Topic 3

Question 1

Chytridiomycota

Answer 1

Fungi are thought to have evolved from an early group known as the Chytridiomycota. The chytrids (sometimes called water moulds) (Figure 3.3) are usually free-swimming in water and their zoospores (motile reproductive cells) still display the single flagellum of their unikont ancestor. Some modern chytrids have evolved alongside ruminant mammals (cattle, sheep, antelope, deer, etc.) and are found in their rumens (stomachs) where they play a key role in breaking down otherwise indigestible plant fibres. Chytrids currently receive a bad press because one of their number is responsible for the disease of amphibians called chytridiomycosis, which is driving many species of frog towards extinction.

Question 2

Zygomycota

Answer 2

The next stage in fungal evolution is represented by the modern Zygomycota, which was perhaps the first group to emerge onto land. They were also the first major group to exhibit the hyphae (long threads) that characterise most modern fungi. Hyphae (pronounced ‘high-fee’) are extraordinary structures because they can have many nuclei within one ‘cell’, a condition that tends not to occur in either plants or animals. The term for a multinucleate structure is coenocytic (meaning ‘cells together’). A hypha (pronounced ‘high-fuh’) several centimetres long may have continuous cytoplasm and therefore be regarded as a single coenocytic ‘cell’ 500 times as long as a basic animal or plant cell. Many species do have cross walls within their hyphae called septa (singular septum), but these walls are perforated and there is no membrane dividing the separate compartments, so the compartments cannot truly be regarded as separate cells.

Question 3

Glomeromycota

Answer 3

The next major group is the Glomeromycota. Glomeromycetes tend to live in association with plant roots, rarely as pathogens, usually as mutualists. The hyphae of glomeromycetes may spread out into the soil for many metres, but they also penetrate plant roots and run between the root cells. Occasionally, a side branch from a hypha penetrates the plant cell wall and creates a structure known as an arbuscule (Figure 3.6) or a vesicle (a storage organ typically filled with lipid) within the plant cell.

Question 4

Ascomycota and Basidiomycota

Answer 4

The final stage of fungal evolution gave rise to two sister orders: the ascomycetes and the basidiomycetes (Figure 3.7). Unlike the glomeromycetes, which just tend to reproduce from single spores that grow at the ends of their hyphae, these two groups produce large, complex fruiting bodies, in which spores are produced and then dispersed. These large bodies, although appearing to be continuous tissue, are in fact still composed of hyphae, but thousands of them grow tightly together to create a solid mass, which may be as soft as jelly or as hard as rock.

Question 5

the main difference between an ascus and a basidium

Answer 5

The key difference is that the ascomycetes create their spores (usually in sets of eight) internally within a specialised cell, known as an ascus, from where they are forcibly shot into the atmosphere. The basidiomycetes in contrast make their spores externally to an equivalent specialised cell, known as a basidium. These basidiospores normally occur in groups of four and are gently released by the basidium to fall under gravity until the wind picks them up.