Dissertation

Aging in basal metazoans: a biodemographic approach

Abstract

Basal metazoans show large variations in life-cycle patterns, including clonal propagation modes, offering unique opportunities to study the evolution of aging from a biodemographic point of view. Comparing age-specific life history traits such as survival and reproduction in experimental laboratory studies and analyzing demographic data across various basal metazoans at both genet and ramet levels this dissertation project aims to shed light on the different resource allocation strategies and diversity of aging patterns across and within species. For the first time ever in this species, we measured age-specific survival, size and reproduction in a longitudinal laboratory experiment with Eleutheria dichotoma (Cnidaria: Hydrozoa), a metagenetic marine hydrozoan with a crawling medusa, under constant conditions for both polyp and medusa stages of one clone (i.e. genet). Established polyp colonies suffered almost no mortality at all within more than 3.5 years of observation whereas larva and primary polyp mortality was rather high, pointing towards a negative senescence pattern at the polyp colony ramet level. Demographic traits of isogenic medusa differed substantially from polyp colonies, exhibiting hump shaped trajectories in survival and both reproduction modes, suggesting a “hump shape senescence” on the medusa ramet level. Medusa size was not correlated with survival and no heritability of lifespan or reproductive output could be found, indicating a stochastic origin of generally high trait variability in medusae. Remarkable was a significant trend towards a qualitative decline in survival and both reproduction outputs with succession of vegetative medusa generations. We reason that the overall aging pattern of E. dichotoma genets is of a negative senescent type. In further sets of laboratory experiments, we tested the resource allocation flexibility of another hydrozoan, a non-senescing and purely asexually reproducing freshwater Hydra strain (Hydra magnipapillata). We examined individual phenotypic variation of isogenic Hydra polyps under constant conditions and Hydra’s phenotypic plasticity in response to various environmental challenges such as temperature gradients, hunger and bisection. We recorded budding rates, size and starvation survival as indicators to changes in the allocation of resources to asexual reproduction and maintenance. Hydra polyps showed highly variable, non-heritable budding phenotypes under constant conditions, hinting towards a random phenotype generation process. Environmental stresses triggered hormetic responses in Hydra without any detectable trade-off costs, showing that variable stressful and fluctuating environments can be salutary for Hydra.