Evolutionary Dynamics of Mating-Type Loci of Mycosphaerella spp. Occurring on Banana

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Abstract
The 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 Mycosphaerella species 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. musicola and M. eumusae . Sequences were compared to the published mating-type loci of M. fijiensis and other Mycosphaerella spp., and a novel organization of the MAT loci was found. The mating-type loci of the examined Mycosphaerella species 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. eumusae contain two additional mating-type-like loci, containing parts of both MAT 1 - 1 - 1 and MAT 1 - 2 - 1 . The data indicate that M. fijiensis , M. musicola , and M. eumusae share an ancestor in which a fusion event occurred between MAT 1 - 1 - 1 and MAT 1 - 2 - 1 sequences 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 Mycosphaerella species in particular.