Chapter 8: Sexual reproduction: the basis of diversity and taxonomy
Fundamentally, sexual reproduction is the fusion of gametes (the differentiated sex cells) or their nuclei to form a diploid that can undergo meiosis. The overall summary 'equation' is:
Plasmogamy → Karyogamy → Meiosis
For most fungi, plasmogamy occurs when hyphal fusion (anastomosis) occurs, and is controlled by the incompatibility systems; growth of the resultant heterokaryon as an independent mycelium prolongs plasmogamy, in some cases indefinitely.
Classical fungal taxonomy is based on the morphology of the sexual reproductive structures, so we should first dispose of those fungi that lack sexual reproduction. Fungi for which sexual reproduction has not been observed are lumped together in a group that has been called the Deuteromycetes or Deuteromycota /Deuteromycotina, all implying formal taxonomic rank though this is not appropriate in a phylogenetically-driven taxonomy because the group is so diverse. It’s best to call them anamorphic (which is used to indicate the asexual or imperfect state), or mitosporic (which refers to their spores being formed via mitosis) fungi. As knowledge accumulates, an anamorphic fungus may be reclassified into the Ascomycota or Basidiomycota following discovery of a sexual phase (known as the teleomorph). For example, a particularly important imperfect fungus, the human pathogenic yeast Candida albicans, has been shown by genomic comparisons to possess homologues of genes known to be involved in meiosis Saccharomyces cerevisiae and some higher eukaryotes, which implies that C. albicans may have a complete sexual cycle (Land, 2001). Taxonomic molecular sequence analysis may also allow imperfect fungi to be classified with their closest sexual (that is, perfect) relatives. This makes it clear that anamorphs have arisen from many different groups of fungi by the loss of sexuality; it is an adaptation to a particular lifestyle. Some of the most frequently encountered fungi are anamorphic, and many of them are of enormous economic significance, either as industrial fungi or as pathogens. Anamorphic fungi can be yeasts or filamentous forms, and the latter fall into three morphological groups known most commonly as:
- hyphomycetes, which are mycelial forms producing conidia on separate hyphae or groups of hyphae but not in fruit bodies (Bärlocher, 2007);
- coelomycetes form conidia in fruiting bodies that may be called pycnidia, acervulae or stromata;
- agonomycetes, which do not form conidia but may produce chlamydospores, sclerotia or other vegetative structures.
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Ordering details: Moore, D., Robson, G.D. & Trinci, A.P.J. (2011). 21st Century Guidebook to Fungi. Cambridge, UK: Cambridge University Press. ISBN: 9780521186957.
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Updated December 23, 2016