Department of Ecology and Evolution, University of Lausanne, Lausanne, Switzerland.
Department of Biology, University of Fribourg, Fribourg, Switzerland.
J Evol Biol. 2018 Sep;31(9):1354-1364. doi: 10.1111/jeb.13310. Epub 2018 Jun 28.
Chromosomal inversions often contribute to local adaptation across latitudinal clines, but the underlying selective mechanisms remain poorly understood. We and others have previously shown that a clinal inversion polymorphism in Drosophila melanogaster, In(3R)Payne, underpins body size clines along the North American and Australian east coasts. Here, we ask whether this polymorphism also contributes to clinal variation in other fitness-related traits, namely survival traits (lifespan, survival upon starvation and survival upon cold shock). We generated homokaryon lines, either carrying the inverted or standard chromosomal arrangement, isolated from populations approximating the endpoints of the North American cline (Florida, Maine) and phenotyped the flies at two growth temperatures (18 °C, 25 °C). Across both temperatures, high-latitude flies from Maine lived longer and were more stress resistant than low-latitude flies from Florida, as previously observed. Interestingly, we find that this latitudinal pattern is partly explained by the clinal distribution of the In(3R)P polymorphism, which is at ~ 50% frequency in Florida but absent in Maine: inverted karyotypes tended to be shorter-lived and less stress resistant than uninverted karyotypes. We also detected an interaction between karyotype and temperature on survival traits. As In(3R)P influences body size and multiple survival traits, it can be viewed as a 'supergene', a cluster of tightly linked loci affecting multiple complex phenotypes. We conjecture that the inversion cline is maintained by fitness trade-offs and balancing selection across geography; elucidating the mechanisms whereby this inversion affects alternative, locally adapted phenotypes across the cline is an important task for future work.
染色体倒位常常有助于跨纬度梯度的局部适应，但潜在的选择机制仍知之甚少。我们和其他人之前已经表明，黑腹果蝇中的一个沿北美和澳大利亚东海岸的体型梯度存在的倒位多态性，In(3R)Payne，是体型梯度的基础。在这里，我们想知道这种多态性是否也有助于其他与适应度相关的性状的梯度变化，即生存性状（寿命、饥饿后生存和冷休克后生存）。我们生成了同核系，要么携带倒位的染色体排列，要么携带标准的染色体排列，这些同核系是从接近北美梯度的种群中分离出来的（佛罗里达州、缅因州），并在两个生长温度下对这些果蝇进行了表型分析（18°C、25°C）。在这两个温度下，来自缅因州的高纬度果蝇比来自佛罗里达州的低纬度果蝇寿命更长，对压力的抵抗力更强，这与之前的观察结果一致。有趣的是，我们发现这种纬度模式部分是由 In(3R)P 多态性的梯度分布所解释的，该多态性在佛罗里达州的频率约为 50%，而在缅因州则不存在：倒位核型的寿命往往比未倒位核型的寿命更短，对压力的抵抗力也更弱。我们还检测到了生存性状与核型和温度之间的相互作用。由于 In(3R)P 影响体型和多种生存性状，因此它可以被视为一个“超级基因”，即一组紧密连锁的基因座，影响多种复杂表型。我们推测，这种倒位梯度是由地理上的适应度权衡和平衡选择维持的；阐明这种倒位如何影响梯度上的替代、适应的表型，是未来工作的一个重要任务。
