Mechanisms of gene death in the Red Queen race revealed by the analysis of de novo microRNAs

The prevalence of de novo coding genes is controversial due to the length and coding constraints. Non-coding genes, especially small ones, are freer to evolve de novo by comparison. The best examples are microRNAs (miRNAs), a large class of regulatory molecules ~22 nt in length. Here, we study 6 de novo miRNAs in Drosophila which, like most new genes, are testis-specific. We ask how and why de novo genes die because gene death must be sufficiently frequent to balance the many new births. By knocking out each miRNA gene, we could analyze their contributions to each of the 9 components of male fitness (sperm production, length, competitiveness etc.). To our surprise, the knockout mutants often perform better in some components, and slightly worse in others, than the wildtype. When two of the younger miRNAs are assayed in long-term laboratory populations, their total fitness contributions are found to be essentially zero. These results collectively suggest that adaptive de novo genes die regularly, not due to the loss of functionality, but due to the canceling-out of positive and negative fitness effects, which may be characterized as quasi-neutrality. Since de novo genes often emerge adaptively and become lost later, they reveal ongoing period-specific adaptations, reminiscent of the Red-Queen metaphor for long term evolution.