| Summary: | Heterothallic, facultatively sexual populations are vulnerable to the loss of a mating type by natural selection during periods of asexual reproduction. Experiments are described which demonstrate a competitive difference between the mating types of Chlamydomonas reinhardtii, a unicellular green alga with two isogamous mating types, mt + and mt $-.$ When grown vegetatively under phototrophic (light) conditions, mt + outcompetes mt $-.$ Assays of the growth parameters of isolated spores suggest that mt + has a higher growth rate than mt $-$ in the light, and that mt $-$ has a higher growth rate than mt + in heterotrophic (dark) growth conditions. === A literature review shows that sampling from natural populations of heterothallic, facultatively sexual species often yields only one mating type or significantly skewed mating-type distributions. This indicates that competition between mating types and the consequent loss of one mating type may be common in these populations. === A discussion of current theories on the evolution of heterothallism as well as the results of a simulation model reveal that heterothallism will spread if any fitness reduction is suffered by in-crossing homothallic individuals. However, fitness differences between the heterothallic alleles allow the invasion of a homothallic allele into a heterothallic population. === The implications of mating type competition on the maintenance and distribution of heterothallic populations in nature are discussed. It is argued that heterothallic, facultatively sexual populations commonly lose the potential for sex because of the loss of one mating-type allele. The prediction is made that homothallism is more common among facultatively sexual organisms than it is among obligately sexual organisms.
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