How new allele specificities of the multiallelic b locus are generated

In many organisms, two component systems have evolved to discriminate self from nonself. In U. maydis sexual (and pathogenic) development is controlled by the multiallelic b locus to ensure outbreeding. Nonself recognition is speciﬁed by bE and bW proteins, encoded by the b locus. The genes are divergently transcribed from a common short promoter region. bE and bW are homeodomain proteins and their allelic differences are restricted to their N-terminal variable regions, which are involved in dimerization. While bE–bW combinations from the same allele do not dimerize, bE and bW proteins originating from different alleles form a heterodimeric complex that functions as master regulator for sexual and pathogenic development. By sequencing all naturally occurring 19 alleles and generating artiﬁcial hybrids, we have now shown that novel speciﬁcities of the b mating type locus have arisen by single homologous recombination events between distinct b alleles. These events lead to the simultaneous exchange of subdomains in bE and bW that govern dimerization. As a consequence the speciﬁcity of both proteins is altered in a single step and there is no constitutively active intermediate generated. Constitutively active strains should arise by a single crossover event in the promoter region and they can easily be generated artiﬁcially by expressing bE and bW genes from different alleles in the same strain. Such strains become solopathogenic, i.e. they can cause disease without a mating partner. However, solo-pathogenic strains produce signiﬁcantly fewer spores than crosses of two compatible haploids, and this likely explains why they have never been isolated from nature.