Evolution of black yeasts: possible adaptation to the human host.

Ascomycetous black yeasts show adaptations to a wide array of environmental conditions. Dothideaceous black yeasts are mostly found on plant leaves, while among herpotrichiellaceous species there are numerous opportunists on humans. Factors which are of ecological significance include the presence of melanin and carotene, formation of thick cell walls and meristematic growth, presence of yeast-like phases, presence of additional forms of conidiogenesis, thermo- and osmotolerance, adhesion, hydrophobicity, production of extracellular polysaccharides, siderophores and acidic or alkaline secondary metabolites. The potential pathogenicity of a species is partly determined by its natural ecological niche. Dothideaceous black yeasts are osmotolerant rather than pathogenic. Herpotrichiellaceous black yeasts probably have low competitive ability and are found in rather special niches as secondary saprophytes, e.g., on bacterial mats, on other fungi or in poor environments. Some species possibly utilize animal vectors for dispersal.