Abstract
BACKGROUND: New genes are constantly formed, sometimes from non-genic sequences, creating what is referred to as de novo genes. Since the total number of genes remains relatively steady, gene deaths likely balance out new births. In metazoan genomes, microRNAs (miRs) genes, small and non-coding, account for the bulk of functional de novo genes and are particularly suited to the investigation of gene death. RESULTS: In this study, we discover a Drosophila-specific de novo miRNA (mir-977) that may be facing impending death. Strikingly, after this testis-specific gene is deleted from D. melanogaster, most components of male fitness increase, rather than decrease as had been expected. These components include male viability, fertility and males' ability to repress female re-mating. Given that mir-977 has a negative fitness effect in D. melanogaster, this de novo gene with an adaptive history for over 60 Myrs may be facing elimination. In some other species where mir-977 is not found, gene death may have already happened. CONCLUSION: The surprising result suggests that de novo genes, constantly rising and falling during evolution, may often be transiently adaptive and then purged from the genome.