Adaptive and non-adaptive responses to toxin-producing cyanobacteria in water fleas

Organisms regularly encounter stressful conditions that negatively affects their fitness. One way to minimize these negative effects is to use cues from the environment in order to develop an appropriate phenotype. Many populations of water fleas (Daphnia spp.) are exposed to algal blooms dominated by toxin- producing cyanobacteria during parts of the summer season, which can reduce growth, reproductive output and survival. Theory suggests that seasonal exposure to toxic cyanobacteria will favour maternal and transgenerational effects that promote tolerance. However, it is poorly understood if, and to what extent, Daphnia can develop tolerance when exposed to toxins over one or several generations.Here, I first investigate the scope and efficacy of maternal (Paper I) and transgenerational (Paper II) effects in this system. Secondly, I examined how the overall fitness, and tolerance to the cyanotoxin microcystin in particular, changes across the season in a natural lake environment (Paper III and IV). Finally, I attempted to link variation in growth, reproductive output and survival to changes in the diversity and composition of the gut microbiome (Paper II and IV), one possible mechanism that enable cumulative transgenerational adaptive change in response to a toxic diet.In agreement with previous studies, I find that exposure to toxin-producing cyanobacteria consistently reduces fitness in Daphnia, both in terms of reproductive output and survival. The ability of mothers to transfer tolerance to their offspring was limited (Paper I), and there was little evidence that these positive effects accumulate across several generations (Paper II). These results fit well with studies of natural seasonal variation in how microcystin-producing cyanobacteria affected fitness, which provided no robust evidence for an increase in tolerance following blooms (Paper III and IV). Instead, the fitness of Daphnia declined steadily over the season in both experiments, an effect that could partly be attributed to natural exposure to microcystin-producing cyanobacteria (Paper III).The gut microbiome was strongly influenced by seasonal changes in a lake environment (Paper IV). The microbiome was also affected by different cyanobacteria environments in the laboratory (Paper II), but these effects were only weakly correlated with the ability to tolerate microcystin-producing cyanobacteria.In summary, this work shows that there are both adaptive and non-adaptive responses to toxin-producing cyanobacteria in Daphnia. The adaptive value of transgenerational effects in this system was consistently small, and does not support the existence of a dedicated machinery to transfer information between generations. This probably also means that induced changes in tolerance within a season have a limited influence on the dynamics of Daphnia populations, even in populations that are recurrently exposed to toxic algal blooms