Evolutionary compromises in fungal fitness: hydrophobins can hinder the adverse dispersal of conidiospores and challenge their survival

Abstract

Fungal evolutionary biology is impeded by the scarcity of fossils, irregular life cycles, immortality, and frequent asexual reproduction. Simple and diminutive bodies of fungi develop inside a substrate and have exceptional metabolic and ecological plasticity, which hinders species delimitation. However, the unique fungal traits can shed light on evolutionary forces that shape the environmental adaptations of these taxa. Higher filamentous fungi that disperse through aerial spores produce amphiphilic and highly surface-active proteins called hydrophobins (HFBs), which coat spores and mediate environmental interactions. We exploited a library of HFB-deficient mutants for two cryptic species of mycoparasitic and saprotrophic fungi from the genus Trichoderma (Hypocreales) and estimated fungal development, reproductive potential, and stress resistance. HFB4 and HFB10 were found to be relevant for Trichoderma fitness because they could impact the spore-mediated dispersal processes and control other fitness traits. An analysis in silico revealed purifying selection for all cases except for HFB4 from T. harzianum, which evolved under strong positive selection pressure. Interestingly, the deletion of the hfb4 gene in T. harzianum considerably increased its fitness-related traits. Conversely, the deletion of hfb4 in T. guizhouense led to the characteristic phenotypes associated with relatively low fitness. The net contribution of the hfb4 gene to fitness was found to result from evolutionary tradeoffs between individual traits. Our analysis of HFB-dependent fitness traits has provided an evolutionary snapshot of the selective pressures and speciation process in closely related fungal species.

Intracellular accumulation and secretion of hydrophobin-enriched vesicles aid the rapid sporulation of molds

“…Located outside of the cell, HFBs assemble at hydrophobic/hydrophilic interfaces, modulate surface properties, and mediate a broad spectrum of fungal interactions with their environment 8 . In molds (Ascomycota), HFBs coat spores, thereby protecting against abiotic stresses 2,38,39 and supporting attachment to hosts or substrates 40 . Because molds can rapidly generate large amounts of spores 16 , HFBs must also be abundantly secreted.…”

“…The deletion of either hfb4 or hfb10 or both genes resulted in reduced conidiation and a "wetted hyphae" phenotype (reduced surface hydrophobicity) in both species 2 . The deletion of hfb2 did not result in any phenotypic alterations (see detailed phenotypic characterization in Supplementary material 5).…”

“…However, fungi reproduce by spores that are not motile and therefore passively dispersed by air or water. Therefore, for reproduction and dispersal, fungi need to rapidly grow out of the substrate and form aerial and hydrophobic sporogenic structures (e.g., fruiting bodies, sporangia, and conidiophores) and spores 1,2 catching suitable dispersal conditions. The hydrophobicity of the spore or hyphal cell wall also influences their adhesion to the substrate and their interactions with symbiotic partners 3,4 .…”

“…These layers significantly decrease the interfacial tension, thus allowing the hyphae to breach the liquid surface and grow into the air. Fruiting bodies, aerial hyphae or spores are also largely coated by HFBs to reduce wetting, provide resilience to environmental stresses 2,12 , promote the adhesion of spores and hyphae to hydrophobic surfaces or interactions with symbiotic partners 4 , and influence growth and development 13,14,15 .…”

“…Compared with HFBs in mushrooms and some airborne Ascomycota, Trichoderma HFBs do not form rodlets or amyloids and are soluble in organic solvents and detergents 7,11,22 . An ecological genetic study of T. guizhouense and T. harzianum revealed that HFB4 on the spore surface controls the preferential dispersal mode and spore survival 2 . However, to date, the biological role of the enrichment of HFBs in the Trichoderma genomes has not been understood 20 .…”