Abstract
Because orthologs diverge after speciation events, and paralogs after gene duplication, it is expected that orthologs should tend to keep their functions, while paralogs have been proposed as a source of new functions. This does not mean that paralogs should diverge much more than orthologs, but it certainly means that, if there is a difference, then orthologs should be more functionally stable. Since protein functional divergence follows from non-synonymous substitutions, here we present an analysis based on the ratio of non-synonymous to synonymous substitutions (dN/dS). The results showed orthologs to have noticeable and statistically significant lower values of dN/dS than paralogs, not only confirming that orthologs keep their functions better, but also suggesting that paralogs are a readily source of functional novelty.
Author summary Homologs are characters diverging from a common ancestor, with orthologs being homologs diverging after a speciation event, and paralogs diverging after a duplication event. Given those definitions, orthologs are expected to preserve their ancestral function, while paralogs have been proposed as potential sources of functional novelty. Since changes in protein function require changes in amino-acids, we analyzed the rates of non-synonymous, mutations that change the encoded amino-acid, to synonymous, mutations that keep the encoded amino-acid, subtitutions (dN/dS). Orthologs showed the lowest dN/dS ratios, thus suggesting that they keep their functions better than paralogs. Because the difference is very evident, our results also suggest that paralogs are a source of functional novelty.