Seasonal variation in sexual readiness in a facultatively sexual freshwater cnidarian with diapausing eggs

Facultative sexuality combines clonal propagation with sexual reproduction within a single life cycle. Clonal propagation enables quick population growth and the occupancy of favorable habitats. Sex, on the other hand, results in the production of offspring that are more likely to survive adverse conditions (such as the resting eggs of many freshwater invertebrates). In seasonal environments, the timing of sex is often triggered by environmental cues signaling the onset of winter (e.g. temperature drop or changes in photoperiod). Organisms switching to sex to produce resting eggs under these conditions face a trade-off: responding too early to an environmental cue increases the chances of missing out in clonal propagation, while having a delayed response to deteriorating conditions entails the risk of parental mortality before sexual reproduction could be completed. To mitigate these risks, increased sensitivity towards environmental cues with the onset of the winter might be an adaptive strategy. To test this hypothesis, we investigated sexual propensity and time to gonadogenesis in clonal strains derived from spring- and autumn-collected polyps of Hydra oligactis , a facultatively sexual freshwater cnidarian where sex only occurs prior to the onset of winter. We show that autumn-collected individuals and their asexual offspring have a higher propensity for sex and require less time for gonad development compared to strains established from spring-collected individuals that were kept under similar conditions in the laboratory. To see if the above results can be explained by phenotypic plasticity in sexual readiness, we exposed cold-adapted lab strains to different lengths of warm periods. We found that sexual propensity increases with warm exposure. Our results suggest that reciprocal cold and warm periods are required for sex induction in H. oligactis , which would ensure proper timing of sex in this species. Increased sensitivity to environmental deterioration might help maximize fitness in environments that have both a predictable (seasonal) and an unpredictable component.