Two mutant clones have been obtained by selfing from closely-related clones of the rotifer, Asplanchna brightwelli. The females of these male-sterile clones are fertile but the males are unable to fertilize females of the mutant or of normal clones. Females from the mutant clones were out-crossed to fertile males of a wildtype clone, and Fz, Fs, and test-cross generations were obtained. The results of these crosses indicate that the male-sterile phenotype i n both mutant clones is determined by a recessive mutation designated "male-sterile" (ms). The mutant ms allele also appears to act as a detrimental, for the viability of zygotes (resting eggs) which are heterozygous for this gene is greater than that of homozygous recessives. This gene shows a maternal effect, for the phenotype of the haploid (or homozygous) males is determined by the genotype of their mothers. Test-crosses of seven individual males born to heterozygous females suggest that each such male carries either the ms or the ms+ allele, but never both. This confirms early cytogenetic studies on oogenesis which indicated that the male rotifer arises by parthenogenetic development from a haploid egg, produced by ordinary meiotic maturation divisions.
1 In part from a thesis submitted in partial fulfillment of the requirements for the degree, Doctor of Philosophy, in the