Evolution of Viral Proteins Originated De Novo by Overprinting

Sabath, Niv; Wagner, Andreas; Karlin, David

Our results suggest that de novo genes do adapt to their genome. More specifically, de novo genes evolve very rapidly shortly after their origin. As they age, they tend to experience increasingly severe selective constraints, and their codon usage tends to approach that of the ancestral gene from which they originate. Our results are consistent with population genetics theory (Hartl and Clark 1997) . Viruses have large population sizes. At such large sizes, natural selection is highly efficient, which has two consequences regarding de novo genes. First, they are likely to become fixed in a population only if they provide some selective advantage. Second, even though a de novo gene might initially only provide a very small fitness benefit, in a large population this fitness benefit can be sufficient to cause the gene's fixation. Immediately after its origin, the sequence of a de novo gene will typically be far from optimal for the (rudimentary) function it provides, unlike a gene originated through modification of an existing gene. Consequently, one would expect a de novo gene to evolve rapidly shortly after its origin and to become better adapted as it ages, resulting in increased selective constraints and a decreased rate of sequence change.