Form and function in the evolution of dermatophytes
Richard C. Summerbell
Mycology, Laboratories Branch, Ontario Ministry of Health, Toronto, Ontario,
The phenotype of dermatophytes has been radically influenced by two very
different evolutionary paths: that of the mostly sexual, soil-associated
species, and that of the mostly asexual, non-soil-associated species. The
former category, including geophiles and some zoophiles, is characterized
by well established conidial dimorphism and the presence of apparent anti-arthropod-grazing
structures such as helical appendages, as well as by growth factor independence,
functional urease enzyme, and hair perforation organs. The latter category,
including some zoophiles and all anthropophiles, is characterized by loss
of some or all of the above characters. Phenotypic characters which are
retained in the non-soil-associated dermatophytes, and likely selected
for in their niche, include the production of infectious "substrate arthroconidia"
and the production of secondary metabolites, often seen as coloured compounds
in culture. The presence of xanthomegnin and other secondary metabolites,
which have no known function in pathogenesis, may reflect deterrence of
bacterial competitors in skin and nails. The diversity of such compounds
may reflect "anomalizing selection" operational in the evolutionary maintenance
of a general inhibition of unspecialized, opportunistic competitors. These
competitors are non-specifically deterred by the saturation of the fungally
colonized area with large quantities of metabolically inaccessible material,
which must be maintained as inaccessible by evolutionarily accelerated
but relatively undirected change.
In: Kushwaha RKS, Guarro J (Eds.). Biology
of Dermatophytes and other Keratinophilic Fungi.
2000-2018@ Revista Iberoamericana de Micología. All rights reserved.
Revista Iberoamericana de Micología, Bilbao, 2000.