Variation in gut microbiota induced by drift affects fitness in the water flea Daphnia magna

It is well documented that perturbation of the gut microbiota can influence the fitness of the host. Less is known about how natural ecological processes can change the composition of the gut microbiota and how such changes influence host fitness. Here we provide novel experimental insights into such processes using the clonally reproducing Daphnia magna. Twenty replicate cultures of a single clone were reared under identical environmental conditions for five weeks to allow for divergence of microbiota communities through drift. After five weeks duplicate cultures were created from each of the twenty replicates by transferring water and animals. These were reared for 21 days while recording offspring production as a measure of fitness. There was a significant repeatability in fitness between replicates originating from the same culture, suggesting that divergence of the microbiota occurring during the first five weeks translated into fitness effects during the subsequent 21 days. This was supported by 16S rDNA amplicon sequencing data, demonstrating significant differences in the relative abundance of gut bacteria between cultures with high and low fitness. These results are consistent with the hypothesis that stochastic processes can cause natural variation in the composition of gut microbiota, which again affect host fitness.