Beyond specialization: re-examining routes of host influence on symbiont evolution

Our understanding of host influence on microbial evolution has focused on symbiont specialization and the genomic streamlining that often accompanies it. However, a vast diversity of symbiotic lineages facultatively interact with hosts or associate with multiple hosts. Yet, there are no clear expectations for how host association influences the niche of these symbionts or their evolution. Here, we discuss how weak or variable selection on microbial symbiotic associations, horizontal transmission, and low costs of adaptation to novel host habitats are predicted to promote the expansion or maintenance of microbial niches. This broad perspective will aid in developing better and more general predictions for evolution in microbial symbioses. Hosts strongly influence the evolution of their associated symbionts.Studies in model systems have emphasized the importance of host specialization and (pseudo)vertical transmission as drivers of selection, resulting in streamlining of symbiont genomes among other outcomes.However, rapidly accumulating data on microbial distributions highlight frequent overlaps in the composition of environmental and host-associated microbial communities and suggest that many symbionts are facultative and/or generalists.Similarly, overlap in the genomic repertoire of free-living and host-associated microbes suggests that symbiosis often contributes to the maintenance or expansion of microbial functional repertoires.Untangling the effects of host specialization and generalization represents a frontier in explaining the influence of host-associated lifestyles on microbial ecology and evolution.