Life history traits and previous exposure predict resistance to UV irradiation in the freshwater cnidarian Hydra oligactis

Abstract

Abiotic stress is an important source of mortality for cnidarians and is likely to be a major factor shaping their life histories. In freshwater hydra, the ability to withstand exogenous sources of stress varies between species and populations, but little is known about the factors responsible for this variation. Here, we investigated resistance to UV irradiation in Hydra oligactis, a common temperate freshwater cnidarian. We collected polyps from 12 populations and propagated these asexually under standard conditions in the laboratory to obtain 69 laboratory clonal lines with a total of 324 polyps of different age. We measured the size of polyps and recorded their budding rate. In addition, a subset of animals was exposed to hormetic treatment, where experimental animals received a short, sublethal irradiation 2 d before testing their resistance to a higher dose. We investigated how life history traits (age, size, and budding rate), hormetic treatment, and the interaction between life history traits and hormetic treatment relate to the ability of hydra polyps to tolerate high doses of UV irradiation. In multivariate models controlling for the effect of other variables, stress tolerance was positively related to age (lower tolerance in freshly detached buds compared to adult hydra) and size (higher tolerance in polyps with a large body column). Budding rate was negatively associated with stress tolerance. Hormetic treatment increased resistance to UV irradiation, but we found no evidence for an interaction between hormetic response and any of the life history traits, suggesting that the ability to upregulate physiological defense mechanisms after exposure to mild stress does not depend on the life history background of the individuals.

Phenotypic plasticity rather than genotype drives reproductive choices in Hydra populations

“…H. oligactis resembles cyclical parthenogenetic cladocerans in that it switches to a sexual mode of reproduction under conditions favouring dormancy (Tessier & Caceres, 2004). Although previous studies have suggested that there may be some genetic differences between different genotypes in this species (Tökölyi, Kozma, et al, 2017; Tomczyk et al, 2015), such a difference was not observed in this study. Hydra individuals engaged in each reproductive strategy are phenotypically very different, but we did not find significant genetic differentiation between sexual and asexual polyps.…”

“…This suggests that there are some internal factors or environmental cues other than temperature that might induce sexual reproduction in this species. First, variation in individual condition due to differences in the availability and predictability of food, population density, age, size or previous exposure to stressors and parasites are factors with a known effect on life history traits in this species (Tökölyi et al, 2016; Tökölyi, Kozma, et al, 2017; Tökölyi, Kozma, et al, 2017). Second, particular microhabitats might occur in the examined water bodies, which could provide favourable conditions for Hydra polyps with either sexual or asexual reproduction.…”

“…This species reproduces asexually under favourable conditions (warm conditions) and does not show signs of senescence, but in unfavourable conditions (cooling), polyps start sexual reproduction (Reisa, 1973), then undergo post‐reproductive senescence and many of them die within a few months after initiating sexual reproduction (Yoshida et al, 2006). The level of sexuality could be genetically determined in this species as well, because, when kept under standard conditions in the laboratory, H. oligactis strains express differences in the probability of initiating sexual reproduction and in post‐sexual survival rates (Tökölyi et al, 2017; Tomczyk et al, 2015). However, actual data on population structure in this species (or any other Hydra species), to our knowledge, does not exist and this lack of data precludes any inference on the interrelationship of different reproductive mode strategies.…”

Age‐dependent plasticity in reproductive investment, regeneration capacity and survival in a partially clonal animal (Hydra oligactis)

“…Based on the results presented in this study, polyp age could be one of the factors determining this plasticity, such that younger polyps reproduce asexually, while older ones initiate sexual reproduction when temperature drops during autumn. In addition to age, external factors such as food availability (Tökölyi, Kozma, Sebestyén, Miklós, & Barta, 2017) or population density (Bell & Wolfe, 1985) might contribute to variation in reproductive strategies, but the relative role of these factors remains to be elucidated.…”

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

“…Furthermore, to see whether differences in sexual readiness between seasons is due to 120 phenotypic plasticity, as predicted by our hypothesis, we performed warm exposure experiments in two 121 lab strains (one male and one female) and looked at changes in sexual readiness in response to 122 exposure to elevated temperature. Hydra (Tökölyi et al 2017a, 2017b, Sebestyén et al 2018. The asexual propagation phase lasted for 10 weeks.…”