The unusual sexual preferences of a chlamydomonas mutant may provide insight into mating-type evolution.

Chlamydomonas monoica undergoes intraclonal mating-type differentiation (homothallism). Although the species differs in this regard from the more commonly studied heterothallic C. reinhardtii, cell-cell interactions and progression of the sexual cycle are similar for many homothallic and heterothallic species of the genus. Regulation of chloroplast gene transmission by the nuclear mating-type alleles (mt+ and mt-) is another common denominator for Chlamydomonas species studied thus far. We have previously reported the use of chloroplast inheritance patterns to identify mutants of C. monoica that have lost the potential to function as the mt+ mating-type. A similar screening procedure led to the isolation of an unusual mutant, mtl-3 whose phenotype is less readily explained. Chloroplast gene transmission patterns in crosses involving mtl-3 suggest that the mtl-3 strain mates preferentially as mt+. However, normal mating efficiencies and high zygospore viability are observed in clonal culture, indicating the unbiased production of functional opposite mating-types. By construction of appropriately marked strains we have been able to show that mtl-3 mt- gametes prefer the mt+ gametes of their own strain. A model is presented which invokes unequal crossing over between highly homologous flagellar agglutinin genes to account for the unusual properties of the mtl-3 strain and for the evolution of mating barriers within the genus.