes (cutinases, cellulases, pectinases). Examples are Magnaporthe oryzae (rice blast), Colletotrichum (anthracnose), Botrytis cinerea and powdery mildews (Erysiphe). Inside the cell, obligate parasites form haustoria. Parasitic plants such as Cuscuta and Orobanche also penetrate directly using haustoria.

Natural openings:

• Stomata: urediniospores of rust (Puccinia), downy mildews, and bacteria such as Pseudomonas syringae.
• Hydathodes: Xanthomonas campestris pv. campestris (black rot of crucifers) and X. oryzae pv. oryzae (bacterial leaf blight of rice).
• Lenticels: Streptomyces scabies (potato scab) and soft-rot bacteria (Pectobacterium).
• Nectarthodes (nectaries): Erwinia amylovora (fire blight of apple and pear).
• Stigma and flowers: loose smut of wheat (Ustilago tritici) infects the developing embryo through the flower and is then carried in the seed.
• Wounds: injuries from pruning, hail, insects or grafting. Examples are Agrobacterium tumefaciens (crown gall), Ceratocystis, many wood-rot fungi, and mechanically transmitted viruses such as TMV.
• Roots and root hairs: Plasmodiophora brassicae (club root), Fusarium wilts entering through root tips and wounds, and root-knot nematodes (Meloidogyne), which penetrate roots and make wounds used by wilt fungi.

Vectors:

• Aphids: potyviruses and cucumber mosaic virus.
• Whiteflies: begomoviruses such as cotton leaf curl and tomato leaf curl.
• Leafhoppers: rice tungro viruses and phytoplasmas.
• Thrips: tospoviruses such as peanut bud necrosis virus.
• Soil fungi and nematodes: Polymyxa transmits beet necrotic yellow vein virus, and Xiphinema transmits grapevine fanleaf virus.
• Seed, pollen and vegetative propagules: these carry many viruses and bacteria into new plants, for example bean common mosaic virus in seed and banana bunchy top in suckers.

(c) Ecological and economic significance of bryophytes

Ecological

• Pioneers: mosses and lichen communities (Grimmia, Tortula) colonise bare rock, trap dust and help form soil in primary succession.
• Water and erosion control: moss carpets act like sponges, holding many times their dry weight in water. They regulate run-off, reduce soil erosion on slopes, and keep forest floors moist for seedlings.
• Peatlands: Sphagnum bogs store about a third of the world's soil carbon while covering only about 3% of land, so draining them releases CO₂. Their acidity and waterlogging slow decomposition.
• Nutrient cycling: mosses capture nutrients from rain and dust. Some host nitrogen-fixing cyanobacteria (for example on Anthoceros and in feather mosses).
• Indicators: they are sensitive to SO₂ and heavy metals, so their absence signals air pollution. They are also used as biomonitors, and some species indicate particular soils (for example "copper mosses").

Economic

• Peat: used as fuel (Ireland, Finland), in horticulture as potting and seed-raising media, and in whisky making.
• Sphagnum: absorbent and mildly antiseptic, it was used as wound dressing in World War I. It is also used as packing material for live plants and in orchid culture.
• Medicinal and chemical uses: Marchantia contains antimicrobial and antifungal compounds (marchantins). Mosses have been used in traditional Chinese and Himalayan medicine and are sources of bioactive terpenoids.
• Other uses: moss gardens and decoration, and model organisms in research, such as Physcomitrium patens and Marchantia polymorpha.
• Conservation: many bryophytes are threatened by habitat loss, including endemic Indian liverworts of the Western Ghats and Eastern Himalaya.

(d) Natural distribution of gymnosperms in India; Bennettitales fossils

Living gymnosperms (about 80 species)

Western and Central Himalaya:

• Pinus roxburghii (chir pine, 900–1,800 m) and P. wallichiana (blue pine).
• P. gerardiana (chilgoza pine, Kinnaur and Chitral).
• Cedrus deodara (deodar), Abies pindrow and A. spectabilis (firs), and Picea smithiana (spruce).
• Cupressus torulosa, Juniperus species (up to the tree line), and Taxus wallichiana (yew, source of taxol, now endangered).
• Dry trans-Himalaya (Ladakh, Lahaul–Spiti): Ephedra gerardiana.

Eastern Himalaya and the North-East:

• Tsuga dumosa (hemlock), Larix griffithii (larch), Picea spinulosa and Abies densa.
• Pinus kesiya (Khasi pine, Meghalaya and Manipur).
• Cephalotaxus, Amentotaxus assamica (endemic to Arunachal Pradesh), Podocarpus neriifolius and Gnetum species.

Peninsular India, Western and Eastern Ghats:

• Cycas circinalis; C. beddomei, endemic to the Seshachalam hills of Andhra Pradesh; Nageia wallichiana (Podocarpus wallichianus); and Gnetum ula, a climber.
• Cycas rumphii grows in the Andaman and Nicobar Islands.

Bennettitales (cycad-like Mesozoic seed plants with flower-like strobili) are well preserved in the Upper Gondwana rocks of Jurassic to Early Cretaceous age:

• Rajmahal Hills, Jharkhand: intertrappean beds of the Rajmahal Formation, Early Cretaceous, about 117 million years old. Birbal Sahni described Williamsonia sewardiana here, along with Ptilophyllum, Otozamites, Pterophyllum and stems (Bucklandia).
• Kachchh (Bhuj Formation), Gujarat.
• South Rewa basin (Jabalpur Formation), Madhya Pradesh.
• East coast Gondwana outcrops: Sriperumbudur and Vemavaram in Tamil Nadu and Andhra Pradesh.

These fossils show that India's Mesozoic flora was dominated by cycadophytes and conifers before angiosperms spread.

(e) Principal directions of floral evolution; the ANA grade

Trends usually regarded as advanced

• Arrangement: from spiral, indefinite and numerous parts on an elongated receptacle to whorled (cyclic) parts of definite, reduced number, often in 3s or 5s.
• Perianth: from undifferentiated tepals to a distinct calyx and corolla.
• Fusion: from free parts to fused ones, including fused sepals (synsepaly), fused petals (sympetaly), stamens joined to petals (epipetaly) and fused carpels (apocarpy to syncarpy).
• Symmetry: from radial (actinomorphic) to bilateral (zygomorphic), as in orchids, the pea family and the mint family.
• Ovary position: from superior (hypogyny) through perigyny to inferior (epigyny).
• Sex and pollination: from bisexual towards unisexual flowers in some wind-pollinated lineages. Pollination moved from generalised beetle or fly pollination to specialised pollination by bees, birds, moths and bats, with nectaries, spurs and landing platforms.
• Aggregation: flowers gathered into compact inflorescences such as capitula (Asteraceae).

These trends (Bessey's "dicta") are useful generalisations, but molecular phylogenies show frequent parallelism and reversals. The earliest-diverging living angiosperm, Amborella, has small, unisexual flowers with spirally arranged tepals, not the large magnolia-like flower once assumed to be primitive.

The ANA grade (the earliest branches of flowering plants, APG IV):

• Amborellales: Amborella trichopoda, a shrub from New Caledonia.
• Nymphaeales: water lilies (Nymphaea, Nuphar, Euryale), Cabomba and Hydatellaceae.
• Austrobaileyales: Austrobaileya, Illicium (star anise), Schisandra, Kadsura and Trimenia.

All other angiosperms, the mesangiosperms (magnoliids, monocots, Ceratophyllum and eudicots), diverged after these three lineages.