Transposable element content is associated with total genome size and host taxonomy in sulfur-oxidizing bacterial endosymbionts

Transposable elements (TEs) are mobile genetic sequences that can disrupt, relocate, or create genes and regulatory elements, thereby contributing to genome evolution. Their potential role in shaping adaptation, including symbiotic relationships, has drawn increasing attention. Most research on TEs to date has focused on terrestrial environments, while less is known about TE function and prevalence in marine habitats. This study investigates the presence and distribution of TEs in marine chemosynthetic symbioses, specifically among the genomes of sulfur-oxidizing Gammaproteobacteria associated with animals living in a diversity of chemically reducing environments. Our data reveal that factors such as phylogenetic relationship and habitat type have no demonstrated association with TE abundance, while factors such as genome size, host family, and lifestyle are significantly correlated. Almost all vesicomyid clam symbionts lacked TEs likely because of reductive genome evolution, while mytilid symbionts had the highest TE abundance possibly due to increased rates of homologous recombination or TE proliferation related to adaptive genome streamlining. Transposase family diversity varied considerably among symbiont groups but was not associated with genome size, indicating that other factors influence transposase composition. Notably, a large fraction of transposase genes was flanked by genes involved in DNA replication and repair, membrane transport, and prokaryotic defense systems, implying a possible role in environmental or host adaptation. This study highlights the dynamic nature of TE content in symbiotic bacteria and allows for the formulation of hypotheses on TE function for the evolution of bacteria in diverse, chemically reducing environments. IMPORTANCE Transposable elements are key drivers of bacterial genome evolution and environmental adaptation by creating new gene functions and altering existing ones, yet they remain comparatively understudied in marine microbes. In this study, we assessed TE content and the function of flanking genes in symbiotic and free-living sulfur-oxidizing bacteria from a variety of aquatic environments. Our analyses show that genome size, bacterial lifestyle, and host association have a significant impact on TE abundance, but not necessarily transposase diversity. Transposases were often flanked by DNA replication and repair genes, membrane transporters, and prokaryotic defense systems, which likely represent transposition cargo that might be relevant for habitat adaptation. The patterns observed within these data further our understanding of the distribution and function of transposases and lay a foundation for future studies investigating the role of transposases in bacterial genome evolution.