Assembly of microbial communities associated with the developing zebrafish intestine
The communities of microorganisms associated with humans and other animals are characterized by a large degree of diversity and unexplained variation across individual hosts. While efforts to explain this variation in host-associated systems have focused heavily on the effects of host selection, community assembly theory emphasizes the role of dispersal and stochastic demographic processes, otherwise known as ecological drift. In this dissertation, I characterize the communities of microorganisms associated with the zebrafish, Danio rerio, intestine, and assess the importance of microbial dispersal and drift to their assembly. First, I describe changes in the composition and diversity of the zebrafish intestinal microbiome over zebrafish development and show that while host development is a major driver of community composition over time, there remains a large amount of unexplained variation among similar hosts of the same age. I go on to show that random dispersal and ecological drift alone in the absence of host selection are sufficient to explain a substantial amount of this variation, but the ability of these processes to predict the distribution of microorganisms across hosts decreases over host development. Finally, I present an experimental test of dispersal in host-associated systems, and show that not only does dispersal among individual zebrafish hosts have a large impact on the composition and diversity of associated microbial communities, but it can also overwhelm the effects of important host factors, such as the innate immune system. As a whole, this work demonstrates that the composition and diversity of microbial communities associated with animal hosts are not solely the result of selection by the host environment, but rather dispersal and stochastic processes have important and often overwhelming effects on their assembly. To fully understand the assembly of host-microbe systems, we must broaden our focus to include scales beyond that of an individual host and their associated microorganisms.