Department of Biology, University of Konstanz, Universitätsstraße 10, 78457 Konstanz, Germany.
Philos Trans R Soc Lond B Biol Sci. 2020 Aug 31;375(1806):20190535. doi: 10.1098/rstb.2019.0535. Epub 2020 Jul 13.
Cichlid fishes are exceptionally species-rich, speciated at explosive rates and, hence, are a model system in speciation research. Yet, their reproductive isolating barriers have, so far, not been comprehensively studied. Here, we review current knowledge on pre- and postzygotic mechanisms in cichlids. While premating isolation is the norm in cichlids, its strength varies across lineages and with the geographical setting. Moreover, manipulations of ambient conditions tended to reduce assortative mating among closely related species, suggesting that premating isolation in cichlids is often fragile and context dependent. The observed lack of complete reproductive isolation is supported by past and present hybridization events that have contributed to diversity by creating novel allelic combinations. On the other hand, our meta-analysis highlights that intrinsic postzygotic isolation might accumulate faster than assumed. Mild forms of genetic incompatibilities, such as sex ratio distortion, can already be observed among closely related species. Therefore, cessation of gene flow by strong reproductive isolation in cichlids requires a combination of premating prezygotic isolation supplemented with intrinsic and extrinsic postzygotic barriers. Further, we suggest crucial next steps to improve our knowledge about reproductive barriers in cichlids to understand the evolutionary dynamics of pre- and postzygotic isolation mechanisms during adaptive radiations. This article is part of the theme issue 'Towards the completion of speciation: the evolution of reproductive isolation beyond the first barriers'.
慈鲷鱼是物种非常丰富的鱼类，以爆炸式的速度形成新物种，因此是物种形成研究的模式系统。然而，它们的生殖隔离障碍迄今为止尚未得到全面研究。在这里，我们回顾了慈鲷鱼中前合子和后合子机制的现有知识。虽然交配前隔离是慈鲷鱼的常态，但它的强度在不同的谱系和地理环境中有所不同。此外，对环境条件的操纵往往会减少密切相关物种的交配选择，这表明慈鲷鱼的交配前隔离通常是脆弱的，并且依赖于环境。过去和现在的杂交事件支持了缺乏完全生殖隔离的观点，这些事件通过创造新的等位基因组合为多样性做出了贡献。另一方面，我们的荟萃分析强调，内在的合子后隔离可能比预期的积累得更快。在密切相关的物种中，已经可以观察到轻微形式的遗传不相容性，例如性别比例扭曲。因此，慈鲷鱼中通过强烈的生殖隔离停止基因流需要前合子前合子隔离与内在和外在的合子后隔离相结合。此外，我们建议采取关键的下一步措施，以提高我们对慈鲷鱼生殖障碍的认识，从而了解适应性辐射过程中前合子和合子后隔离机制的进化动态。本文是主题为“迈向生殖隔离的完成：适应辐射中前合子和合子后隔离机制的进化”的一部分。
