Several recent studies have shown that ectosymbiotic bacterial microbiota, including gut microbes, affect the growth and reproduction of Daphnia, a key organism in lake ecosystems. However, these studies examined specific species, such as the model organism, D. magna, and used green algae as food. It is unclear if symbiotic bacteria affect fitness in other Daphnia species common in lakes and ponds in Japan. In this study, we examined the growth rates of sterilized individuals of two D. pulex genotypes with D. magna using the green algae Scenedesmus and the diatom Cyclotella as algal diets. In addition, we examined the growth rate of the sterilized Daphnia pulex individuals infected by Limnohabitans spp., previously reported to promote the fitness of D. magna as ectosymbiotic bacteria. We found that the effects of ectosymbiotic bacteria on growth rate differed not only between different genotypes of D. pulex but also between individuals fed the different algal diets. The results suggest that the genotype- and diet-specific differences in fitness-dependency on the ectosymbiotic microbiota can be factors affecting the genetic structures of D. pulex populations.

Detailed methods are described in the paper published in Ecological Research.