Gene expression has a key role in reproductive isolation, and studies of hybrid gene expression have identified mechanisms causing hybrid sterility. Here, we review the evidence for altered gene expression following hybridization and outline the mechanisms shown to contribute to altered gene expression in hybrids. Transgressive gene expression, transcending that of both parental species, is pervasive in early generation sterile hybrids, but also frequently observed in viable, fertile hybrids. We highlight studies showing that hybridization can result in transgressive gene expression, also in established hybrid lineages or species. Such extreme patterns of gene expression in stabilized hybrid taxa suggest that altered hybrid gene expression may result in hybridization‐derived evolutionary novelty. We also conclude that while patterns of misexpression in hybrids are well documented, the understanding of the mechanisms causing misexpression is lagging. We argue that jointly assessing differences in cell composition and cell‐specific changes in gene expression in hybrids, in addition to assessing changes in chromatin and methylation, will significantly advance our understanding of the basis of altered gene expression. Moreover, uncovering to what extent evolution of gene expression results in altered expression for individual genes, or entire networks of genes, will advance our understanding of how selection moulds gene expression. Finally, we argue that jointly studying the dual roles of altered hybrid gene expression, serving both as a mechanism for reproductive isolation and as a substrate for hybrid ecological adaptation, will lead to significant advances in our understanding of the evolution of gene expression.

中文翻译：

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杂交和基因表达：超越差异表达基因

基因表达在生殖隔离中起着关键作用，对杂种基因表达的研究已经确定了导致杂种不育的机制。在这里，我们回顾了杂交后基因表达改变的证据，并概述了导致杂交种基因表达改变的机制。超越两个亲本物种的越界基因表达在早期不育杂交种中普遍存在，但在有活力的可育杂交种中也经常观察到。我们重点研究表明杂交可能导致基因表达越界，在已建立的杂交谱系或物种中也是如此。稳定的杂交类群中基因表达的这种极端模式表明，改变的杂交基因表达可能会导致杂交衍生的进化新颖性。我们还得出结论，虽然杂种中的错误表达模式有充分的记录，但对导致错误表达的机制的理解却是滞后的。我们认为，除了评估染色质和甲基化的变化之外，联合评估杂交体中细胞组成的差异和基因表达的细胞特异性变化，将显着增进我们对基因表达改变基础的理解。此外，揭示基因表达的进化在多大程度上导致单个基因或整个基因网络的表达改变，将增进我们对选择如何塑造基因表达的理解。最后，我们认为，共同研究改变的杂种基因表达的双重作用，既作为生殖隔离的机制，又作为杂种生态适应的基础，将导致我们对基因表达进化的理解取得重大进展。