Grup de Genòmica, Bioinformàtica i Biologia Evolutiva, Departament de Genètica i Microbiologia, Universitat Autònoma de Barcelona, Cerdanyola del Vallès, Bellaterra, Spain.
J Hered. 2019 Jan 7;110(1):124-136. doi: 10.1093/jhered/esy040.
Contrary to the view that hybrids are lineages devoid of evolutionary value, a number of case studies that have been lately reported show how hybrids are at the origin of many species. Some well-documented cases demonstrate that bursts of transposition often follow hybridization, generating new genetic variability. Studies in hybrid transposition strongly suggest that epigenetic changes and divergence in piRNA pathways drive deregulation in TE landscapes. Here, I have focused on mechanisms acting in Drosophila hybrids between two cactophilic species. The results reported here show that while hybrid instability by transposition is a genome-wide event, deregulation by TE overexpression in hybrid ovaries is not a general rule. When piRNA pools of ovaries are studied, results show that TEs with parental differences higher than 2-fold in their piRNA amounts are not more commonly deregulated in hybrids than TEs with similar levels, partially discrediting the generality of the maternal cytotype hypothesis. Some promising results on the piRNA pathway global failure hypothesis, which states that accumulated divergence of piRNA effector proteins is responsible for hybrid TE deregulation, have also been obtained. Altogether, these results suggest that TE deregulation might be driven by several interacting mechanisms. A natural scenario is proposed in which genome instability by transposition leads to hybrid genome reorganization. Small hybrid populations, subjected to natural selection helped by genetic drift, evolve new adaptations adapted to novel environments. The final step is either introgression or even a new hybrid species.
与杂种没有进化价值的观点相反，最近报道的一些案例研究表明，杂种是许多物种的起源。一些有充分文献记录的案例表明，转座子的爆发常常伴随着杂交，产生新的遗传可变性。杂种转座的研究强烈表明，表观遗传变化和 piRNA 途径的分歧导致 TE 景观的失调。在这里，我专注于在两个嗜热仙人掌物种之间的果蝇杂种中起作用的机制。这里报道的结果表明，虽然杂种通过转座的不稳定性是一个全基因组事件，但杂种卵巢中 TE 的过度表达导致的失调并不是普遍规律。当研究卵巢中的 piRNA 池时，结果表明，与具有相似水平的 TE 相比，piRNA 数量差异超过 2 倍的 TE 在杂种中并没有更经常地失调，部分否定了母体细胞型假说的普遍性。关于 piRNA 通路全局失效假说的一些有希望的结果也已获得，该假说指出，piRNA 效应蛋白的累积分歧是杂种 TE 失调的原因。总之，这些结果表明，TE 的失调可能是由几种相互作用的机制驱动的。提出了一个自然场景，即转座导致杂种基因组重组的基因组不稳定性。小的杂种群体，在遗传漂变帮助下的自然选择下，进化出适应新环境的新适应性。最后一步是种间渗入，甚至是一个新的杂种物种。
