Understanding the evolutionary forces that shape symbiotic partnerships is a challenging task. Arguably, host-microbiota symbioses bring the endeavour to another level as multiple factors such as diet, age, sex and host genetics likely play a role in determining the composition and function of the symbioses. Ideally, we would want to understand how animal microbial communities are assembled and how they function in nature. Yet, as it is often the case when answering complex questions, a controlled setup is an obvious staring point. One interesting hypothesis, known as phylosymbiosis, states that hosts and their microbial communities can co-evolve resulting in a match between the phylogeny of the host group and the ecological relatedness of their microbial communities. Evidence for phylosymbiosis in nature is not universal. For example, while there is evidence for phylosymbiosis in sponges, ants and apes, patterns consistent with such co-evolutionary pattern have not been observed in flies, birds and mice. In a recent study published in PloS Biology, Brooks, Kohl and co-authors present an ambitious approach that characterizes the microbiota of 24 animal species including mammals and insects under highly controlled conditions. Their study is designed to look for evidence of phylosymbiosis in these taxa and they carefully test several predictions. First, they show that hosts harbour different microbial communities and that the composition of microbial communities is more similar between related taxa (i.e. insect microbial communities clustered from the mammalian ones). Second, clades with higher divergence time between their species have more dissimilar microbial communities than clades that have diverged more recently. Third, they show topological similarity between host phylogenetic trees and microbiota dendrograms, which reflects congruence between host evolutionary proximity and microbial community diversity. This host-microbiota congruence was observed in Nasonia wasps, mosquitoes, Peromyscus mice and hominids but not in Drosophila. Having demonstrated strong evidence for phylosymbiosis in wasps, mosquitoes, mice and hominids, the authors ask about its functional significance by conducting interspecific microbiota transplants. They find that mice that received transplants have decreased ability to digest food and transplanted wasps have lower survival. Moreover, the negative effects of transplants in performance or fitness of the host were more pronounced when microbiota were transplanted from a distant related species. Together these data provide compelling evidence for an intimate relationship between host evolution and the composition and structure of symbiotic microbial communities in insects and mammals alike. The mechanism by which phylosymbiosis is established is still an open question. While it is likely that the host immune system plays an important role in determining microbiota communities it is also conceivable that microbes play an active part.

Brooks AW, Kohl KD, Brucker RM, van Opstal EJ, Bordenstein SR (2016) Phylosymbiosis: Relationships and Functional Effects of Microbial Communities across Host Evolutionary History. PLoS Biol 14(11): e2000225. doi:10.1371/journal. pbio.2000225