Comparative population genomics of Pacific deep-sea tubeworms and their bacterial endosymbionts: cryptic diversity and host-symbiont specificity

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Vestimentiferan tubeworms are dominant taxa in vent and seep habitats worldwide. Lacking a digestive system as adults, they are nutritionally dependent on their sulfur-oxidizing bacterial endosymbionts, which are acquired from the local environment. Although horizontal transmission should allow uptake of various symbiotic microbes, traditional 16S rRNA sequence analyses suggest that every tubeworm harbors a clonal population of only one bacterial phylotype. The same studies have shown that different host taxa from the same location can be infected by the same symbiotic phylotype, indicating that there is no exclusive association (specificity) between a particular symbiont and a host species. However, evidence for multiple infections and subtype variation from multi-gene analyses has recently challenged the finding of genetic homogeneity in the tubeworm endosymbionts, implying that their diversity has so far been largely underestimated. Should these assumptions hold true, it is likely that also patterns of host-symbiont specificity have been overlooked due to limited variability of the investigated marker genes and insufficient information on the genetic variation in both partners. My hypotheses are that (1) tubeworms are infected by a heterogeneous mixture of bacterial types and that (2) hosts and symbionts are specific to each other, both at the species and genotype level. To test these hypotheses, I propose to analyze symbiotic associations in geographically widespread populations of the tubeworm species Lamellibrachia barhami and Escarpia spicata, which co-occur at cold seep systems in the eastern Pacific Ocean. By combining high-throughput restriction-site associated DNA sequencing with metagenomics, I will identify single-nucleotide polymorphisms (SNP) in hosts and symbionts to get an estimate of the genomic diversity and population structure in both partners. The genotypic data from different SNP subsets will then be used in multivariate and correlative statistics to determine the degree of host-symbiont specificity. I expect that my study will advance our understanding of symbioses between animals and bacteria in the deep sea and provide a basis for future research on how host-symbiont interactions influence ecological adaptation, genetic population differentiation and evolutionary diversification of both partners.