O'Neill R J, O'Neill M J, Graves J A
Department of Genetics and Human Variation, La Trobe University, Bundoora, Victoria, Australia.
Nature. 1998 May 7;393(6680):68-72. doi: 10.1038/29985.
Genetic models predict that genomic rearrangement in hybrids can facilitate reproductive isolation and the formation of new species by preventing gene flow between the parent species and hybrid (sunflowers are an example). The mechanism underlying hybridization-induced chromosome remodelling is as yet unknown, although mobile element activity has been shown to be involved in DNA rearrangement in some dysgenic Drosophila hybrids. It has been proposed that DNA methylation evolved as a means of repressing the movement of mobile elements (the host defence model). If such a protective mechanism were to fail, mobile elements could be activated, and could cause major and rapid genome alterations. Here we demonstrate the occurrence of genome-wide undermethylation, retroviral element amplification and chromosome remodelling in an interspecific mammalian hybrid (Macropus eugenii x Wallabia bicolor). Atypically extended centromeres of Macropus eugenii derived autosomes in the hybrid were composed primarily of an unmethylated, amplified retroviral element not detectable in either parent species. These results, taken with the observation of deficient methylation and de novo chromosome change in other mammalian hybrids, indicate that the failure of DNA methylation and subsequent mobile-element activity in hybrids could facilitate rapid karyotypic evolution.
遗传模型预测，杂种中的基因组重排可通过阻止亲本物种与杂种之间的基因流动来促进生殖隔离和新物种的形成（向日葵就是一个例子）。尽管已证明转座因子活性参与了某些发育异常的果蝇杂种中的DNA重排，但杂交诱导的染色体重塑的潜在机制尚不清楚。有人提出，DNA甲基化的进化是作为一种抑制转座因子移动的手段（宿主防御模型）。如果这种保护机制失效，转座因子可能会被激活，并可能导致主要且快速的基因组改变。在这里，我们证明了在种间哺乳动物杂种（尤金袋鼠×双色瓦拉比袋鼠）中发生了全基因组低甲基化、逆转录病毒元件扩增和染色体重塑。杂种中源自尤金袋鼠的常染色体的异常延长着丝粒主要由一种未甲基化的、扩增的逆转录病毒元件组成，在两个亲本物种中均未检测到。这些结果，结合在其他哺乳动物杂种中观察到的甲基化缺陷和新生染色体变化，表明杂种中DNA甲基化的失败和随后的转座因子活性可能促进快速的核型进化。
