Evolutionary dynamics and selection modeling of climate-driven Vibrio vulnificus expansion into high-latitude environments

<title>Abstract</title> <p> In the era of rapid climate change and the unprecedented rate of increasing sea temperatures, the geographic range for the opportunistic marine pathogen <italic>Vibrio vulnificus</italic> is expanding into the brackish Baltic Sea. However, the genomic mechanisms enabling regional adaptation and human infection in Baltic strains remain largely uncharacterized. To that end, this study conducted a comparative genomic analysis of 1,315 non-clonal genomes, stratified across geographical regions and ecological origins, using recombination-masked phylogenies, macro-synteny analysis, and selection modeling. The results demonstrate that Baltic strains comprise multiple phylogenetic clades characterized by the complete phylogenetic interspersion of environmental and clinical isolates. This close relationship within the Baltic cohort suggests the cold, brackish Baltic ecosystem serves as the primary reservoir for human infections without requiring a specialized clinical sub-clade. A noteworthy finding was that host colonization by these endemic strains is associated with a measurable genomic trade-off, which was absent from the broader global population. Specifically, the transition of the Baltic strains from environmental to clinical settings is accompanied by a drastic reduction in the complexity of the bacterial defense system—decreasing from 36,645 to 6,435 co-occurring defense-system interactions—and coincides with increased integration of mobile genetic elements. In addition, population genetic metrics reveal that this regional divergence is linked to specific loci, highlighting highly differentiated candidate genes under positive selection, namely the outer-membrane synthesis gene <italic>opsX</italic> (F _ST = 0.475) and the metabolic gene <italic>menC</italic> (F <italic>ST</italic> = 0.463). Ultimately, these data support a model where adaptation to the cold, brackish Baltic environment may constrain the genome, driving a potential trade-off between defense systems and mobile genetic elements during opportunistic host invasion. </p>