Intrahost Genetic Diversity of Bacterial Symbionts Exhibits Evidence of Mixed Infections and Recombinant Haplotypes

Even the simplest microbial-eukaryotic mutualisms are comprised of entire populations of symbionts at the level of the host individual. Early work suggested that these intrahost populations maintain low genetic diversity as a result of transmission bottlenecks or to avoid competition between symbiont genotypes. However, the amount of genetic diversity among symbionts within a single host remains largely unexplored. To address this, we investigated the chemosynthetic symbiosis between the bivalve Solemya velum and its intracellular bacterial symbionts, which exhibits evidence of both vertical and horizontal transmission. Intrahost symbiont populations were sequenced to high coverage (200-1,000×). Analyses of nucleotide diversity revealed that the symbiont genome sequences were largely homogeneous within individual host specimens, consistent with vertical transmission, except for particular regions that were polymorphic in ∼20% of host specimens. These variant sites were also found segregating in other host individuals from the same population, colocalized to several regions of the genome, and consistently co-occurred on the same short read pairs (derived from the same chromosome). These results strongly suggest that these variant haplotypes originated through recombination events, potentially during prior mixed infections or in the external environment, rather than as novel mutations within symbiont populations. This abundant genetic diversity could have a profound influence on symbiont evolution as it provides the opportunity for selection to limit the extent of reductive genome evolution commonly seen in obligate intracellular bacteria and to enable the evolution of adaptive genotypes.