Urban Sabine, Nater Alexander, Meyer Axel, Kratochwil Claudius F
Department of Biology, University of Konstanz, Konstanz, Germany.
Mol Biol Evol. 2021 Jan 23;38(2):465-477. doi: 10.1093/molbev/msaa237.
The adaptive radiations of East African cichlid fish in the Great Lakes Victoria, Malawi, and Tanganyika are well known for their diversity and repeatedly evolved phenotypes. Convergent evolution of melanic horizontal stripes has been linked to a single locus harboring the gene agouti-related peptide 2 (agrp2). However, where and when the causal variants underlying this trait evolved and how they drove phenotypic divergence remained unknown. To test the alternative hypotheses of standing genetic variation versus de novo mutations (independently originating in each radiation), we searched for shared signals of genomic divergence at the agrp2 locus. Although we discovered similar signatures of differentiation at the locus level, the haplotypes associated with stripe patterns are surprisingly different. In Lake Malawi, the highest associated alleles are located within and close to the 5' untranslated region of agrp2 and likely evolved through recent de novo mutations. In the younger Lake Victoria radiation, stripes are associated with two intronic regions overlapping with a previously reported cis-regulatory interval. The origin of these segregating haplotypes predates the Lake Victoria radiation because they are also found in more basal riverine and Lake Kivu species. This suggests that both segregating haplotypes were present as standing genetic variation at the onset of the Lake Victoria adaptive radiation with its more than 500 species and drove phenotypic divergence within the species flock. Therefore, both new (Lake Malawi) and ancient (Lake Victoria) allelic variation at the same locus fueled rapid and convergent phenotypic evolution.
东非维多利亚湖、马拉维湖和坦噶尼喀湖中的丽鱼科鱼类的适应性辐射以其多样性和反复进化的表型而闻名。黑色水平条纹的趋同进化与一个携带刺鼠相关肽2（agrp2）基因的位点有关。然而，该性状潜在的因果变异在何处及何时进化，以及它们如何推动表型分化仍不清楚。为了检验现存遗传变异与从头突变（在每个辐射中独立起源）的替代假说，我们在agrp2位点寻找基因组分化的共享信号。尽管我们在基因座水平发现了相似的分化特征，但与条纹模式相关的单倍型却惊人地不同。在马拉维湖，最高相关等位基因位于agrp2的5'非翻译区内及附近，可能是通过近期的从头突变进化而来。在较年轻的维多利亚湖辐射中，条纹与两个内含子区域相关，这两个区域与先前报道的顺式调控区间重叠。这些分离单倍型的起源早于维多利亚湖辐射，因为它们也存在于更原始的河流和基伍湖物种中。这表明，在拥有500多个物种的维多利亚湖适应性辐射开始时，这两种分离单倍型均作为现存遗传变异存在，并推动了物种群内的表型分化。因此，同一基因座上新的（马拉维湖）和古老的（维多利亚湖）等位基因变异推动了快速且趋同的表型进化。
