Cardazzo Barbara, Bargelloni Luca, Toffolatti Luisa, Patarnello Tomaso
Dipartimento di Biologia, Campus di Agripolis, Università di Podova, Podova, Italy.
Mol Biol Evol. 2003 Dec;20(12):2034-41. doi: 10.1093/molbev/msg213. Epub 2003 Aug 29.
The enlargement of the genome size and the decrease in genome compactness with increase in the number and size of introns is a general pattern during the evolution of eukaryotes. Among the possible mechanisms for modifying intron size, it has been suggested that the insertion of transposable elements might have an important role in driving intron evolution. The analysis of large portions of the human genome demonstrated that a relatively recent (50 to 100 MYA) accumulation of transposable elements appears to be biased, favoring a preferential insertion of LINE1 transposons into sex chromosomes rather than into autosomes. In the present work, the effect of chromosomal location on the increase in size of introns was evaluated with a comparative analysis performed on pairs of human paralogous genes, one located on the X chromosome and the second on an autosome. A phylogenetic analysis was also performed on the X-encoded proteins and their paralogs to confirm orthology-paralogy and to approximately estimate the time of gene duplication. Statistical analysis of total intron length for each pair of paralogous genes provided no evidence for a larger size of introns in the gene copies located on the X chromosome. On the opposite, introns of autosomal genes were found to be significantly longer than introns of their X-linked paralogs. Likewise, LINE1 elements were not significantly more frequent in X-chromosome introns, whereas the frequency of SINE elements showed a marginally significant bias toward autosomal introns.
随着内含子数量和大小的增加，基因组大小的增大以及基因组紧凑性的降低是真核生物进化过程中的一种普遍模式。在改变内含子大小的可能机制中，有人提出转座元件的插入可能在驱动内含子进化中起重要作用。对人类基因组大部分区域的分析表明，转座元件相对较新（50至1亿年前）的积累似乎存在偏差，有利于LINE1转座子优先插入性染色体而非常染色体。在本研究中，通过对一对人类旁系同源基因进行比较分析来评估染色体位置对内含子大小增加的影响，其中一个基因位于X染色体上，另一个位于常染色体上。还对X染色体编码的蛋白质及其旁系同源物进行了系统发育分析，以确认直系同源 - 旁系同源关系并大致估计基因复制的时间。对每对旁系同源基因的总内含子长度进行统计分析，没有证据表明位于X染色体上的基因拷贝中的内含子更大。相反，发现常染色体基因的内含子明显长于其X连锁旁系同源物的内含子。同样，LINE1元件在X染色体内含子中并不明显更频繁，而SINE元件的频率显示出对常染色体内含子的轻微显著偏向。
