The divergence-homogenization duality in the evolution of the b1 mating type gene of Coprinus cinereus

The A mating type locus of the fungus Coprinus cinereus is a complex, multi genic locus which regulates compatibility and subsequent sexual development . Genes within the A locus such as the b1 gene studied here exhibit extreme sequence variation. In this work, we asked how b1 alleles have evolved hig h levels of variation and, at the same time, conserved function. We compare d sequence Variation in 17 alleles characterized as belonging to seven diff erent compatibility classes. Comparison of sequence variation between repre sentatives of these seven classes shows that different regions of the b1 ge ne have been subject to varying levels of substitution, recombination, and structural/functional constraints. The N-terminal region of the encoded pro tein, which has been previously demonstrated to govern self/nonself recogni tion, exhibited hypervariability with levels of amino acid identity as low as 41%. We used a novel analysis of neutral mutations accumulating in this gene to rule out the possibility that the N-terminal region is hypermutable . In contrast, the C-terminal region displayed heterogeneous levels of vari ation, with functional motifs being better conserved. In fact, there is a d uality in the b1 gene between variability and conservation; recombination e vents have homogenized the C-terminal region, while recombination events ar e undetectable in the N-terminal region. The ability to regulate sexual dev elopment is maintained in all of the mating compatibility alleles studied, and these data suggest that some functional motifs may tolerate high levels of substitution.