Selection on nuclear genes in a Pinus phylogeny.

In this study, we investigate natural selection in a pine phylogeny. DNA sequences from 18 nuclear genes were used to construct a very well-supported species tree including 10 pine species. This tree is in complete agreement with a previously reported supertree constructed from morphological and molecular data, but there are discrepancies with previous chloroplast phylogenies within the section Pinus. A significant difference in evolutionary rate between Picea and Pinus was found, which could potentially indicate a lower mutation rate in Picea, but other scenarios are also possible. Several approaches were used to study selection patterns in a set of 21 nuclear genes in pines and in some cases in Picea and Pseudotsuga. The overall pattern suggests efficient purifying selection resulting in low branch-specific d(n)/d(s) ratios with an average of 0.22, which is similar to other higher plants. Evidence for purifying selection was common and found on at least 55% of the branches. Evidence of positive selection at several sites was found in a phytocyanin homolog and significant differences in d(n)/d(s) among the branches in the gene tree in dehydrin 1. Several genes suitable for further phylogenetic analysis at various levels of divergence were identified.