Department of Biological Sciences, DePaul University, Chicago, IL 60614, USA.
School of Biological Sciences, Area of Ecology and Biodiversity, University of Hong Kong, Hong Kong, SAR 999077, China.
Integr Comp Biol. 2022 Aug 25;62(2):388-405. doi: 10.1093/icb/icac071.
The Threespine Stickleback is ancestrally a marine fish, but many marine populations breed in fresh water (i.e., are anadromous), facilitating their colonization of isolated freshwater habitats a few years after they form. Repeated adaptation to fresh water during at least 10 My and continuing today has led to Threespine Stickleback becoming a premier system to study rapid adaptation. Anadromous and freshwater stickleback breed in sympatry and may hybridize, resulting in introgression of freshwater-adaptive alleles into anadromous populations, where they are maintained at low frequencies as ancient standing genetic variation. Anadromous stickleback have accumulated hundreds of freshwater-adaptive alleles that are disbursed as few loci per marine individual and provide the basis for adaptation when they colonize fresh water. Recent whole-lake experiments in lakes around Cook Inlet, Alaska have revealed how astonishingly rapid and repeatable this process is, with the frequency of 40% of the identified freshwater-adaptive alleles increasing from negligible (∼1%) in the marine founder to ≥50% within ten generations in fresh water, and freshwater phenotypes evolving accordingly. These high rates of genomic and phenotypic evolution imply very intense directional selection on phenotypes of heterozygotes. Sexual recombination rapidly assembles freshwater-adaptive alleles that originated in different founders into multilocus freshwater haplotypes, and regions important for adaptation to freshwater have suppressed recombination that keeps advantageous alleles linked within large haploblocks. These large haploblocks are also older and appear to have accumulated linked advantageous mutations. The contemporary evolution of Threespine Stickleback has provided broadly applicable insights into the mechanisms that facilitate rapid adaptation.
三刺鱼的祖先是海鱼，但许多海洋种群在淡水中繁殖（即溯河洄游），这使得它们在形成后几年就能成功定居于孤立的淡水生境中。至少在 1000 万年的时间里，三刺鱼持续不断地适应淡水环境，导致其成为研究快速适应的主要系统。溯河洄游和淡水三刺鱼在同域繁殖，可能发生杂交，导致淡水适应等位基因逐渐渗入溯河洄游种群，这些等位基因以古老的遗传变异形式维持在低频率。溯河洄游三刺鱼积累了数百个淡水适应等位基因，这些基因在每个海洋个体中分布在少数几个位点上，为它们在定居淡水环境时的适应提供了基础。最近在阿拉斯加库克湾周围的湖泊进行的全湖实验揭示了这个过程是多么惊人地迅速和可重复，40%的已鉴定的淡水适应等位基因的频率从海洋祖先中的微不足道（~1%）增加到淡水环境中的 10 代内≥50%，相应地，淡水表型也随之进化。这些高比例的基因组和表型进化意味着杂合子表型受到非常强烈的定向选择。性重组迅速将源自不同祖先的淡水适应等位基因组装成多基因淡水单倍型，而对适应淡水至关重要的区域抑制了重组，从而使有利等位基因在大单倍型块内保持连锁。这些大的单倍型块也更古老，似乎积累了有利的连锁突变。三刺鱼的当代进化为快速适应的机制提供了广泛适用的见解。
