Characterization of a Newly Discovered Symbiosis between Bacteria and Leeches

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DEB- 0108163Mark A. SiddallSusan PerkinsA grant has been awarded to Drs. Mark Siddall and Susan Perkins of the American Museum of Natural History to study the historical relationship between leeches and their bacterial symbionts. Species that rely on nutritionally poor diets are often reliant on obligate bacterial endosymbionts. Vertebrate blood is deficient in B vitamins and some amino acids. A broad array of blood-feeding invertebrates harbor bacterial symbionts providing some of these essential nutrients to their hosts. Leeches are known to have symbionts in organs that open into the esophagus just in front of the crop where blood is stored. A diversity of leeches will be collected from North America, South America and South Africa. Using DNA sequences amplified and sequenced from the isolated bacteria and from leeches, the project will answer significant issues regarding the evolutionary origin of the bacteria and the degree to which they have coevolved with leeches that harbor them. Modern phylogenetic analyses will determine the degree of cospeciation through time by comparing the evolutionary tree topologies from the two associated groups.Preliminary data indicate that these bacteria living inside the leech epithelial cells are closely related to Agrobacterium, Rhizobium and Sinorhizobium species including those forming symbioses for nitrogen fixation in leguminous plants (e.g., beans). If bacteria from different species of leech are each others' closest relatives it would suggest that they have retained a tight coevolutionary association with each other through time. The alternative is independent acquisition of different bacterial associates by different groups of leeches. What is particularly intriguing about this symbiotic association and the phylogenetic position of the bacteria is the implication that these intracellular symbionts may be playing a role in nitrogen metabolism for their leech hosts. Furthermore the results should help to answer questions regarding the evolution of blood-feeding in annelid worms.