The Suhomyces clade: from single isolate to multiple species to disintegrating sex loci

Candida tanzawaensis clade members are now placed in Suhomyces. The group was virtually unknown until the early 2000s. Here, we review progress made on Suhomyces over the last two decades and provide data from reports of new members of the group from distant localities worldwide, their habitats and a new study of mating loci that helps explain earlier failed compatibility tests. Phylogenetic studies indicate early diverging members are mostly associated with plants, but later diverging species are usually fungus-feeding insect associates. The genome of S. tanzawaensis was known to have a heterothallic mating allele arrangement with a single MAT α idiomorph. For this review, we generate sequence data and compare the MAT gene arrangement of 30 strains from nine Suhomyces species. These varied from MAT α loci containing mating genes α1 and α2, hypothetical MAT a loci without detectable mating genes a1 and a2 to truncated, possibly completely dissociated MAT loci with intraspecific variation. The absence of a second MAT in a genome locus precludes the possibility of mating type switching. Sympatric speciation likely occurred after MAT locus deterioration began in isolated habitats. Although asexual reproduction may be an effective short-term strategy, theory predicts it will not endure over the extreme long term.