Abstract
Vertebrate gut microbiota mediate critical physiological processes known to affect host fitness, but the mechanisms that expose wildlife to pioneer members of this important microbial community are not well understood. For example, oviparous vertebrates are thought to acquire gut microbiota through post-natal exposure to the external environment, but recent evidence from placental mammals suggests that the vertebrate reproductive tract harbours microbiota that may inoculate offspring in utero These findings suggest that oviparous vertebrates may be capable of acquiring pioneer microbiota in ovo, but this phenomenon remains unexplored. To fill this knowledge gap, we used culture-independent inventories to determine if the eggs of wild birds and lizards harboured in ovo microbial communities. Our approach revealed distinct in ovo bacterial communities, but fungal communities were indistinguishable from controls. Further, lizard eggs from the same clutch had bacterial community structures that were more similar to each other than to unrelated individuals. These results suggest that oviparous vertebrates may acquire maternal microbiota in ovo, possibly through the inoculation of egg yolk prior to shelling. Therefore, this study may provide a first glimpse of a phenomenon with substantial implications for our understanding of the ecological and evolutionary factors shaping gut microbial communities.