Polymorphism and reproductive mode in the rotifer, Asplanchna sieboldi: relationship between meiotic oogenesis and shape of body-wall outgrowths.

Sexuality and polymorphism are closely coupled in the rotifer Asplanchna sieboldi. In a graded response to dietary tocopherol, embryos develop body-wall outgrowths of various sizes and shapes. Also in a graded response to this compound, some of the affected females produce eggs undergoing meiotic instead of mitotic oogenesis. The haploid eggs of such mictic females develop parthenogenetically into males instead of females. The incidence of mictic females among animals with different shapes was studied among cohorts from mothers subjected to different inducing conditions. The hypothesis that external tocopherol concentrations absolutely fix the probability of meiotic oogenesis was rejected. The other extreme hypothesis, that the probability of meiotic oogenesis is fixed by morphotype, was rejected for animals at the low end of the morphotypic scale but accepted for the more strongly-affected individuals. The probability of meiotic oogenesis is thus constant for the higher morphotypes. The ascertainment of morphotype frequencies in natural or laboratory populations may suffice for estimating the incidence of sexual forms; furthermore studies of factors affecting the body-wall-outgrowth response may also bear directly upon regulation of sexual reproduction in this species.