Department of Biological Sciences, Virginia Tech, Blacksburg, VA, USA.
Department of Biology, University of Virginia, Charlottesville, VA, USA.
Heredity (Edinb). 2020 May;124(5):647-657. doi: 10.1038/s41437-020-0300-5. Epub 2020 Feb 28.
The arms race between tetrodotoxin-bearing Pacific newts (Taricha) and their garter snake predators (Thamnophis) in western North America has become a classic example of coevolution, shedding light on predator-prey dynamics, the molecular basis of adaptation, and patterns of convergent evolution. Newts are defended by tetrodotoxin (TTX), a neurotoxin that binds to voltage-gated sodium channels (Na proteins), arresting electrical activity in nerves and muscles and paralyzing would-be predators. However, populations of the common garter snake (T. sirtalis) have overcome this defense, largely through polymorphism at the locus SCN4A, which renders the encoded protein (Na1.4) less vulnerable to TTX. Previous work suggests that SCN4A commonly shows extreme deviations from Hardy-Weinberg equilibrium (HWE) in these populations, which has been interpreted as the result of intense selection imposed by newts. Here we show that much of this apparent deviation can be attributed to sex linkage of SCN4A. Using genomic data and quantitative PCR, we show that SCN4A is on the Z chromosome in Thamnophis and other advanced snakes. Taking Z-linkage into account, we find that most apparent deviations from HWE can be explained by female hemizygosity rather than low heterozygosity. Sex linkage can affect mutation rates, selection, and drift, and our results suggest that Z-linkage of SCN4A may make significant contributions to the overall dynamics of the coevolutionary arms race between newts and snakes.
西半球的带河豚毒素的太平洋蝾螈（Taricha）与它们的束带蛇捕食者（Thamnophis）之间的“军备竞赛”已经成为协同进化的经典范例，揭示了捕食者-猎物动态、适应的分子基础以及趋同进化的模式。蝾螈受到河豚毒素（TTX）的保护，河豚毒素是一种神经毒素，可与电压门控钠通道（Na 蛋白）结合，阻止神经和肌肉中的电活动，使潜在的捕食者瘫痪。然而，普通束带蛇（T. sirtalis）的种群已经克服了这种防御，这主要是通过 SCN4A 基因座的多态性实现的，这种多态性使编码的蛋白质（Na1.4）对 TTX 的敏感性降低。先前的研究表明，在这些种群中，SCN4A 通常表现出与 Hardy-Weinberg 平衡（HWE）的极端偏离，这被解释为蝾螈施加的强烈选择的结果。在这里，我们表明，这种明显的偏离很大程度上可以归因于 SCN4A 的性连锁。使用基因组数据和定量 PCR，我们表明 SCN4A 在 Thamnophis 和其他高级蛇中位于 Z 染色体上。考虑到 Z 连锁，我们发现大多数明显偏离 HWE 的情况可以用雌性半合子解释，而不是低杂合性。性连锁会影响突变率、选择和漂变，我们的结果表明，SCN4A 的 Z 连锁可能对蝾螈和蛇之间协同进化军备竞赛的整体动态产生重大贡献。
