Evidence for adaptive evolution in a stress inducible gene of tomato (genus Lycopersicon)

FRANKEL N; HASSON E; ROSSI MS; IUSEM N

Villa Carlos Paz, Córdoba, Argentina

Congreso; XXXVII Reunión anual de la Sociedad Argentina de Investigaciones en Bioquímica y Biología Molecular (SAIB); 2001

The Asr2 gene encodes a highly hydrophilic protein expressed upon abscicic acid treatment, water stress and fruit ripening. Asr2 is a member of a gene family with three known members up to date. It was first cloned and characterized in cultivated tomato (L. esculentum). In this work, we obtained the complete coding sequence of the orthologous Asr2 genes in three tomato wild species: L. hirsutum, L. peruvianum and L. chilense. With these four sequences, we computed the Ka/Ks ratio (nonsynonymous to synonymous substitutions) for the six possible pairwise comparisons within the genus Lycopersicon. We have found Ka/Ks values larger than 1, suggesting that positive selection is shaping the evolution of this locus. To test if these values are unusual for this locus we calculated the Ka/Ks ratio between tomato sequences and the orthologous gene ci21b in potato (Solanum tuberosum). Ka/Ks within the genus Lycopersicon is on average 12 times greater than that found between the tomato sequences and potato ci21b. Another piece of evidence for positive selection is an aminoacidic substitution (leucine instead of proline) found only in L. chilense, a species that grows in the Antofagasta desert, an extremely dry environment. This aminoacidic residue is conserved among all known Asr-like proteins, from Gymnosperms to Angiosperms