RAPID EVOLUTION OF SEX-RELATED GENES IN CHLAMYDOMONAS

Biological speciation ultimately results in prezygotic isolation-the i nability of incipient species to mate with one another-but little is u nderstood about the selection pressures and genetic changes that gener ate this outcome. The genus Chlamydomonas comprises numerous species o f unicellular green algae, including numerous geographic isolates of t he species C. reinhardtii. This diverse collection has allowed us to a nalyze the evolution of two sex-related genes: the mid gene of C. rein hardtii, which determines whether a gamete is mating-type plus or minu s, and the fus1 gene, which dictates a cell surface glycoprotein utili zed by C. reinhardtii plus gametes to recognize minus gametes. Low str ingency Southern analyses failed to detect any fus1 homologs in other Chlamydomonas species and detected only one mid homolog, documenting t hat both genes have diverged extensively during the evolution of the l ineage. The one mid homolog was found in C. incerta, the species in cu lture that is most closely related to C. reinhardtii. Its mid gene car ries numerous nonsynonymous and synonymous codon changes compared with the C. reinhardtii mid gene. In contrast, very high sequence conserva tion of both the mid and fus1 sequences is found in natural isolates o f C. reinhardtii, indicating that the genes are not free to drift with in a species but do diverge dramatically between species. Striking div ergence of sex determination and mate recognition genes also has been encountered in a number of other eukaryotic phyla, suggesting that uni que, and as yet unidentified, selection pressures act on these classes of genes during the speciation process.