Molecular and Genetic Evidence for a Tetrapolar Mating System in the Basidiomycetous Yeast Kwoniella mangrovensis and Two Novel Sibling Species

Kwoniella mangrovensis has been described as a sexual species with a bipolar mating system. Phylogenetic analysis of multiple genes places this species together withKwoniella heveanensis in theKwoniella clade, a sister clade to that containing two pathogenic species of global importance,Cryptococcus neoformans andCryptococcus gattii , within the Tremellales. Recent studies defining the mating type loci (MAT ) of species in these clades showed that, with the exception ofC. neoformans andC. gattii , which are bipolar with a single biallelic multigeneMAT locus, several other species feature a tetrapolar mating system with two unlinked loci (homeodomain [HD] and pheromone/receptor [P/R] loci). We characterized several strains from the original study describingK. mangrovensis ; twoMAT regions were amplified and sequenced: theSTE20 gene (P/R locus) and the divergently transcribedSXI1 andSXI2 genes (HD locus). We identified five different mating types with differentSTE20 /SXI allele combinations that together with results of mating experiments demonstrate thatK. mangrovensis is not bipolar but instead has a tetrapolar mating system. Sequence and gene analysis for a 43-kb segment of theK. mangrovensis type strainMAT locus revealed remarkable synteny with the homologousK. heveanensis MAT P/R region, providing new insights into slower evolution ofMAT loci in theKwoniella compared to theCryptococcus clade of the Tremellales. The study of additional isolates from plant substrates in Europe and Botswana using a combination of multilocus sequencing withMAT gene analysis revealed two novel sibling species that we nameKwoniella europaea andKwoniella botswanensis and which appear to also have tetrapolar mating systems.