Plants adapt to their local environment through complex interactions between genes, gene networks, and hormones. Although the impact of gene expression on trait regulation and evolution has been recognized for many decades, its role in the evolution of adaptation is still a subject of intense exploration. We used a Multi-parent Advanced Generation Inter-Cross (MAGIC) population, which we derived from crossing multiple parents from two distinct coastal ecotypes of an Australia wildflower, Senecio lautus. We focused on studying the contrasting gravitropic behaviors of these ecotypes, which have evolved independently multiple times and show strong responses to natural selection in field experiments, emphasizing the role of natural selection in their evolution. Here, we investigated how gene expression differences have contributed to the adaptive evolution of gravitropism. We studied gene expression in 600 pools at five time points (30, 60, 120, 240, and 480 minutes) after rotating half of the pools 90°. We found 428 genes with differential expression in response to the 90° rotation treatment. Of these, 81 genes (~19%) have predicted functions related to the plant hormones auxin and ethylene, which are crucial for the gravitropic response. By combining insights from Arabidopsis mutant studies and analyzing our gene networks, we propose a preliminary model to explain the differences in gravitropism between ecotypes. This model suggests that the differences arise from changes in the transport and availability of the hormones auxin and ethylene. Our findings indicate that the genetic basis of adaptation involves interconnected signaling pathways that work together to give rise to new ecotypes.

Unravelling the interplay among genes, networks, and signalling molecules is key to understanding how many natural populations adapt. Although the impact of gene expression on trait regulation and evolution has been recognised for many decades, its role in the evolution of adaptations is still a subject of intense exploration. Using a hybrid population derived from two contrasting ecotypes of an Australian wildflower, Senecio lautus, we investigated the role of gene expression divergence in their origins. Coastal ecotypes of S. lautus have contrasting vegetative heights and gravitropic behaviours that evolved independently many times, highlighting the role of natural selection in their evolution. We examined gene expression in 10 gravitropic and 10 agravitropic hybrid families from the hybrid population of Senecio at Lennox Head, NSW. We found 428 genes that showed differential expression between the gravitropic control and treatment groups when we rotated the hybrids 90 degrees. Of these, 81 genes (~19%) had predicted functions linked to several plant hormones. Using knowledge from Arabidopsis mutant screens and assessing our gene networks, we construct a model for differences in gravitropism between ecotypes that relies on modulating the movement and accessibility of the hormone auxin, known to control the gravitropic response across plants. Our findings suggest a role for the hormonal control of gravitropism in plant adaptation to coastal environments, where ecotypes are known to differ from their counterparts in other habitats. More generally, we posit that the genetics of adaptation encompasses the evolution of intertwined signalling pathways that ultimately contribute to the origin of new ecotypes.