转座子是一些小片段DNA，它们能在一个细胞的基因组中移动，在这个过程中急剧的增加了基因组的大小。多年以来，研究人员一直认为这种DNA是没有任何作用的“垃圾”，对其知之甚少。然而最新的发现却揭开了转座子那变幻莫测的神奇世界。格鲁吉亚大学的科学家发现基因功能的自然选择在不断的推动着一种LTR逆转座子（LTR-R）的进化。LTR-R是转座子的一个子类，它在植物中数目尤其众多，可通过反转录实现复制（RNA介导复制）。
研究人员分析了水稻LTR-R基因的遗传变异模式，从而研究可能限制核基因组扩增和数目的选择性压力类型。他们发现LTR-R高度纯化选择参与自身复制和生命周期的基因，因而受到严重的进化限制。研究人员还发现，不管LTR-R序列属于哪一家族，它们均表现出相似的“生活史”。
首席研究员Regina Baucom说：“科学家的这些发现有助于解释为什么这些遗传因素能在沉寂了数百万年后还能突然在基因组内扩增，它们不会导致更多的长期伤害，而是占据基因组内的空间。”（生物谷Bioon.com）
生物谷推荐原始出处：
Genome Research Published in Advance November 24, 2008, doi: 10.1101/gr.083360.108 Genome Res. 2009.
Natural selection on gene function drives the evolution of LTR retrotransposon families in the rice genome
Regina S. Baucom13, James C. Estill2, Jim Leebens-Mack2 and Jeffrey L. Bennetzen1
1Department of Genetics, University of Georgia, Athens, Georgia 30602-7223, USA
2Plant Biology Department, University of Georgia, Athens, Georgia 30602-7223, USA
Although the proliferation of LTR retrotransposons can cause major genomic modification and reorganization, the evolutionary dynamics that affect their frequency in host genomes are poorly understood. We analyzed patterns of genetic variation among LTR retrotransposons from Oryza sativa to investigate the type of selective forces that potentially limit their amplification and subsequent population of a nuclear genome. We performed both intra- and interfamily analyses of patterns of molecular sequence variation across multiple LTR retrotransposon genes. This analysis involved more than 1000 LTR retrotransposon sequences from 14 separate families that varied in both their insertion dates and full-length copy numbers. We uncovered evidence of strong purifying selection across all gene regions, but also indications that rare episodes of positive selection and adaptation to the host genome occur. Furthermore, our results indicate that LTR retrotransposons exhibit different but predictable patterns of sequence variation depending on their date of transposition, suggesting that LTR retrotransposons, regardless of superfamily and family classifications, show similar “life-histories.”
