Adaptive evolution of the water stress-induced gene Asr2 in Lycopersicon species dwelling in arid habitats

The Asr2 gene encodes a putative transcription factor that is up-regulated in leaves and roots of tomato plants exposed to water-deficit stress. This gene was first cloned and characterized in a cultivar of commercial tomato (Lycopersicon esculentum cv. Ailsa Craig). In this work, we report the complete coding sequences of the orthologous Asr2 genes in six wild tomato lineages: L. hirsutum, L. cheesmanii, L. esculentum v. cerasiforme, L. chilense, L. peruvianum v. humifusum and L. peruvianum f. glandulosum. Estimates of the K-a/K-s ratio (omega) in pairwise comparisons within the genus Lycopersicon were equal or greater than I (a signature of adaptive evolution) when involving L. chilense and L, peruvianum v. humifusum. Interestingly, these two species are distinct from the others in their adaptation to dry habitats. We also mapped the detected substitutions onto a phylogenetic tree of the genus Lycopersicon. Remarkably, there are two and three amino acid substitutions, which contrast with the absence of synonymous substitutions along the terminal branches leading to L. chilense and L. peruvianum v. humifusum, respectively. Likelihood ratio tests confirmed that omega values in the branches leading to these species are significantly different from the remaining branches of the tree. Moreover, inferred changes in the branches leading to these species that inhabit dry areas are nonconservative and may be associated with dramatic alterations in ASR2 protein conformation. In this work, we demonstrate accelerated rates of amino acid substitutions in the Asr2 gene of tomato lineages living in dry habitats, thus giving support to the hypothesis of adaptive Darwinian evolution.