The significance of conidiogenesis
One of the revolutionary criteria for identification of Asco- and Basidiomycetes half a century ago was conidiogenesis: the way in which asexual propagules are formed (Cole 1976). The most important process of spore production in fungi is certainly the sexual type, which generates new genotypes by recombination during meiosis. But once a fungus has found a permissive habitat, it rapidly colonizes this environment by clonal reproduction. This exclusively mitotic form is what we mostly see in the culture plate. The great majority of fungi are encountered in clinical and hygienic practice are asexual. As axenic culture is an easy substrate for the fungus, it is colonized by the conidial form of propagation. There are many ways in which conidia can be produced, and therefore precise observation of the early phase of production has been a significant aid to distinguish superficially similar fungi. On p. 120 and following, the Atlas of Clinical Fungi provides keys for identification of asexual asco- and basidiomycetous fungi, which apply conidiogenesis as a main criterion.
Is conidiogenesis still useful today, when we are used to identification by molecular sequencing or Maldi-tof? That depends on the purpose. Purely morphological identification down to species level is no longer realistic for 90% of the fungi, although basic knowledge of the microscopic appearance of fungi remains useful to confirm whether the correct isolate has been sequenced, or whether it was a casual contaminant. More important, however, is the basic understanding of evolution and ecology of the fungus at hand, where sexual and asexual propagation are driving forces.
Conidiogenesis is rather consistent over larger groups (Walther et al. 2005). In the Atlas on p. 518 and following, the Ascomycota are introduced with short descriptions of the orders which contain the described pathogens and opportunists. Major differences in life style are noted between orders. Sexual reproduction is instrumental for the generation of novel genotypes that are needed under environmental stress conditions, when the fungus has trouble to survive. In vitro, ascigerous fruitbodies can therefore often be produced by “mistreatment” of the culture, by e.g. high or low temperature, salt, or dryness. In contrast, when the fungus has a good life, conidia serve multiplication of the same genotype, leading to fast reproduction. Conidial characteristics are Indicate their transmission vectors. For example, conidia in Eurotiales (Aspergillus, Penicillium) are hydrophobic and easily detached, promoting airborne dispersal (Martinez Herrera et al. 2016). Many Ophiostomatales (relatives of Sporothrix) have complex interactions with bark beetles and other arthropods (Roets et al. 2009), which may be informative for the origin of the opportunistic members of the order. The slimy conidia in Hypocreales (such as Fusarium) enhance distribution by adhesion to objects (Garibaldi et al. 2004) or to moisture droplets (Moretti et al. 2018) and are vectored in the hospital by aerosols; those of Agaricales (mushrooms) are dry and airborne, matching with their pulmonary occurrence (Hayes & Rooney 2014). Capnodiales are extremotolerant fungi, therefore strongly melanized and often produce meristematic, clumpy conidia (Gostinčar et al. 2018). Yeasts are single-celled because of their prevalence in liquid substrates, whereby ascomycetes (Candida) are different from basidiomycetes (Cryptococcus) by being sympodial or annellidic, respectively. The phylogenetic tree below of the fungal Kingdom (p. 139-140 of the Atlas) summarizes the prevalent types of asexual propagation in the orders that include the majority of clinical fungi. A systematic overview of types of conidiogenesis with informative diagrams has been added to the Atlas website; type “conidiogenesis” in chapter search.
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- Garibaldi A, Gilardi G, Pasquali M, et al. (2004) Seed transmission of Fusarium oxysporum of Eruca vesicaria and Diplotaxis muralis. Z. PflKrankh. PflSchutz 111: 345-350.
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- Walther G, Garnica S, Weiß M (2005) The systematic relevance of conidiogenesis modes in the gilled Agaricales. Mycol. Res. 109: 525-544.