Connecting the selective forces that drive the evolution of phenotypes to their underlying genotypes is key to understanding adaptation, but such connections are rarely tested experimentally. Threespine stickleback (Gasterosteus aculeatus) are a powerful model for such tests because genotypes that underlie putatively adaptive traits have been identified. For example, a regulatory mutation in the Ectodysplasin (Eda) gene causes a reduction in the number of bony armor plates, which occurs rapidly and repeatedly when marine sticklebacks invade freshwater. However, the source of selection on plate loss in freshwater is unknown. Here, we tested whether dietary reduction of phosphorus can account for selection on plate loss due to a growth advantage of low‐plated fish in freshwater. We crossed marine fish heterozygous for the 16 kilobase freshwater Eda haplotype and compared the growth of offspring with different genotypes under contrasting levels of dietary phosphorus in both saltwater and freshwater. Eda genotype was not associated with growth differences in any treatment, or with mechanisms that could mitigate the impacts of phosphorus limitation, such as differential phosphorus deposition, phosphorus excretion, or intestine length. This study highlights the importance of experimentally testing the putative selective forces acting on phenotypes and their underlying genotypes in the wild.

中文翻译：

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磷限制不会导致淡水刺鱼（Gasterosteus aculeatus）骨侧板的损失


将驱动表型进化的选择力与其潜在的基因型联系起来是理解适应的关键，但这种联系很少经过实验测试。三刺刺鱼（Gasterosteus aculeatus）是此类测试的强大模型，因为已经确定了假定的适应性特征背后的基因型。例如，外生蛋白（Eda）基因的调节突变会导致骨装甲板数量减少，当海洋刺鱼入侵淡水时，这种情况会迅速且反复地发生。然而，淡水中板损失的选择来源尚不清楚。在这里，我们测试了膳食中磷的减少是否可以解释由于淡水中低板鱼的生长优势而造成的板损失的选择。我们对 16 kilobase 淡水 Eda 单倍型的杂合海水鱼进行了杂交，并在咸水和淡水中膳食磷水平对比的情况下比较了不同基因型的后代的生长情况。 Eda 基因型与任何处理中的生长差异无关，也与可以减轻磷限制影响的机制（例如差异磷沉积、磷排泄或肠道长度）无关。这项研究强调了通过实验测试作用于野生表型及其潜在基因型的推定选择力的重要性。