Fungi Reproduction

2 Key excerpts on "Fungi Reproduction"

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  eBook - ePub

  Essential Microbiology

  • Stuart Hogg(Author)
  • 2013(Publication Date)
  • Wiley-Blackwell

    (Publisher)

  Rhizopus reproduces in this way; each spore, upon finding a suitable substratum for growth, is capable of germinating and initiating a new mycelium.

  Figure 8.8

  The life cycle of Rhizopus .

  Both sexual and asexual cycles involve the production of sporangiospores. In sexual reproduction, hyphae from different mating strains fuse to form a diploid zygospore, via a short-lived dikaryotic intermediate. Germination of the zygospore gives rise to an aerial sporangium; this contains many haploid sporangiospores, which give rise to another vegetative mycelium.

  Sexual reproduction occurs when environmental conditions are unfavourable. Most species of Rhizopus are heterothallic ; that is, there exist two distinct mating strains known as + and −. When hyphae of opposite mating types come into contact, a cross-wall develops a short distance behind each tip, and the regions thus isolated swell to produce gametangia. These fuse to form a zygospore, which can survive extremes of drought and temperature, and may remain dormant for months. When conditions are favourable again, the nuclei from each strain fuse in pairs, to give a fully diploid zygote. Just before germination, meiosis occurs, and an aerial sporangiophore emerges, terminating in a sporangium. Production and dispersal of haploid spores then occurs as in the asexual life cycle, and a new mycelium forms following spore germination.

  1. Frequently still referred to by its previous name, Chalara fraxinea .

  Passage contains an image 9 The Protista

  Although not such an all-embracing a term as originally envisaged by Haeckel, the Protista represents a very diverse group of organisms, united by their possession of eukaryotic characteristics, and failure to fit satisfactorily into the animal, plant or fungal kingdoms. Some scientists limit use of the name to unicellular organisms, while others also include organisms such as the macroscopic algae, which are not accommodated conveniently elsewhere.

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  eBook - ePub

  The Fungi

  • Sarah C. Watkinson, Lynne Boddy, Nicholas Money(Authors)
  • 2015(Publication Date)
  • Academic Press

    (Publisher)

  Aspergillus species hitherto considered to be asexual, whole genome sequences have revealed the presence of suites of genes associated with parts of the sexual cycle in other ascomycetes, including for mating and pheromone response, meiosis, and development of fruit bodies. Population genetic studies have also shown evidence of genetic recombination (linkage disequilibrium) which indicates past sexual activity.

  Many plant and animal pathogens are considered to be asexual, but this is likely due to the fact that only one clone has the set of genes needed to infect the appropriate plant or animal. Although Magnaporthe oryzae is, for example, considered to have an asexual life cycle, there appears to be a sexual population in India, and the global distribution of clones and clonal lineages probably reflects rare ‘escapes’ from the sexual population. The human pathogen Candida albicans was, until recently, thought to be completely asexual, always being diploid, however, mating between cells with opposite mating types (see below) does occur. However, as well as this heterothallism (see below), homothallism also occurs.

  Mating Systems

  Fungi have mating systems or breeding systems that determine whether or not individuals of the same species can mate. Some fungi are self-fertile but many fungi have genetic systems that prevent mating between very genetically similar individuals, (i.e. self-sterile) so that genetic diversity will be increased. With basidiomycetes, since somatic (vegetative) incompatibility (pp. 102–104) often prevents non-self mycelium from fusing to form a stable connection containing nuclei from both, this rejection mechanism has to be overcome before successful mating can occur. With ascomycetes, somatic (vegetative) incompatibility is suppressed during mating, provided that mating occurs between female reproductive structures (protoperithecia) and a male cell, and does not require hyphal fusion (heterokaryon formation) prior to perithecia formation. Compatible matings are determined by mating-type (MAT) factors. The MAT loci have a complex genetic structure. In Coprinus cinereus , for example, there are four sites each having two closely linked loci with multiple alleles. However, in population genetics, these complex loci can usually be treated as if they were simply multiple alleles at two loci, A and B, i.e. A1 …An and B1 …Bn , or in the case of some basidiomycetes one locus A1 …An

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