Adaptive divergence in shoot gravitropism creates intrinsic reproductive isolation in an Australian wildflower
AbstractAdaptation to the local environment is a major driver of speciation. Yet, it remains largely unknown whether natural selection directly causes intrinsic reproductive isolation (hybrid sterility or inviability) between locally adapted 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 intrinsic reproductive isolation 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 in multi-year reciprocal transplant experiments using hybrid populations. We show that shoot gravitropism and plant height respond to natural selection in the expected direction of the locally adapted population. Remarkably, we find that crossing F11 hybrid individuals with extreme differences in gravitropism significantly reduces their ability to produce seeds, providing strong evidence that this adaptive trait is genetically correlated with intrinsic reproductive isolation. Our results suggest that natural selection can drive the evolution of locally adaptive traits that incidentally create intrinsic reproductive isolation, thus increasing our understanding of the origin and maintenance of new species.