Genus-wide comparison of <i>Pseudovibrio</i> bacterial genomes reveal diverse adaptations to different marine invertebrate hosts

<div><p>Bacteria belonging to the genus <i>Pseudovibrio</i> have been frequently found in association with a wide variety of marine eukaryotic invertebrate hosts, indicative of their versatile and symbiotic lifestyle. A recent comparison of the sponge-associated <i>Pseudovibrio</i> genomes has shed light on the mechanisms influencing a successful symbiotic association with sponges. In contrast, the genomic architecture of <i>Pseudovibrio</i> bacteria associated with other marine hosts has received less attention. Here, we performed genus-wide comparative analyses of 18 <i>Pseudovibrio</i> isolated from sponges, coral, tunicates, flatworm, and seawater. The analyses revealed a certain degree of commonality among the majority of sponge- and coral-associated bacteria. Isolates from other marine invertebrate host, tunicates, exhibited a genetic repertoire for cold adaptation and specific metabolic abilities including mucin degradation in the Antarctic tunicate-associated bacterium <i>Pseudovibrio</i> sp. Tun.PHSC04_5.I4. Reductive genome evolution was simultaneously detected in the flatworm-associated bacteria and the sponge-associated bacterium <i>P</i>. <i>axinellae</i> AD2, through the loss of major secretion systems (type III/VI) and virulence/symbioses factors such as proteins involved in adhesion and attachment to the host. Our study also unraveled the presence of a CRISPR-Cas system in <i>P</i>. <i>stylochi</i> UST20140214-052 a flatworm-associated bacterium possibly suggesting the role of CRISPR-based adaptive immune system against the invading virus particles. Detection of mobile elements and genomic islands (GIs) in all bacterial members highlighted the role of horizontal gene transfer for the acquisition of novel genetic features, likely enhancing the bacterial ecological fitness. These findings are insightful to understand the role of genome diversity in <i>Pseudovibrio</i> as an evolutionary strategy to increase their colonizing success across a wide range of marine eukaryotic hosts.</p>