| Summary: | <p>Organisms host diverse and complex bacterial communities (microbiotas), often with fundamental roles in host ecology and evolution. While microbiotas of the gut have received considerable research focus, relatively little is known about reproductive microbiotas, (i.e. those communities living within the reproductive tract and potentially transmitted at mating), despite the potentially critical role they may play in host sexual selection and mating system evolution. This thesis seeks to address this gap by laying the foundations for the systematic study of reproductive microbiotas in bird populations, and the patterns and drivers of intra- and inter-individual variation. I studied a captive population of red junglefowl, Gallus gallus, a sexually promiscuous species with well-characterised mating systems. Developing a pipeline based on a next-generation sequencing approach I show that junglefowl ejaculates contain diverse bacterial communities of bacteria (seminal microbiotas), which reflect the gut microbiota and vary in structure across males. I then use in vitro culturing to demonstrate that seminal microbiotas are alive and ecologically complex. Next, focusing on patterns of variation in seminal microbiotas, I show an association between inter-male variation in composition and aspects of the male immune-genotype, and find evidence that variation in seminal microbiota structure might be influenced by the sexual dynamics of social groups. Finally, I focus on the female cloacal microbiota and the potential effects that insemination of male semen might have on these communities. Using a series of controlled artificial insemination experiments, I show some evidence that the microbiota of the female cloaca appears to be disrupted by the first inseminations, but might revert back to its initial structure following cessation of inseminations. These effects might be due to the disruption caused by the insemination, rather than by the transmission, and subsequent colonisation of male bacteria. Collectively, these results demonstrate an ecologically complex and dynamic avian reproductive microbiota, providing a platform and direction for future work, by emphasising the need for an understanding of composition, function and variation in reproductive microbiota in order to better understand microbiotas and contextualise the evolutionary ecology of host mating systems. </p>
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