Natural selection is a significant driver of speciation. Yet it remains largely unknown whether local adaptation can drive speciation through the evolution of hybrid sterility between populations. Here, we show that adaptive divergence in shoot gravitropism, the ability of a plant’s shoot to bend upwards in response to the downward pull of gravity, contributes to the evolution of hybrid sterility in an Australian wildflower, Senecio lautus. We find that shoot gravitropism has evolved multiple times in association with plant height between adjacent populations inhabiting contrasting environments, suggesting that these traits have evolved by natural selection. We directly tested this prediction using a hybrid population subjected to eight rounds of recombination and three rounds of selection in the field. It revealed that shoot gravitropism responds to natural selection in the expected direction of the locally adapted population. This provided an ideal platform to test whether genetic differences in gravitropism contribute to hybrid sterility in S. lautus. Using this advanced hybrid population, we discovered that crossing individuals with extreme differences in gravitropism reduce their ability to produce seed by 21%, providing strong evidence that this adaptive trait is genetically correlated with hybrid sterility. Our results suggest that natural selection can drive the evolution of locally adaptive traits that also create hybrid sterility, thus indicating an evolutionary connection between local adaptation and the origin of new species.

中文翻译：

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地心引力的适应差异在澳大利亚野花中产生了杂交不育

自然选择是物种形成的重要驱动力。然而，在当地进行适应能否通过种群间杂种不育的发展来驱动物种形成仍是一个未知数。在这里，我们表明，地心引力的适应性发散（一种植物的新梢响应重力的向下拉动而向上弯曲的能力）有助于澳大利亚野花千里光（Senecio lautus）杂种不育的进化。。我们发现，地心引力与居住在相反环境中的相邻种群之间的株高相关联地进化了许多次，表明这些性状是通过自然选择而进化的。我们使用杂种种群在野外进行了八轮重组和三轮选择，直接检验了这一预测。结果表明，地心引力在本地适应种群的预期方向上对自然选择有反应。这提供了一个理想的平台，以测试引力性的遗传差异是否有助于S. lautus的杂交不育。。使用这个先进的杂交种群，我们发现在重力趋向方面具有极大差异的杂交个体将其产生种子的能力降低了21％，提供了有力的证据表明这种适应性状与杂交不育遗传相关。我们的研究结果表明，自然选择可以驱动局部适应性状的进化，这也造成了杂交不育，从而表明了局部适应性与新物种起源之间的进化联系。