Highly improved single contig genome assembly of thioauto-trophic symbiont of hydrothermal vent mussel Bathymodiolus thermophilus

Mytilid mussel <i>Bathymodiolus thermophilus</i> lives in the deep-sea hydrothermal vent regions through a symbiotic relationship with chemosynthetic Gammaproteobacteria. The mussel depends on the nutrient provided by the symbiont residing inside the specialized gill cells. The symbionts in the gill bacteriocytes oxidize the reduced sulfur amply available in the vent environments. Here we sequenced the gill symbiont genome of the mussel from the East Pacific Rise, using PacBio sequencing technology, and assembled the complete genome with a hierarchical genome-assembly process (HGAP3) pipeline. The final symbiont assembly consists of a single contig size of 2.83 Mb, 126x genome coverage, a GC content of 38.6%, 2,156 protein-coding genes, and CheckM completeness of 97.19%. Comparative analysis with other <i>B. thermophilus</i> thiotrophic gill symbiont genomes indicates 1,514 shared gene clusters comprised of core gene sets and common metabolic pathways among thiotrophic mussel symbionts. In contrast, only four unique gene clusters consists of several transposase families and cadherin repeat domain-containing proteins. This complete mussel symbiont genome will be invaluable for further comparative genomic analyses studying structural genome evolution, symbiont strain population diversity, and their physiological adaptive features.