Department of Biology, University of Kentucky, Lexington, KY 40506.
Department of Computational Medicine and Bioinformatics, University of Michigan, Ann Arbor, MI 48109.
Proc Natl Acad Sci U S A. 2021 Apr 27;118(17). doi: 10.1073/pnas.2014719118.
The North American tiger salamander species complex, including its best-known species, the Mexican axolotl, has long been a source of biological fascination. The complex exhibits a wide range of variation in developmental life history strategies, including populations and individuals that undergo metamorphosis; those able to forego metamorphosis and retain a larval, aquatic lifestyle (i.e., paedomorphosis); and those that do both. The evolution of a paedomorphic life history state is thought to lead to increased population genetic differentiation and ultimately reproductive isolation and speciation, but the degree to which it has shaped population- and species-level divergence is poorly understood. Using a large multilocus dataset from hundreds of samples across North America, we identified genetic clusters across the geographic range of the tiger salamander complex. These clusters often contain a mixture of paedomorphic and metamorphic taxa, indicating that geographic isolation has played a larger role in lineage divergence than paedomorphosis in this system. This conclusion is bolstered by geography-informed analyses indicating no effect of life history strategy on population genetic differentiation and by model-based population genetic analyses demonstrating gene flow between adjacent metamorphic and paedomorphic populations. This fine-scale genetic perspective on life history variation establishes a framework for understanding how plasticity, local adaptation, and gene flow contribute to lineage divergence. Many members of the tiger salamander complex are endangered, and the Mexican axolotl is an important model system in regenerative and biomedical research. Our results chart a course for more informed use of these taxa in experimental, ecological, and conservation research.
北美的虎螈物种复合体，包括其最著名的物种——墨西哥蝾螈，长期以来一直是生物学研究的焦点。该复合体在发育生活史策略方面表现出广泛的变化，包括经历变态的种群和个体、能够避免变态并保留幼体、水生生活方式（即幼态持续）的个体以及同时具有这两种特征的个体。幼态持续生活史状态的进化被认为会导致种群遗传分化增加，并最终导致生殖隔离和物种形成，但它对种群和物种水平分化的影响程度还知之甚少。利用来自北美的数百个样本的大量多基因数据集，我们在虎螈复合体的地理范围内确定了遗传聚类。这些聚类通常包含幼态持续和变态类群的混合物，这表明在这个系统中，地理隔离在谱系分化中发挥的作用比幼态持续更大。这一结论得到了以下支持：地理信息分析表明生活史策略对种群遗传分化没有影响，基于模型的种群遗传分析表明相邻的变态和幼态持续种群之间存在基因流。这种对生活史变异的精细遗传视角为理解可塑性、局部适应和基因流如何促进谱系分化提供了框架。虎螈复合体的许多成员都处于濒危状态，而墨西哥蝾螈是再生和生物医学研究中的重要模型系统。我们的研究结果为更明智地利用这些类群进行实验、生态和保护研究提供了指导。
