Studies on the physiology and genetics of the rotifer, Asplanchna. II. The genic basis of a case of male sterility

Abstract 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 F2, F3, and test‐cross generations were obtained. The results of these crosses indicate that the male‐sterile phenotype in 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 effects, 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.