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Many populations of water fleas (Daphnia) are exposed to algal blooms dominated by microcystin-producing cyanobacteria. However, the severity of these effects on Daphnia fitness remain poorly understood in natural populations. We investigated seasonal changes in body size, reproduction and survival of D. longispina individuals from five eutrophic lakes in southern Sweden. We tested whether individuals collected before, during or following algal blooms differed in their reproduction and survival when experimentally exposed to microcystin-producing cyanobacteria. The concentration of microcystin in the lakes was significantly higher during summer and autumn compared to spring, but there were substantial differences between lakes. The reproductive output of individuals declined consistently over the season, and this decline was stronger for Daphnia collected during periods of, or from lakes with, high microcystin concentration. There was little evidence that individuals adapted to the toxin over the season. The strong seasonal changes in body size, reproduction and survival in these D. longispina appear to be caused partly by variation in the abundance of toxin-producing cyanobacteria. Populations were unable to adapt sufficiently quickly during summer and autumn to recover from the negative effects of microcystin. We therefore suggest that seasonal increases in tolerance to microcystin-producing cyanobacteria have limited effects on the eco-evolutionary dynamics between Daphnia and phytoplankton.

The gut microbiome is increasingly recognized for its impact on host fitness, but it remains poorly understood how naturally variable environments influence gut microbiome diversity and composition. We studied changes in the gut microbiome of ten genotypes of water fleas Daphnia magna in submerged mesocosm enclosures in a eutrophic lake over a period of 16 weeks, from early summer to autumn. The microbial diversity increased when Daphnia were reintroduced from the laboratory to the lake, and the composition of gut microbes drastically changed. Both gut microbiome diversity and composition continued to change over the 16-week period, with alpha diversity peaking in late summer. The gut microbiome community was clearly distinct from that of the surrounding water, and temporal changes in the two communities were independent of each other. There were no consistent differences in the gut microbiomes among Daphnia genotypes in the lake environment. The change in gut microbiome over the season was accompanied by a decline in reproductive output and survival. There were weak, but statistically supported, effects of microbiota composition on Daphnia fitness, but there was no evidence that natural variation in microbiome diversity or composition was associated with tolerance to the cyanotoxin microcystin. We conclude that the gut microbiome of Daphnia is highly dynamic in a natural lake environment, but that host genetic effects on microbiome diversity and composition between genotypes within a population can be vanishingly small. These results emphasize that establishing the ecological effects of gut microbiota will require large-scale experiments under natural conditions. (Less)

Predation risk is often invoked to explain variation in stress responses. Yet, the answers to several key questions remain elusive, including the following: (1) how predation risk influences the evolution of stress phenotypes, (2) the relative importance of environmental versus genetic factors in stress reactivity and (3) sexual dimorphism in stress physiology. To address these questions, we explored variation in stress reactivity (ventilation frequency) in a post-Pleistocene radiation of live-bearing fish, where Bahamas mosquitofish (Gambusia hubbsi) inhabit isolated blue holes that differ in predation risk. Individuals of populations coexisting with predators exhibited similar, relatively low stress reactivity as compared to low-predation populations. We suggest that this dampened stress reactivity has evolved to reduce energy expenditure in environments with frequent and intense stressors, such as piscivorous fish. Importantly, the magnitude of stress responses exhibited by fish from high-predation sites in the wild changed very little after two generations of laboratory rearing in the absence of predators. By comparison, low-predation populations exhibited greater among-population variation and larger changes subsequent to laboratory rearing. These low-predation populations appear to have evolved more dampened stress responses in blue holes with lower food availability. Moreover, females showed a lower ventilation frequency, and this sexual dimorphism was stronger in high-predation populations. This may reflect a greater premium placed on energy efficiency in live-bearing females, especially under high-predation risk where females show higher fecundities. Altogether, by demonstrating parallel adaptive divergence in stress reactivity, we highlight how energetic trade-offs may mould the evolution of the vertebrate stress response under varying predation risk and resource availability. CC BY 4.0Correspondence: Jerker Vinterstare