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[转贴]进化故事13:果蝇热爱转座子(from PNAS,2004)
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以下故事将要由PNAS，2004年2月发表，现在网上能看到
如果有人告诉你在中国有两个人长得一模一样，也许这没有什么特别的。
但如果有人告诉你在中国东北所有的人都长得一模一样的话，这事就有点奇怪了。
类似的事情发生在美国加州的一种果蝇（Drosophila simulans）基因组上。那里所有的果蝇，其基因组上一个近100 kb的片段上，几乎没有什么变异－－大家全都一模一样。
换句专业的话来说，这个片段是“固定”在果蝇种群里了。令人惊讶的是，这可是一个长100000 bp的片断啊。
只有两种可能：其一，这个片段是单元起源的。然而，即使如些，在以后的种群分化中不产生变异也是一个问题。
另一种可能性，则是有某种遗传学或进化机制，比如强烈的选择压力，维持着如此之高的同源性。
事情正是如此。研究者在这个片断里发现一个参与脂肪酸氧化的基因（cyp6g，我对这样的基因太熟悉了，因为我刚克隆了一个这样的基因，它参与细胞的解毒功能）。联想到加州常用杀虫剂，这下你有点明白了吧。
研究者发现在这个基因的promoter上游存在一个转座子，正是这个转座子，使该基因的表达大为提高（与其它果蝇相比），增加了细胞的解毒能力。所有加州这样的果蝇基因组相对位置上都有这么一个转座子。
所以事情很明显了。这个转座子有可能是偶然插入cyp6g基因上游的。由于它能使该基因表达量提高，从而使果蝇获得了更多对付杀虫剂的能力。在如此之高的选择压力（杀虫）下，这种新的调控得以在种群中传递，最后被固定下来。
这就是转座子对进化所做出的贡献。
顺附论文摘要和网址http://www.pnas.org/cgi/content/abstract/0303793101v1
Strong selective sweep associated with a transposon insertion in Drosophila simulans
Todd A. Schlenke * and David J. Begun
Section of Evolution and Ecology, Division of Biological Sciences, University of California, Davis, CA 95616
Edited by Margaret G. Kidwell, University of Arizona, Tucson, AZ, and approved December 7, 2003 (received for review June 19, 2003)
We know little about several important properties of beneficial mutations, including their mutational origin, their phenotypic effects (e.g., protein structure changes vs. regulatory changes), and the frequency and rapidity with which they become fixed in a population. One signature of the spread of beneficial mutations is the reduction of heterozygosity at linked sites. Here, we present population genetic data from several loci across chromosome arm 2R in Drosophila simulans. A 100-kb segment from a freely recombining region of this chromosome shows extremely reduced heterozygosity in a California population sample, yet typical levels of divergence between species, suggesting that at least one episode of strong directional selection has occurred in the region. The 5' flanking sequence of one gene in this region, Cyp6g1 (a cytochrome P450), is nearly fixed for a Doc transposable element insertion. Presence of the insertion is correlated with increased transcript abundance of Cyp6g1, a phenotype previously shown to be associated with insecticide resistance in Drosophila Drosophila melanogaster. Surveys of nucleotide variation in the same genomic region in an African D. simulans population revealed no evidence for a high-frequency Doc element and no evidence for reduced polymorphism. These data are consistent with the notion that the Doc element is a geographically restricted beneficial mutation. Data from D. simulans Cyp6g1 are paralleled in many respects by data from its sister species D. melanogaster. epigenetics? [标签:content1][标签:content2]
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作者:admin@医学,生命科学 2011-05-28 05:14
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