Abstract
Although several commensal alphaproteobacteria form close relationships with plant hosts where they aid in (e.g.,) nitrogen fixation and nodulation, only a few inhabit animal hosts. Among these, Reichenowia picta, R. ornata and R. parasitica, are currently the only known mutualistic, alphaproteobacterial endosymbionts to inhabit leeches. These bacteria are harbored in the epithelial cells of the mycetomal structures of their freshwater leech hosts, Placobdella spp., and these structures have no other obvious function than housing bacterial symbionts. However, the function of the bacterial symbionts has remained unclear. Here, we focused both on exploring the genomic makeup of R. parasitica and on performing a robust phylogenetic analysis, based on more data than previous hypotheses, to test its position among related bacteria. We sequenced a combined pool of host and symbiont DNA from 36 pairs of mycetomes and performed an in silico separation of the different DNA pools through subtractive scaffolding. The bacterial contigs were compared to 50 annotated bacterial genomes and the genome of the freshwater leech Helobdella robusta using a BLASTn protocol. Further, amino acid sequences inferred from the contigs were used as queries against the 50 bacterial genomes to establish orthology. A total of 358 orthologous genes were used for the phylogenetic analyses. In part, results suggest that R. parasitica possesses genes coding for proteins related to nitrogen fixation, iron/vitamin B translocation and plasmid survival. Our results also indicate that R. parasitica interacts with its host in part by transmembrane signaling and that several of its genes show orthology across Rhizobiaceae. The phylogenetic analyses support the nesting of R. parasitica within the Rhizobiaceae, as sister to a group containing Agrobacterium and Rhizobium species.
Highlights
Hematophagous leeches (Hirudinida) of the family Glossiphoniidae posses specialized organs related to the esophagous whose only known function is to house intracellular bacterial symbionts [1,2,3]
Beyond corroborating and solidifying the hypothesis that Reichenowia parasitica, a mutualistic, intracellular bacterial symbiont of the fresh-water leech Placobdella parasitica, places phylogenetically among the alphaproteobacterial Rhizobiaceae, the present study reveals several interesting features of the genomic makeup of the bacterium
Cation Pump Membrane Proteins (Nitrogen Fixation) Because of the close relationship of R. parasitica to each of Rhizobium and Sinorhizobium, it is likely that these taxa share genes by virtue of having a rather recent common ancestor
Summary
Hematophagous leeches (Hirudinida) of the family Glossiphoniidae posses specialized organs related to the esophagous whose only known function is to house intracellular bacterial symbionts [1,2,3]. It has been hypothesized that the lack of essential nutrients, such as vitamins and enzymes, brought by the leeches’ restricted diet of vertebrate blood [5], is ameliorated by the provision of nutrients by bacterial symbionts housed in the mycetomes [6]. In both plants and animals, obligate bacterial symbionts (primary symbionts) are housed in a distinct set of host-cells, known as bacteriocytes, and are strongly associated with these cells, to the point that they cannot invade unspecialized tissues [7]. The importance of the leech bacterial symbionts is suggested by their vertical transovarial transmission [4]
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