Adaptive Evolution of the Water Stress-Induced Gene Asr2 in Lycopersicon Species Dwelling in Arid Habitats

NICOLÁS FRANKEL; ESTEBAN HASSON; NORBERTO D. IUSEM; MARÍA SUSANA ROSSI

MOLECULAR BIOLOGY AND EVOLUTION

OXFORD UNIV PRESS

Año: 2003 vol. 20 p. 1955 - 1962

0737-4038

The Asr2 gene encodes a putative transcription factor that is up-regulated in leaves and roots of tomato plants exposed towater-deficit stress. This gene was first cloned and characterized in a cultivar of commercial tomato (Lycopersiconesculentum cv. Ailsa Craig). In this work, we report the complete coding sequences of the orthologous Asr2 genes in sixwild tomato lineages: L. hirsutum, L. cheesmanii, L. esculentum v. cerasiforme, L. chilense, L. peruvianum v. humifusumand L. peruvianum f. glandulosum. Estimates of the Ka/Ks ratio (x) in pairwise comparisons within the genusLycopersicon were equal or greater than 1 (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 twoand three amino acid substitutions, which contrast with the absence of synonymous substitutions along the terminalbranches leading to L. chilense and L. peruvianum v. humifusum, respectively. Likelihood ratio tests confirmed that xvalues 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 beassociated with dramatic alterations in ASR2 protein conformation. In this work, we demonstrate accelerated rates ofamino acid substitutions in the Asr2 gene of tomato lineages living in dry habitats, thus giving support to the hypothesis ofadaptive Darwinian evolution.