The short life cycle of orphan genes in the Drosophila obscura group explains the paradox of conserved gene number across species. Christian W. Schloetterer, Nicola Palmieri, Carolin Kosiol, Viola Nolte. Inst f Populationsgenetik, Vetmeduni Vienna, Wien, Austria.

   It is well understood that orphan genes emerge at a high rate, frequently from previously non-coding DNA. Despite that a high fraction of the recently emerged genes quickly acquires essential functions, the total number of genes remains remarkably stable across species. To shed more light on this apparent paradox we studied orphan genes in the Drosophila obscura group. Using RNA-Seq we identified 1143 putatively protein coding orphan genes in D. pseudoobscura, which is consistent with the previously published high emergence rate of orphan genes. Through phylogenetic analysis of D. affinis, D. lowei, D. miranda and D. persimilis we dated the birth and death process of these orphan genes. Contrary to neutral expectations, we found that most orphan genes pseudogenized (lost function) shortly after their emergence and only few orphans are phylogenetically conserved. This short life cycle of orphan genes reconciles their high birth rate with the stability of gene number across species.