Abstract
Similarly to other strict blood feeders, leeches from the Haementeria genus (Hirudinida: Glossiphoniidae) have established a symbiotic association with bacteria harbored intracellularly in esophageal bacteriomes. Previous genome sequence analyses of these endosymbionts revealed co-divergence with their hosts, a strong genome reduction, and a simplified metabolism largely dedicated to the production of B vitamins, which are nutrients lacking from a blood diet. 'Candidatus Providencia siddallii' has been identified as the obligate nutritional endosymbiont of a monophyletic clade of Mexican and South American Haementeria spp. However, the Haementeria genus includes a sister clade of congeners from Central and South America, where the presence or absence of the aforementioned symbiont taxon remains unknown. In this work, we report on a novel bacterial endosymbiont found in a representative from this Haementeria clade. We found that this symbiont lineage has evolved from within the Pluralibacter genus, known mainly from clinical but also environmental strains. Similarly to Ca. Providencia siddallii, the Haementeria-associated Pluralibacter symbiont displays clear signs of genome reduction, accompanied by an A+T-biased sequence composition. Genomic analysis of its metabolic potential revealed a retention of pathways related to B vitamin biosynthesis, supporting its role as a nutritional endosymbiont. Finally, comparative genomics of both Haementeria symbiont lineages suggests that an ancient Providencia symbiont was likely replaced by the novel Pluralibacter one, thus constituting the first reported case of nutritional symbiont replacement in a leech without morphological changes in the bacteriome. IMPORTANCE: Obligate symbiotic associations with a nutritional base have likely evolved more than once in strict blood-feeding leeches. Unlike those symbioses found in hematophagous arthropods, the nature, identity, and evolutionary history of these remains poorly studied. In this work, we further explored obligate nutritional associations between Haementeria leeches and their microbial symbionts, which led to the unexpected discovery of a novel symbiosis with a member of the Pluralibacter genus. When compared to Providencia siddallii, an obligate nutritional symbiont of other Haementeria leeches, this novel bacterial symbiont shows convergent retention of the metabolic pathways involved in B vitamin biosynthesis. Moreover, the genomic characteristics of this Pluralibacter symbiont suggest a more recent association than that of Pr. siddallii and Haementeria. We conclude that the once-thought stable associations between blood-feeding Glossiphoniidae and their symbionts (i.e., one bacteriome structure, one symbiont lineage) can break down, mirroring symbiont turnover observed in various arthropod lineages.