Diversified bet-hedging strategy in the desiccation tolerant tardigrade Paramacrobiotus richtersi

In variable environmental conditions a genotype may reduce the risk associated with reproduction by two main types of bet-hedging strategies: conservative and diversified bet-hedging. The former involves an avoidance of extremes to minimize fluctuations in a life-history trait that will assure an optimal mean for energetic constraints. The latter entails probabilistic risk spreading among individuals of the same genotype that express a range of diversified phenotypes to sample a range of different environments through time. Diversified bet-hedging is generally considered the most viable strategy for a long-term performance in a habitat varying in an unpredictable way. To date, only a few empirical studies provide evidence that bet-hedging occurs in nature. The timing of many phenological events (e.g. egg hatching) results from a complex interplay among organism genotype, environmental factors and maternal effects (e.g. egg size). The relationship between egg size and development time is known and it is generally accepted that larger eggs take longer to develop than smaller ones. The production of eggs with variable size may represent a strategy by which a mother spreads the risk connected with life in a temporary habitat. As regards tardigrades, they evolved a large variety of dormant stages that can be ascribed to diapause (encystment, resting eggs) and cryptobiosis (anhydrobiosis, cryobiosis, anoxibiosis). Therefore, their life-cycle can be generally divided into two very distinct stages: the active stage and the dormant stage, which are characterized by substantially different requirements and risks. During the active stage, tardigrades may exhibit high plasticity in life history traits (e.g. egg number, egg size and hatching time), as an adaptive measure to cope with risks linked to unpredictable habitat conditions. Clonal lineages from an apomictic population of Paramacrobiotus richtersi (Eutardigrada, Macrobiotidae) may produce up to four kinds of eggs: subitaneous eggs, delayed-hatching eggs, abortive eggs and diapause resting eggs, the last ones require a stimulus to hatch (e.g. rehydration after a period of desiccation). The lack of genetic variation expected among clonal organisms make them ideal material for investigating diversified bet-hedging that is, by definition, life history trait variance expressed within genotypes, and maternal effects. We compared the proportion of different kinds of eggs and analysed the correlation between egg size and hatching time within clonal eggs. Our preliminary results provide possible empirical support of the occurrence of diversified bet-hedging strategy due to maternal effect in tardigrades living in stochastic environments.