Evolutionary Dynamics of Mating-Type Loci of Mycosphaerellaspp. Occurring on Banana

ABSTRACTThe devastating Sigatoka disease complex of banana is primarily caused by three closely related heterothallic fungi belonging to the genus Mycosphaerella: M. fijiensis, M. musicola, and M. eumusae. Previous phylogenetic work showing common ancestry led us to analyze the mating-type loci of these Mycosphaerellaspecies occurring on banana. We reasoned that this might provide better insight into the evolutionary history of these species. PCR and chromosome-walking approaches were used to clone the mating-type loci of M. musicolaand M. eumusae. Sequences were compared to the published mating-type loci of M. fijiensisand other Mycosphaerellaspp., and a novel organization of the MAT loci was found. The mating-type loci of the examined Mycosphaerellaspecies are expanded, containing two additional Mycosphaerella-specific genes in a unique genomic organization. The proteins encoded by these novel genes show a higher interspecies than intraspecies homology. Moreover, M. fijiensis, M. musicola, and M. eumusaecontain two additional mating-type-like loci, containing parts of both MAT1-1-1and MAT1-2-1. The data indicate that M. fijiensis, M. musicola, and M. eumusaeshare an ancestor in which a fusion event occurred between MAT1-1-1and MAT1-2-1sequences and in which additional genes became incorporated into the idiomorph. The new genes incorporated have since then evolved independently in the MAT1-1 and MAT1-2 loci. Thus, these data are an example of the evolutionary dynamics of fungal MAT loci in general and show the great flexibility of the MAT loci of Mycosphaerellaspecies in particular.