|Answers to review questions – chapter 37
1. What features distinguish fungi from other organisms? (pp. 908–909)
Fungi are eukaryotic heterotrophs that digest their food externally and reproduce by forming spores.
2. What are the similarities and key differences between fungal-like protists, red algae (phylum Rhodophyta) and fungi? (p. 907)
The red algae (phylum Rhodophyta) and some fungi have similar morphologies and life cycles. These reflect convergent evolution rather than any taxonomic relationship.
The fungal-like protists have superficial morphological resemblances to fungi, producing a network of coenocytic hyphae (finely branched non-septate filaments that permeate their food substrate). However, there are clear differences between fungi and the fungal-like protists because the cell walls of the protists contain cellulose, not chitin as in fungi; the morphological structure of the mitochondria of the two groups are quite different; and molecular data of important DNA sequences indicate they are not closely related.
The similarities between the fungal-like protists, red algae (phylum Rhodophyta) and fungi appear to be the result of convergent evolution towards the fungal mode of life rather than evolutionary affinities. Important evidence for this concept includes comparisons of ribosomal DNA sequences which show these two groups, the fungal-like protists and the Rhodophyta, are quite different from fungi.
3. What evidence suggests that fungi have evolved from a protistan ancestor not unlike the choanoflagellates? (p. 907)
Molecular evidence from comparison of nucleic acid sequences and ultrastructural details indicate close affinities between fungi and the choanoflagellates.
4. What evidence suggests a close affinity between the ascomycetes and the basidiomycetes?
Evidence suggesting a close affinity between the ascomycetes and the basidiomycetes includes the formation of regular septa in the hyphae of both groups, a dikaryotic phase in both life cycles, and the production of complex macroscopic reproductive structures by species in both these groups of fungi.
5. Describe some of the unusual features of nuclear behaviour in fungi. (pp. 912–913)
Most fungi are haploid and multinucleate because successive mitotic divisions produce multinucleate hyphae. These multinucleate organisms are homokaryons if nuclei are all the same, or heterokaryons if the nuclei are different. Heterokaryotic form when two hyphae containing different nuclei fuse, or when there is a mutation in one of the nuclei in a homokaryotic cell, and the mutated nucleus multiplies by successive mitotic divisions. Fungi are often dikaryotic organisms because of delayed karyogamy.
6. What features allow fungi to survive in all environments where life is possible? Why is this important to us? (pp. 913–914)
Fungi tolerate a wide range of environmental extremes. Xerotolerant fungi can grow at low water potentials because they can release water through the interconversion of storage compounds (e.g. mannitol being metabolised to glycerol and water), or from hydrolysis of cellulose to glucose and water. Thermophilic fungi can grow at temperatures of 50ºC, by adjusting the lipid composition of their membranes to maintain constant fluidity, but prevent melting at the high temperatures. Psychrophilic fungi can tolerate temperatures of –5ºC.
It is important that fungi can be found wherever other organisms are present because fungi are the primary decomposers of organic matter, especially cellulose and other plant components that require extracellular enzymes for their degradation.
7. What is a ‘fungal succession’? Give an example. (p. 913)
‘Fungal succession’ occurs when a natural substrate is colonised sequentially by different metabolic types of fungi. A very good example can be shown by thinking about the decomposition of animal dung by the sequential metabolic activities of different fungi. Dung is often first colonised by fast-growing zygomycetes that are able to metabolise simple compounds and reproduce rapidly, then slower growing ascomycetes able to metabolise cellulose as well as simple sugars appear on the dung, and finally the slowest growing fungi, usually basidiomycetes that can metabolise simple sugars, cellulose and lignin, are able to grow. Often the slower growing species inhibit the growth of competitors by secreting antibiotics.
8. How do anomorphic fungi generate variability and diversity without sexual reproduction?
The anomorphic fungi produce incredibly large numbers of asexual spores. Rare mutational events and mitotic recombinations generate genetic diversity in variant spores. The heterokaryotic nature of many of these fungi allows mutations to be carried until unusual environmental pressures select for their expression.
9. What features do fungal symbioses have in common? Why are fungi particularly well suited to their roles in these partnerships? (pp. 923–926).
Fungal symbioses involve nutrient exchange between the host organism and the fungus; with the fungus being provided with carbon substrates by the host and the host often gaining nutrients.
As heterotrophic organisms, fungi depend on autotrophic organisms to provide them with organic sources of carbon. The photoautotrophic partner in the many symbioses (alga or cyanobacteria in lichens, higher plants in mycorrhizal and endophytic associations) captures light energy to produce sugars using photosynthesis, and some of the carbohydrates are supplied to the fungal partner. The hyphal growth habit of mycorrhizal fungi allows them to explore more volume of soil than the roots of the host plant, and in nutrient-poor soils the fungus can supply the plant with additional mineral nutrients.
10. What features of terrestrial fungi support the proposition that they are capable of interplanetary travel? (pp. 913–914).
Fungi are able to tolerate extreme environmental conditions. Fungi have very low water potentials compared with plants, animals or bacteria. They may grow in temperatures as low as –5C and greater than 50C. They are non-photosynthetic and therefore do not require light to grow. Fungi are able to sense gravity to orient their fruiting bodies vertically so that spores can be dispersed by air currents.