Molecular population genetic studies on the evolution of retroposons

为了阐明逆转录因子的进化机制，从黑腹果蝇及其兄弟种中克隆了果蝇I元件并测定了它们的序列。分析结果表明，I元素至少两次入侵黑腹田鼠种群，分别发生在黑腹田鼠和拟象田鼠多样化之前和之后，第二次入侵发生在近期。我们将调查范围扩大到果蝇和蚕蛾其他类群的i -同源元件，并确定了它们在染色体上的序列和分布。它们与d.m anogaster不同，但有时具有gag^-和具有TAA重复的逆转录样序列。一般来说，I元素既存在于染色体臂上，也存在于染色体中心上，但更可能存在于染色体臂上。因为它们在很多物种中都被发现，所以它们的起源被认为相当古老。为了理解逆转录子进化的动力学，从理论上研究了一个假设主拷贝模型的逆转录子进化的瞬时群体遗传模型。计算一致频率、核苷酸纯合性和被认为是由主拷贝谱系中发生的突变引起的共有差异的数量的均值和方差。所研究的所有量都显示为膨胀周期持续时间的单调函数。利用理论结果，对人类Alu序列的Sb亚族进行了分析。与亚族扩展结束后的时间相比，估计扩展期较短。然而，观察到的共享差异数量约为主拷贝模型下预期的一半，估计的扩展期表明Sb亚家族中存在多个主拷贝位点。

In order to elucidate evolutionary mechanisms of retroposons, Drosophila I elements were cloned from Drosophila melanogaster and its sibling species and their sequences were determined. Analysis of them showed that I elements invaded populations of D.melanogaster at least twice, before and after the diversification of D.melanogaster and D.simulans and the second invasion occurred recently. We extended the survey to include I-homologous elements in other species groups of Drosophila and silk moth and determined their sequences and distribution on the chromosome. They were diverged from those of D.melanogaster but sometimes have gag^- and revesetranscriptase-like sequnces with TAA repeats. Generally, I elements are found both on chromosome arms and chromocenters but more likely to be on the latter. Because they were found in a wide range of species, their origin is considered to be fairly old.To understand the dynamics of retroposon evolution, a transient population genetic model of SINE evolution assuming the master copy model was theoretically inverstigated. Means and variances of consensus frequency, nucleotide homozygosity, and the number of shared differences which are considered to have been caused by mutations occurring in the master copy lineages are computed. All quantities investigated are shown to be monotone functions of the duration of the expansion period. Using the theoretical results, the Sb subfamily of fuman Alu sequences is analyzed. The expansion period is estimated to be short compared to the time since the end of the expansion of the subfamily. However, the observed number of the shared differences is about one half of that expected under the master copy model with the estimated expansion period suggesting the presence of multiple master copy loci in the Sb subfamily.