Degnan Patrick H, Moran Nancy A
Department of Ecology and Evolutionary Biology, Biosciences West, Room 310, 1041 E. Lowell Street, University of Arizona, Tucson, AZ 85721-0088, USA.
Mol Ecol. 2008 Feb;17(3):916-29. doi: 10.1111/j.1365-294X.2007.03616.x. Epub 2007 Dec 20.
The facultative endosymbiont of aphids, Hamiltonella defensa, kills parasitoid wasp larvae, allowing aphid hosts to survive and reproduce. This protection may depend on toxins that are encoded by the genomes of H. defensa and of its bacteriophage (APSE). Strains of H. defensa vary in degree of protection conferred upon Acyrthosiphon pisum (pea aphid). Although H. defensa is known to undergo some horizontal transmission among aphid maternal lineages, divergence, recombination, and population structure in H. defensa and APSE have not been characterized. We performed a multilocus sequence analysis of 10 bacterial and five phage loci for strains isolated from A. pisum and other aphid species. The H. defensa chromosome was found to be largely clonal, allowing us to generate a well-resolved H. defensa strain phylogeny. In contrast, APSE chromosomes undergo recombination and numerous H. defensa strains have probably lost the phage. Within a set of H. defensa strains that are indistinguishable on the basis of chromosomal genes or restriction digests of chromosomal fragments, loss of APSE is associated with decreased protection, strongly suggesting that APSE-encoded genes contribute to the defensive phenotype. Thus, homologous recombination of APSE genes and sexual transmission of symbionts and phage are likely factors influencing the exchange of ecologically important genes among symbionts. Although H. defensa has been lost, transferred and gained within A. pisum, one subclade of H. defensa appears to be universal within a subclade of the aphid genus Uroleucon, suggesting a transition from facultative, horizontal transmission to strictly vertical inheritance.
蚜虫的兼性内共生菌——防御汉氏菌（Hamiltonella defensa），能杀死寄生蜂幼虫，使蚜虫宿主得以存活和繁殖。这种保护作用可能依赖于由防御汉氏菌及其噬菌体（APSE）基因组编码的毒素。防御汉氏菌菌株对豌豆蚜（Acyrthosiphon pisum）提供的保护程度各不相同。尽管已知防御汉氏菌会在蚜虫母系谱系间进行一定程度的水平传播，但防御汉氏菌和APSE的分化、重组及种群结构尚未得到描述。我们对从豌豆蚜和其他蚜虫物种中分离出的菌株的10个细菌基因座和5个噬菌体基因座进行了多位点序列分析。结果发现，防御汉氏菌的染色体在很大程度上是克隆性的，这使我们能够构建出一个分辨率良好的防御汉氏菌菌株系统发育树。相比之下，APSE染色体发生重组，并且许多防御汉氏菌菌株可能已经失去了噬菌体。在一组基于染色体基因或染色体片段的限制性酶切无法区分的防御汉氏菌菌株中，APSE的缺失与保护作用减弱相关，这强烈表明APSE编码的基因有助于防御表型的形成。因此，APSE基因的同源重组以及共生菌和噬菌体的有性传播可能是影响共生菌之间生态重要基因交换的因素。尽管防御汉氏菌在豌豆蚜体内有丢失、转移和获得的情况，但防御汉氏菌的一个亚分支似乎在蚜属Uroleucon的一个亚分支中普遍存在，这表明其从兼性水平传播向严格垂直遗传的转变。
