H. Badrane et G. May, The divergence-homogenization duality in the evolution of the b1 mating type gene of Coprinus cinereus, MOL BIOL EV, 16(7), 1999, pp. 975-986
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.