Responses to simulated winter conditions differ between threespine stickleback ecotypes

Abiotic factors can act as barriers to colonization and drive local adaptation. During colonization, organisms may cope with changes in abiotic factors using existing phenotypic plasticity, but the role of phenotypic plasticity in assisting or hindering the process of local adaptation remains unclear. To address these questions, we explore the role of winter conditions in driving divergence during freshwater colonization and the effects of plasticity on local adaptation in ancestral marine and derived freshwater ecotypes of threespine stickleback (Gasterosteus aculeatus). We found that freshwater-resident stickleback had greater tolerance of acute exposure to low temperatures than marine stickleback, but these differences were abolished after acclimation to simulated winter conditions (9L:15D photoperiod at 4°C). Plasma chloride levels differed between the ecotypes, but showed a similar degree of plasticity between ecotypes. Gene expression of the epithelial calcium channel (ECaC) differed between ecotypes, with the freshwater ecotype demonstrating substantially greater expression than the marine ecotype, but there was no plasticity in this trait under these conditions in either ecotype. In contrast, growth (assessed as final mass) and the expression of an isoform of the electroneutral Na+/H+ exchanger (NHE3) exhibited substantial change with temperature in the marine ecotype that was not observed in the freshwater ecotype under the conditions tested here, which is consistent with evolution of these traits by a process such as genetic assimilation. These data demonstrate substantial divergence in many of these traits between freshwater and marine stickleback, but also illustrate the complexity of possible relationships between plasticity and local adaptation.

非生物因子（abiotic factors）可成为物种定殖的障碍，并驱动局部适应。在定殖过程中，生物可借助已有的表型可塑性应对非生物因子的变化，但表型可塑性在辅助或阻碍局部适应进程中所扮演的角色仍未明确。为解答上述问题，我们探究了冬季环境在淡水定殖过程中驱动性状分化的作用，以及表型可塑性对三棘刺鱼（Gasterosteus aculeatus）祖先海洋种群与衍生淡水生态型局部适应的影响。研究发现，相较于海洋刺鱼，淡水定居型刺鱼对低温急性暴露的耐受性更强，但在模拟冬季环境（4℃下光周期为9L:15D）驯化后，该差异不复存在。血浆氯离子水平在两种生态型间存在显著差异，但二者的表型可塑性程度相近。上皮钙通道（epithelial calcium channel, ECaC）的基因表达在两种生态型间存在分化：淡水生态型的基因表达量显著高于海洋生态型，但在本实验设置的条件下，两种生态型的该性状均未表现出可塑性。与之相反，在本次测试的条件下，海洋生态型的生长情况（以最终体重为衡量指标）及电中性钠氢交换体（electroneutral Na+/H+ exchanger, NHE3）的同工型基因表达随温度发生了显著变化，而淡水生态型则未出现此类变化，这与遗传同化等过程驱动相关性状演化的结论相符。本研究数据表明，淡水与海洋刺鱼的诸多性状均存在显著分化，同时也揭示了可塑性与局部适应间潜在关联的复杂性。