Local synteny and codon usage contribute to asymmetric sequence divergence of Saccharomyces cerevisiae gene duplicates

BACKGROUND: Duplicated genes frequently experience asymmetric rates of sequence evolution. Relaxed selective constraints and positive selection have both been invoked to explain the observation that one paralog within a gene-duplicate pair exhibits an accelerated rate of sequence evolution. In the majority of studies where asymmetric divergence has been established, there is no indication as to which gene copy, ancestral or derived, is evolving more rapidly. In this study we investigated the effect of local synteny (gene-neighborhood conservation) and codon usage on the sequence evolution of gene duplicates in the S. cerevisiae genome. We further distinguish the gene duplicates into those that originated from a whole-genome duplication (WGD) event (ohnologs) versus small-scale duplications (SSD) to determine if there exist any differences in their patterns of sequence evolution. RESULTS: For SSD pairs, the derived copy evolves faster than the ancestral copy. However, there is no relationship between rate asymmetry and synteny conservation (ancestral-like versus derived-like) in ohnologs. mRNA abundance and optimal codon usage as measured by the CAI is lower in the derived SSD copies relative to ancestral paralogs. Moreover, in the case of ohnologs, the faster-evolving copy has lower CAI and lowered expression. CONCLUSIONS: Together, these results suggest that relaxation of selection for codon usage and gene expression contribute to rate asymmetry in the evolution of duplicated genes and that in SSD pairs, the relaxation of selection stems from the loss of ancestral regulatory information in the derived copy.