Phenotypic influences on the reproductive strategy of the facultative sexual rotifer <Emphasis Type="Italic">Brachionus rubens</Emphasis> (Monogononta)

Facultative sexual species employ a dual reproductive strategy (heterogony) comprising primarily asexual reproduction with intermittent sexual reproduction. Given the higher relative costs of sexual reproduction, elucidating the triggers underlying these transitions might help our understanding of the evolution of (obligate) sex in general. Existing hypotheses into how and when facultative sexuals invest into sex focus largely either on environmental (habitat-deterioration and resource-demanding hypotheses) or genetic factors (condition-dependent hypothesis), but tend to lack experimental evidence, especially with respect to within-population variation. To address this deficit, we examined the influence of several variables that potentially affect fitness (food quality, water temperature, physiological acclimation, and all combinations thereof) on both the lifetime reproduction (total number of offspring) and investment into sexual offspring per female in a clonal population of the monogonont rotifer Brachionus rubens. Investment into sex, both absolutely and relative to lifetime reproduction, was tied most closely to and positively correlated with individual fitness (i.e., lifetime reproduction): individuals with higher fitness invested more into sexual reproduction. These results run contra to the condition-dependent hypothesis and indicate an energy-budget analogue of the resource-demanding hypothesis. Furthermore, investment into sex increased after a period of physiological acclimation to the new conditions, probably because of the amelioration of short-term stress effects or clonal selection. Our results underscore that life history and general phenotypic considerations—here, energetic provisioning of offspring, the presence of a sexual resting stage, and the relative timing of sexual versus asexual reproduction—can modify existing hypotheses based either on environmental or genetic factors alone.