Bacterial endosymbionts of Placobdella (Annelida: Hirudinea: Glossiphoniidae): phylogeny,genetic distance,and vertical transmission

Blood-feeding leeches of the genus Placobdella have acquired intracellular alphaproteobacterial endosymbionts of the genus Reichenowia that potentially aid in the production of B vitamins, thereby ameliorating the lack of these essential nutrients in the diet of the hosts. For Placobdella associates, little is still known about the diversity, genetic makeup, and the mode of transmission of bacteria between leeches. We aimed to (i) place the bacterial symbionts in a phylogenetic context, (ii) compare patterns of cladogenesis between the bacteria and hosts to search for evidence of co-speciation, and (iii) explore the mode of bacterial transmission between leech parent and offspring. DNA sequencing of the bacterial 16S rDNA and 23S rDNA loci suggests that, whereas Reichenowia forms a monophyletic group within the alphaproteobacterial family Rhizobiaceae, no evidence for co-speciation between hosts and bacteria can be traced. Attempts at DNA amplification for ovarial tissues were negative for a range of species, but two 16S rDNA sequences retrieved from the testisacs of P. rugosa showed very high similarity with Reichenowia. Although we cannot rule out that this may be a contamination, or a different, potentially free-living species of bacteria, our results may indicate that Reichenowia is transferred from leech parent to offspring via the testisacs.