Dpto de Anatomía Patolóxica e Ciencias Forenses, Grupo de Medicina Xenómica-CIBERER, Universidade de Santiago de Compostela, Rúa de San Francisco s/n, Santiago de Compostela, 15782, Spain.
Genome Biol. 2009 Feb 18;10(2):R22. doi: 10.1186/gb-2009-10-2-r22.
Horizontal transfer (HT) could play an important role in the long-term persistence of transposable elements (TEs) because it provides them with the possibility to avoid the checking effects of host-silencing mechanisms and natural selection, which would eventually drive their elimination from the genome. However, despite the increasing evidence for HT of TEs, its rate of occurrence among the TE pools of model eukaryotic organisms is still unknown.
We have extracted and compared the nucleotide sequences of all potentially functional autonomous TEs present in the genomes of Drosophila melanogaster, D. simulans and D. yakuba - 1,436 insertions classified into 141 distinct families - and show that a large fraction of the families found in two or more species display levels of genetic divergence and within-species diversity that are significantly lower than expected by assuming copy-number equilibrium and vertical transmission, and consistent with a recent origin by HT. Long terminal repeat (LTR) retrotransposons form nearly 90% of the HT cases detected. HT footprints are also frequent among DNA transposons (40% of families compared) but rare among non-LTR retroelements (6%). Our results suggest a genomic rate of 0.04 HT events per family per million years between the three species studied, as well as significant variation between major classes of elements.
The genome-wide patterns of sequence diversity of the active autonomous TEs in the genomes of D. melanogaster, D. simulans and D. yakuba suggest that one-third of the TE families originated by recent HT between these species. This result emphasizes the important role of horizontal transmission in the natural history of Drosophila TEs.
水平转移（HT）可能在转座元件（TEs）的长期存在中发挥重要作用，因为它使它们有可能避免宿主沉默机制和自然选择的检查效应，而这些效应最终会导致它们从基因组中被消除。然而，尽管有越来越多的证据表明 TEs 发生了 HT，但在模式真核生物 TE 库中，其发生频率仍不清楚。
我们已经提取并比较了黑腹果蝇、D. simulans 和 D. yakuba-1 基因组中所有潜在功能自主 TE 的核苷酸序列——141 个不同家族的 1436 个插入序列——并表明，在两个或更多物种中发现的许多家族显示出的遗传分化和种内多样性水平明显低于假设拷贝数平衡和垂直传递时的预期水平，与最近由 HT 引起的起源一致。长末端重复（LTR）反转录转座子构成了检测到的 HT 案例的近 90%。HT 足迹在 DNA 转座子中也很常见（比较的家族占 40%），但在非 LTR 反转座子中很少见（6%）。我们的结果表明，在所研究的三个物种之间，每个家族每百万年发生 0.04 次 HT 事件的基因组率，以及主要元件类之间的显著差异。
黑腹果蝇、D. simulans 和 D. yakuba 基因组中活性自主 TE 的序列多样性的全基因组模式表明，三分之一的 TE 家族是由这些物种之间最近的 HT 产生的。这一结果强调了水平转移在果蝇 TEs 自然史中的重要作用。
