Molecular Studies of Cospeciation in a Host-Parasite Assemblage

尽管做了大量的研究，生物学家们对远亲生物群体（如哺乳动物和昆虫）DNA变化的相对速度仍然知之甚少。在这项研究中，Mark Hafner和Xuhua Xia将研究宿主-寄生虫系统，从中他们将产生关于DNA变化率的比较数据。这项研究的宿主是口袋地鼠，这是一种小型的食草啮齿动物，分布在北美和中美洲的大部分地区。这种寄生虫是咀嚼虱，是一种生活在宿主皮毛中的小型外寄生昆虫。咀嚼虱和它们的宿主之间的联系被认为是有义务的，因为虱子不能离开它们的自然宿主生存。口袋地鼠和咀嚼虱子之间的这种密切而长期的联系为研究这些生物DNA分子变化的模式和速度提供了一个不寻常的机会。先前的研究表明，口袋地鼠的每一个物种形成事件都伴随着它们的虱子的一个物种形成事件。这意味着人们可以测量生活在这些地鼠身上的虱子物种之间的分子变化量。由于时间是一个常数，这样的比较可以直接衡量地鼠和它们的虱子DNA变化的相对速度。两个线粒体基因（细胞色素氧化酶I和细胞色素b）序列将用于构建地鼠和虱子的独立系统发育。这些树将进行统计比较，以评估总体分支相似性，从而在地鼠-鼠组合的共种程度。最后，将地鼠和虱子树中同源分支的长度进行比较，以确定地鼠和虱子的相对DNA变化率。

9527583 HAFNER Despite considerable study, biologists still know very little about relative rates of DNA change in distantly related groups of organisms, such as mammals versus insects. In this research, Mark Hafner and Xuhua Xia will investigate a host-parasite system, from which they will generate comparative data about rates of DNA change. The hosts in this study are pocket gophers, which are small, herbivorous rodents found throughout much of North and Central America. The parasites are chewing lice, which are small, ectoparasitic insects that live in the fur of their hosts. The association between chewing lice and their hosts is considered obligate, because the lice cannot survive off their natural hosts. This intimate and long-term association between pocket gophers and chewing lice presents an unusual opportunity to examine both the pattern and rate of molecular change in the DNA of these organisms. Previous work has shown that each speciation event in the pocket gophers is accompanied by a speciation event in their lice. This means that one can measure the amount of molecular change between the species of lice that live on these gophers. Since time is a constant, such a comparison yields a direct measure of relative rates of DNA change in the gophers versus their lice. Sequences of two mitochondrial genes (cytochrome oxidase I and cytochrome b) will be used to construct independent phylogenies of gophers and lice. These trees will be compared statistically to evaluate overall branching similarity, hence degree of cospeciation in the gopher-louse assemblage. Finally, lengths of homologous branches in the gopher and louse trees will be compared to determine relative rates of DNA change in gophers versus lice.