Female Polymorphism and Sexual Reproduction in the Rotifer Asplanchna: Evolution of Their Relationship and Control by Dietary Tocopherol

Abstract

The adaptive significance of the control of sexual reproduction in heterogonic life cycles by different types of environmental factors is considered. In several of the more carnivorous species of the rotifer Asplanchna, the incidence of sexuality is directly associated with a nongenetic female polymorphism controlled by dietary tocopherol (vitamin E). Knowledge of this relationship is reviewed, and 2 theories attempting to explain how such a system might have evolved are evaluated. The hypothesis that tocopherol is required for male fertility and serves as an adaptive signal for male production is untenable. The initial response to tocopherol was a general growth increase which permitted Asplanchna to more easily ingest larger, and possibly more nutritious, herbivorous prey. The tocopherol molecule is an adaptive cue for this growth response. It is synthesized only by photosynthetic organisms and readily assimilated by herbivores, and its ingestion by the predatory Asplanchna indicates the availability of herbivorous rotifer and crustacean prey. Since it is readily assimilated by Asplanchna and efficiently transmitted without degradation to its embryos, it is in a position to exert a well-buffered, inductive effect over several generations during embryogenesis and postnatal development; it can itself promote growth in Asplanchna through increased cell enlargement, cell division, and possibly DNA endoreduplication. Four closely related species of Asplanchna respond very differently to tocopherol and provide some evidence for a proposed scheme describing the evaluation of tocopherol-dependent morphotypes. A. girodi does not respond to tocopherol and his monomorphic females. A. brightwelli, A. intermedia and A. sieboldi have increasingly more complex and extensive responses to tocopherol. The selective pressures resulting in this progression seem to be related to increasing availabilities of zooplankton prey. Growth responses to tocopherol evolved only in those species which do not depend on the direct utilization of relatively small tocopherol-rich algae and which can ingest copious quantities of zooplankton prey needed for sustaining populations of large individuals.