SELF‐STERILE AND MATURATION‐DEFECTIVE MUTANTS OF THE HOMOTHALLIC ALGA, CHLAMYDOMONAS MONOICA (CHLOROPHYCEAE).

Homothallic sexual reproduction in Chlamydomonas monoica Strehlow culminated in the formation of mature, chloroform‐resistant zygospores (zygotes) in clonal culture. Early in the zygote maturation process, a distinctive “primary zygote wall” was released into the culture medium where it remained stable for at least several days. This wall appeared as a rigid, darkly‐outlined, and often multilayered structure, as viewed by phase contrast microscopy. From a sample, of 2500 individual clones isolated after ethyl methanesulfonate mutagenesis, five maturation‐defective strains zym produced abnormal zygotes which failed to release a primary zygote wall, failed to develop the normal reticulate zygospore wall, and disintegrated within five days. These strains were utilized to identify additional mutants which were sexually competent, but self‐sterile (het). Mixed cultures of the zym and het mutant strains were found to contain numerous, fully‐matured, chloroform‐resistant zygospores and discarded primary zygote walls. In combination, the two types of mutants provided a useful system for the selective recovery of heterozygous zygospores, thus facilitating genetic studies on a homothallic Chlamydomonas.