Characterization of a Newly Discovered Symbiosis between Bacteria and Leeches

DEB- 0108163Mark A.美国自然历史博物馆的马克·西德尔博士和苏珊·帕金斯博士获得了一笔赠款，用于研究水蛭和它们的细菌共生体之间的历史关系。依赖于营养不良饮食的物种通常依赖于专性细菌内共生体。 脊椎动物的血液缺乏B族维生素和一些氨基酸。 大量的吸血无脊椎动物体内都有细菌共生体，为它们的宿主提供这些必需的营养物质。众所周知，水蛭的器官中有共生体，这些器官通向食道，就在储存血液的作物前面。 将从北美、南美和南非收集各种水蛭。利用从分离的细菌和水蛭中扩增和测序的DNA序列，该项目将回答有关细菌进化起源的重要问题，以及它们与窝藏它们的水蛭共同进化的程度。 现代系统发育分析将通过比较两个相关类群的进化树拓扑结构来确定随时间推移的共同物种形成程度。初步数据表明，这些生活在水蛭上皮细胞内的细菌与土壤杆菌属、根瘤菌属和中华根瘤菌属物种密切相关，包括那些在豆科植物中形成固氮共生体的物种（例如，豆）。 如果来自不同种类水蛭的细菌是彼此最近的亲戚，这将表明它们在时间上彼此保持着紧密的共同进化联系。 另一种方法是由不同的水蛭群独立地获得不同的细菌。 关于这种共生关系和细菌的系统发育位置特别有趣的是，这些细胞内共生体可能在水蛭宿主的氮代谢中发挥作用。此外，这些结果应该有助于回答有关环节动物蠕虫吸血进化的问题。

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.