The adaptive cline at Ldh (lactate dehydrogenase) in killifish Fundulus heteroclitus remains stationary after 40 years of warming estuaries

Since the 1970s, water temperatures along the Atlantic seaboard of the United States have risen by an average of 0.5ºC in summer months and 2.2ºC in winter months. In response, the distribution and abundance of several nearshore species have changed dramatically, but no study has attempted to document whether estuarine populations have evolved greater thermal tolerance. Here, we re-examine the classic latitudinal cline at lactate dehydrogenase in the killifish Fundulus heteroclitus that was originally described by Dennis Powers and associates from samples collected between 1970-72. Laboratory and field evidence indicated that northern and southern isozymes at muscle lactate dehydrogenase (LDH) are locally adapted to cold and warm temperatures, respectively. Despite the potential for evolutionary response at this adaptive locus, we detected no significant shift of the LDH cline between the early 1970s and 2010. We conclude that microevolution of LDH-mediated thermal tolerance has not occurred, shifts in alleles are too incremental to be distinguished from random processes, or that F. heteroclitus uses phenotypic and genetic mechanisms besides LDH to respond to warmer waters.

自20世纪70年代以来，美国大西洋沿岸海域夏季表层水温平均升高0.5℃，冬季升高2.2℃。受此影响，多种近岸物种的分布范围与种群丰度均发生了显著变化，但目前尚无研究探讨河口种群是否已演化出更强的耐热性。本研究重新分析了由丹尼斯·鲍尔斯（Dennis Powers）及其团队于1970至1972年采集样本所首次确立的经典纬度渐变群——斑纹底鳉（Fundulus heteroclitus）肌肉乳酸脱氢酶（lactate dehydrogenase, LDH）的分布特征。实验室与野外观测证据显示，该酶的北方与南方同工酶分别适应当地的低温与高温环境。尽管该适应性位点存在演化响应的潜在可能，但我们并未检测到1970年代早期至2010年间LDH渐变群发生显著偏移。据此我们得出三种可能的结论：一是LDH介导的耐热性未发生微进化；二是等位基因频率的变化过于平缓，无法与随机过程区分开来；三是斑纹底鳉通过LDH以外的表型与遗传机制来应对水温升高。