Flóra Sebestyén, Zoltán Barta & Jácint Tökölyi
Hydras are small sessile predators, living in freshwater lakes and rivers. They have been used as laboratory model organisms since the 18th century, when Abraham Trembley observed two fully regenerating hydra polyps after cutting one in half. Today, we know that the body of hydra contains intensively dividing stem cells, which are responsible for the production of both body (somatic) and reproductive cells. Stem cells are needed for reproduction whether it is sexual or asexual. Hydra oligactis polyps can switch between these modes and uniquely in the Hydra genus, they show signs of aging after sexual reproduction and eventually die, possibly due to the exhaustion of stem cells in reproductive cell production. A conflict between reproduction and survival may cause this exhaustion/depletion, since stem cells are available only in a limited amount. But this phenomenon has been studied only in laboratory hydras and competition for stem cells is generally underinvestigated.
To investigate whether limited availability of stem cells results in competition between the cells needed for self-maintenance and reproduction, we collected polyps from their natural habitat during the autumn sexual reproduction period. We measured the number of stem cells, along with their descendant stinging cells (for food capture) and stem cell dependent regeneration rate (proportion of animals regenerating heads after decapitation) in polyps with different reproductive modes: asexual, sexual or non-reproductive. We found that sexual individuals contained fewer stem cells, fewer somatic derivatives and lower regeneration ability compared to non-reproductives or asexuals, suggesting that stem cell sources are used up during sexual reproduction. We also found that more reproductive cells is correlated with fewer stem cells, supporting the negative relation between somatic and reproductive investment in hydra polyps. These results show that the cellular (stem cell) costs of reproduction are higher for sexual individuals, highlight the importance of stem cells and confirm the theory behind senescence following sexual reproduction, described in Hydra oligactis individuals from a natural population for the first time.