(a) Enumerate the types of fruiting bodies of Ascomycota and Basidiomycota. Write the various steps of ascus formation in…

Fruiting Bodies and Ascus Formation

Ascomycota produce sexual fruiting bodies known as ascocarps, which are categorized into three main types based on their morphology and spore release mechanism. The Apothecium is a cup-shaped, open structure where asci are exposed on the hymenium, facilitating passive spore dispersal; examples include Peziza and Neurospora. The Perithecium is a flask-shaped, closed structure with a narrow opening called an ostiole, protecting the asci; Penicillium and Aspergillus (in some sexual stages) exhibit this form. The Cleistothecium is a completely closed, spherical sac that ruptures to release spores, as seen in Erysiphe (powdery mildews).

Basidiomycota produce basidiocarps, primarily classified by their shape and spore-bearing surface. The Agaric (mushroom) features a pileus and stipe with gills, e.g., Agaricus bisporus. Polypores (bracket fungi) are woody, shelf-like structures with pores on the underside, e.g., Polyporus. Puffballs (Lycoperdon) are spherical and closed, releasing spores like dust. Clavarioid forms are club-shaped, e.g., Clavaria. Phalloid forms, such as stinkhorns (Phallus), have a distinctive stalk and cap, often with foul odors to attract vectors.

Ascus Formation Ascus development in Ascomycetes is a highly regulated process. It begins with the differentiation of ascogenous hyphae, where a hook or crozier forms at the tip. This hook bends back, and the nucleus divides mitotically. One nucleus migrates to the tip, while the other remains in the foot cell. The tip cell undergoes karyogamy (fusion of two haploid nuclei) to form a diploid nucleus. This is followed by meiosis I, producing two haploid nuclei, and meiosis II, resulting in four haploid nuclei. A subsequent mitotic division yields eight haploid nuclei. These nuclei become delimitated by cell walls to form eight ascospores within the ascus. The ascus wall may be vegetative (thin) or thick-walled, and in some species, the ascospores are forcibly discharged.

Viral Infection and Dissemination

Plant viruses infect hosts through specific entry points and disseminate via various vectors. Mechanical inoculation occurs when plant sap containing virions is transferred to wounds caused by pruning, harvesting, or natural abrasions. Grafting transmits viruses through vascular connections, particularly in perennial crops like citrus and grapes. Seed transmission is rare but significant in legumes (e.g., Bean Common Mosaic Virus) and cereals. Pollen transmission allows viruses to move from male to female flowers during pollination. Dodder (Cuscuta) acts as a parasitic vector, connecting to host plants and transmitting viruses through its haustoria.

Dissemination is heavily reliant on insect vectors. Aphids transmit viruses in non-persistent (short duration, e.g., Tobacco Mosaic Virus), semi-persistent, or persistent (long duration, e.g., Beet Yellows Virus) styles. Leafhoppers and whiteflies are key vectors for many rice and vegetable viruses. Nematodes (e.g., Xiphinema) transmit viruses like Beet Necrotic Yellow Vein Virus through soil. Fungal vectors such as Olpidium and Polymyxa transmit viruses in sedges and other monocots. Human agency also contributes to dissemination through the movement of infected planting material, seeds, and tools across regions. In India, the Tungro virus of rice is disseminated by the green rice leafhopper (Nephotettix spp.), causing significant yield losses in paddy fields.

Gnetum Strobili and Gymnosperm Status

Male Strobilus The male strobilus of Gnetum is a compact, strobiloid structure. It consists of a central axis bearing decussate bracts. Each bract bears a microsporangiophore on its inner surface. A single whorl of 3–6 microsporangiophores is present per collar. Each microsporangiophore bears 2–3 microsporangia on its inner surface. The microsporangia produce microspores that develop into male gametophytes with prothallial cells, which eventually produce two sperm cells.

Female Strobilus The female strobilus is larger and more complex. It is a compound structure with an outer envelope formed by bracts. Inside, there are nucelli (ovules) arranged in whorls. Each nucellus is surrounded by an integument and possesses a micropylar tube for pollen entry. The ovules are naked, lacking an ovary wall.

Gymnosperm Justification Gnetum is classified as a gymnosperm primarily because its ovules are naked, not enclosed within an ovary. Although it exhibits double fertilization (the second male gamete fuses with the ventral canal nucleus), it does not form triploid endosperm via triple fusion; instead, it produces a free-nuclear endosperm. The presence of archegonia (though reduced) and the retention of tracheids with bordered pits alongside vessels in its xylem further support its gymnosperm status, distinguishing it from angiosperms which lack archegonia and have true flowers and fruits.