Environmental and Genetic Determinants of Bacterial Microbiota and Parasites Associated with Cottus Across Natural Hybrid Zones

Natural hybrid zones often emerge at habitat clines where distinct populations meet, mate and hybridize. The possible factors that govern the dynamics of a hybrid zone are insufficiently explored. For example symbiotic bacteria and also parasites can induce selection pressure by influencing the host’s immune system, nutrition, development and behavior. These interactions are in turn influenced by environmental factors making it difficult to disentangle all involved components. Here, we use a secondary contact zone where two populations of European freshwater sculpins (pisces), Cottus rhenanus and invasive Cottus, hybridize at a habitat ecotone. This system enables to study the association of symbiotic microbiota, habitat and host genotype to infer whether host-bacterial interaction may act as evolutionary force. The goal was to determine whether skin bacterial communities of hybrid and parental sculpins differ which would then suggest a significant role for microbiota in influencing the sculpins’ distribution. Further teleost fish, Barbatula barbatula and Phoxinus phoxinus, both inhabiting either habitat of the contact zone, served as control for environmental variation. The bacterial community structure of the fish was examined by applying high-throughput sequencing of the 16S rDNA gene from fin clips. The host genetic makeup was inferred based on nuclear SNP markers that permit to classify all fish into parental forms and recent hybrids. By means of additional laboratory experiments we examined the host genetic influence on bacterial communities under controlled environments. Phylogenetic and statistical analyses revealed major differences in bacterial diversity and abundance in distinct host habitats. However, we also identified minor host-specific variation being independent from the environmental variation. These results indicate that bacterial communities in our system are shaped by both, host genetic and environmental factors, and can at least not solely act as selective force against migrants in a Cottus spp. hybrid zone