Abstract
Horizontally transmitted symbioses usually house multiple and variable symbiont genotypes that are acquired from a much more diverse environmental pool via partner choice mechanisms. However, in the deep-sea hydrothermal vent tubeworm Riftia pachyptila (Vestimentifera, Siboglinidae), it has been suggested that the Candidatus Endoriftia persephone symbiont is monoclonal. Here, we show with high-coverage metagenomics that adult R. pachyptila house a polyclonal symbiont population consisting of one dominant and several low-frequency variants. This dominance of one genotype is confirmed by multilocus gene sequencing of amplified housekeeping genes in a broad range of host individuals where three out of four loci ( atpA, uvrD and recA) revealed no genomic differences, while one locus ( gyrB) was more diverse in adults than in juveniles. We also analysed a metagenome of free-living Endoriftia and found that the free-living population showed greater sequence variability than the host-associated population. Most juveniles and adults shared a specific dominant genotype, while other genotypes can dominate in few individuals. We suggest that although generally permissive, partner choice is selective enough to restrict uptake of some genotypes present in the environment.
Highlights
Most ecosystems harbour a large diversity of mutualistic interactions in which microbes live in physical contact with a eukaryotic host and the partners cooperate to increase each other’s fitness compared with solitary life [1,2]
Applying high-coverage metagenomics on one environmental sample taken underneath a tubeworm clump, and one Riftia specimen from this clump as well as multilocus gene sequencing on a broad range of hostassociated and environmental populations we show that host specimens at the hydrothermal vent environment, contain polyclonal populations dominated by one genotype and that symbiont strain diversity in the environment exceeds diversity in the host
To investigate the variability within the symbiont population in Riftia from the four vent sites Tica and P-Vent at 98500 N East Pacific Rise (EPR) and Janine and Genesis at 138 N EPR, we analysed a total of 16 specimens, including nine adults and seven juveniles using a multilocus gene framework by comparing the allelic profiles of the four housekeeping loci atpA, uvrD, recA and gyrB
Summary
Most ecosystems harbour a large diversity of mutualistic interactions in which microbes live in physical contact with a eukaryotic host and the partners cooperate to increase each other’s fitness compared with solitary life [1,2]. Mechanisms are required to create a positive assortment of symbiont genotypes, so that cooperating partners preferentially interact with each other [11]. Theory predicts that partner choice mechanisms through signalling between partners or screening of the partner play a crucial role in maintaining the associations [3,12,13]. The structure and abundance of symbiont populations in the environment as well as dispersal and contact of both partners were identified as important factors influencing which symbiont strains are transmitted [14,15].
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