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

Facultative sexual organisms combine sexual and asexual reproduction within a single life cycle, often switching between reproductive modes depending on environmental conditions. These organisms frequently inhabit variable seasonal environments, where favourable periods alternate with unfavourable periods, generating temporally varying selection pressures that strongly influence life history decisions and hence population dynamics. Due to the rapidly accelerating changes in our global environment today, understanding the dynamics of and genetic changes in facultative sexual populations inhabiting seasonal environments is critical to assess and prepare for additional challenges that will affect such ecosystems. In this study we aimed at obtaining insights of the seasonal population dynamics of the facultative sexual freshwater cnidarian Hydra oligactis through a combination of Restriction-site Associated Sequencing (RAD-Seq) genotyping and the collection of phenotypic data on the reproductive strategy of field-collected hydra strains. We found no significant genetic change during the two years in the study population. Clone lines were detected between seasons and even years, suggesting that clonal lineages can persist for a long time in a natural population. We also found that distinct genotypes differ in sexual reproduction frequency, but these differences did not affect whether genotypes reappeared across samplings. Our study describes changes in population genetic structure across the seasons in a hydra population for the first time, providing key insights into the biology of the species, while also contributing to understanding the population biology of facultative sexual species inhabiting freshwater ecosystems.