Sexual Reproduction in the Rotifer Asplanchna brightwellii from a Temporary Habitat: Tests for Transgenerational and Population-Density Effects

1. This study examines endogenous and environmental factors that might affect the timing and propensity for sex in a population of the rotifer Asplanchna brightwellii living in a Chihuahuan Desert habitat with hydroperiods of about 2–3 weeks. Multiple clones were initiated from stem females hatched from resting eggs in the sediment and cultured on Cryptomonas erosa, an alga containing tocopherol (vitamin E) required for induction of mictic females and hence production of males and fertilised resting eggs. Two hypotheses regarding the commitment of A. brightwellii to sexual reproduction were considered. First, does its propensity for sex differ in early and late parthenogenetic generations after stem females hatch? Second, is its propensity for sex affected by its population density?
2. Experiments testing eight clones over 10 successive generations from the stem female showed no significant effect of generation on propensity for mictic-female production. Percentages of mictic females in small populations initiated by juvenile females from each generation generally were low (grand mean of 9%).
3. Experiments with five clones showed that the propensity for mictic-female production was density independent. Percentages of mictic daughters produced by single amictic females cultured from birth in 70 mL or 2 mL did not differ significantly and generally were low (0%–18%).
4. The persistent and low propensity for sexual reproduction in laboratory populations of these A. brightwellii clones suggests a low, fixed propensity for sex whenever the diet contains tocopherol and allows induction of mictic females. Such a life cycle strategy appears suitable for an ephemeral habitat, where the duration of hydroperiods and growing seasons is very short and unpredictable. It assures the production of some resting eggs soon after population development without greatly limiting the potential for rapid population growth via female parthenogenesis. Clones with a relatively high propensity for sex may have an advantage during shorter growing seasons, and vice versa.