Different evolutionary patterns between young duplicate genes in the human genome

Background: Following gene duplication, two duplicate genes may experience relaxed functional constraints or acquire different mutations, and may also diverge in function. Whether the two copies will evolve in different patterns remains unclear, however, because previous studies have reached conflicting conclusions. In order to resolve this issue, by providing a general picture, we studied 250 independent pairs of young duplicate genes from the whole human genome. Results: We showed that nearly 60% of the young duplicate gene pairs have evolved at the amino-acid level at significantly different rates from each other. More than 25% of these gene pairs also showed significantly different ratios of nonsynonymous to synonymous rates (K-a/K-s ratios). Moreover, duplicate pairs with different rates of amino-acid substitution also tend to differ in the K-a/K-s ratio, with the fast-evolving copy tending to have a slightly higher K-s than the slow-evolving one. Lastly, a substantial portion of fast-evolving copies have accumulated amino-acid substitutions evenly across the protein sequences, whereas most of the slow-evolving copies exhibit uneven substitution patterns. Conclusions: Our results suggest that duplicate genes tend to evolve in different patterns following the duplication event. One copy evolves faster than the other and accumulates amino-acid substitutions evenly across the sequence, whereas the other copy evolves more slowly and accumulates amino-acid substitutions unevenly across the sequence. Such different evolutionary patterns may be largely due to different functional constraints on the two copies.