Comparing patterns of nucleotide substitution rates among chloroplast loci using the relative ratio test

Even when several genetic loci are used in molecular evolutionary studies, each locus is typically analyzed independently of the others. This type of approach makes it difficult to study mechanisms and processes that affect multiple genes. In this work we develop a statistical approach for the joint analysis of two or more loci. The tests we propose examine whether or not nucleotide substitution rates across evolutionary lineages have the same relative proportions at two loci. Theses procedures are applied to 33 genes from the chloroplast genomes of rice, tobacco, pine, and liverwort. With the exception of five clearly distinct loci, we find that synonymous substitution rates tend to change proportionally across genes. We interpret these results to be consistent with a "lineage effect" acting on the entire chloroplast genome. In contrast, nonsynonymous rates do not change proportionally across genes, suggesting that locus-specific evolutionary effects dominate patterns of nonsynonymous substitution.