Molecular population genetic studies on the evolution of retroposons

为了阐明反转座子的进化机制，从黑腹果蝇及其近缘种中克隆了Drosophila I元件，并测定了其序列。分析表明，在黑腹果蝇和拟果蝇的分化前后，碘元素至少入侵黑腹果蝇种群两次，最近发生了第二次入侵。我们扩大调查，包括在其他物种组的果蝇和蚕蛾的I-同源元件，并确定其序列和染色体上的分布。它们与黑腹果蝇的序列不同，但有时具有带有TAA重复的gag^-和逆转录酶样序列。一般来说，I元件既存在于染色体臂上，也存在于染色中心上，但更可能存在于后者上。为了理解反转录子进化的动力学，从理论上研究了一个假定主拷贝模型的SINE进化的瞬时群体遗传模型。计算共有频率、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。研究的所有数量被证明是单调函数的膨胀期的持续时间。利用理论结果，对fuman Alu序列的Sb子族进行了分析.与亚科扩展结束后的时间相比，扩展期估计很短。然而，所观察到的共享差异的数量是根据主拷贝模型与估计的扩展期表明在Sb亚家族中存在多个主拷贝基因座的预期的大约一半。

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.