Evolutionary analysis of S-RNase genes from Rosaceae species

Eight new cDNA sequences for S-RNases were cloned and analysed from almond (Prunus dulcis) cultivars of European origin, and compared to published sequences from other Rosaceae species. Insertions/deletions of 10-20 amino acid residues were detected in the RC4 and C5 domains of S-RNases from almond and sweet cherry. The S-RNases of the Prunus species and those of the genera Malus and Pyrus formed two distinct groups on phylogenetic analysis. Nucleotide substitutions were analysed in the S-RNase genes of these species. The S-genes of almond and sweet cherry have a lower Ka/Ks value than those of apple, pear and wild apple do. The fact that there is no fixed difference between the S-RNase genes of almond and sweet cherry, or between apple and pear, suggests that nucleotide substitutions only introduce transient polymorphism into the two groups, and rarely became fixed and contribute to divergence. Through the comparative study of 17 S-RNase genes from the genus Prunus and 18 from the genera Malus and Pyrus, some fixed nucleotide differences between the two groups were identified. These differences do not appear to be the result of selection for adaptive mutations, since the number of replacement substitutions is not significantly greater than the number of synonymous substitutions. S-RNase genes of almond and sweet cherry, and of apple and pear, showed little heterogeneity in nucleotide substitution rates. However, heterogeneity was observed between the two groups of S-alleles, with the Prunus alleles exhibiting a lower rate of non-synonymous substitutions than alleles from Malus and Pyrus. The evolutionary relationships between these species are discussed.