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

Abstract

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 contrary, 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 toward 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 with strains established from spring‐collected individuals that were kept under similar conditions in the laboratory. To see whether the above results can be explained by phenotypic plasticity in sexual readiness, we exposed cold‐adapted laboratory 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.

Seasonal variation of genotypes and reproductive plasticity in a facultative clonal freshwater invertebrate animal (Hydra oligactis) living in a temperate lake

“…The water temperature in the lake can rise above 25 ° C in the warmest months (even though the lake is surrounded by woody vegetation, which provides substantial shade), while it stays below 12°C between October and April. The data on laboratory strains used here were collected as part of a previous study aimed at comparing spring‐ and autumn‐collected hydra strains (Tökölyi et al, 2021). Briefly, we collected polyps from multiple locations along the shoreline (distance between locations at least 2 meters), with the following sampling intensity: spring sampling in 2018 from 19 points; autumn sampling in 2018 from 11 points; spring sampling in 2019 from 20 points; autumn sampling in 2019 sampling from 13 points (these are shown in Figure 1).…”

“…The water temperature in the lake can rise above 25 ° C in the warmest months (even though the lake is surrounded by woody vegetation, which provides substantial shade), while it stays below 12°C between October and April. The data on laboratory strains used here were collected as part of a previous study aimed at comparing spring-and autumn-collected hydra strains (Tökölyi et al, 2021).…”

“…Now, however, it has also been shown that there is a high degree of plasticity in the expression of the mode of reproduction between the different genotypes.Different genotypes show different propensity to initiate sexual reproduction in the same environmental conditions. Thus, due to the fact that individuals in the same genotypes respond differently to the same environmental stimulus (which may even be due to the different physiological status of the individuals; Hadany & Otto, 2007), it may create diverse reproductive strategies in populations(Tökölyi et al, 2021). Therefore, in fact, the observed diversity in reproductive patterns results from a combination of genetic diversity and phenotypic plasticity, which may result in a better adaptability of the population to the changing environment.…”

Seasonal variation of genotypes and reproductive plasticity in a facultative clonal freshwater invertebrate animal (Hydra oligactis) living in a temperate lake

“…Experimental animals were asexually propagated for 10 weeks in the first phase and then placed into cold circumstances to induce sexual reproduction in the second phase. Details of the standard living conditions of the hydra polyps, both for asexual reproduction phase and cooling phase, can be found in Tökölyi et al, (2021). Up to N=18 of polyps/strain were retained to collect data on reproductive mode.…”

“…Reproductive mode was inferred based on data from N = 921 polyps (this is a subset of individuals analysed in Tökölyi et al, (2021) that belong to the 121 genotyped strains). There were on average 7.74 polyps per strain to estimate reproductive mode (range: 1-18).…”