Strong transgenerational effects but no genetic adaptation in zooplankton 24 years after an abrupt +10°C climate change

The climate is currently warming fast, threatening biodiversity all over the globe. Adaptation is often rapid when the environment changes quickly, but for climate warming very little evidence is available. Here, we investigate the pattern of adaptation to an extreme +10{degrees}C climate change in the wild, following the introduction of brine shrimp Artemia franciscana from San Francisco Bay, USA, to Vinh Chau saltern in Vietnam. We use a resurrection ecology approach, hatching diapause eggs from the ancestral population and the introduced population after 13 and 24 years (resp. ~54 and ~100 generations). In a series of coordinated experiments, we determined whether the introduced Artemia show increased tolerance to higher temperatures, and the extent to which genetic adaptation, developmental plasticity, transgenerational effects, and local microbiome differences contributed to this tolerance. We find that introduced brine shrimp do show increased phenotypic tolerance to warming. Yet strikingly, these changes do not have an additive genetic component, are not caused by mitochondrial genetic variation, and are not caused by epigenetic marks set by adult parents exposed to warming. Further, we do not find any developmental plasticity in response to warming, nor any protective effect of heat-tolerant local microbiota. We conclude that the evolution of shrimps extreme thermal tolerance is only due to transgenerational (great)grandparental effects, possibly epigenetic marks set by parents who were exposed to high temperatures as juveniles. This finding challenges standard models of genetic and plastic adaptive responses, and our conception of how species may cope with climate warming. Significance statementAdaptation is often rapid when environments change quickly, but for climate warming little evidence is available. Many studies report no genetic responses due to pre-existing plasticity, while others point towards epigenetics and microbiota effects. In this study, we take advantage of a natural experiment to study all of these effects. We use a set of coordinated experiments and a resurrection ecology approach, reviving resting eggs of brine shrimp up to100 generations after their introduction from a temperate to a tropical saltern. We find that heat adaptation occurs, but heritability is fully "missing". Plasticity and microbiota play no role either, indicating that only transgenerational (great)grandmaternal effects are involved. This finding prompts us to reconsider the relative importance of the different possible mechanisms by which phenotypic change can occur, especially in response to temperature variation.