Neofunctionalization of young duplicate genes in Drosophila. Raquel Assis, Doris Bachtrog. Integrative Biology, University of California, Berkeley, Berkeley, CA.

   Gene duplication is a key mechanism by which novel phenotypes arise. There are two major hypotheses for how this occurs. According to the neofunctionalization hypothesis, one gene copy acquires a novel function, while the other retains the ancestral function. In contrast, the subfunctionalization hypothesis proposes that there is a division of functions between duplicate genes, such that each copy performs a subset of the ancestral functions. To disentangle these two processes in Drosophila, we studied the phenotypic evolution of recent gene duplicates in D. melanogaster by using gene expression profiles as a proxy for function. Comparison of gene expression profiles in D. melanogaster and D. pseudoobscura revealed that ancestral genes tend to have conserved and broadly expressed functions, whereas new copies frequently have novel tissue-specific functions. Moreover, we found that new copies evolve significantly faster at the sequence and expression level than ancestral copies, which evolve at similar rates to single-copy genes. In particular, new genes that are testis-specific and/or male-biased evolve the fastest at the sequence and expression levels and display evidence of positive selection. Thus, our findings are consistent with the neofunctionalization hypothesis and suggest that the origin of novel phenotypes by neofunctionalization in Drosophila is driven by strong positive selection on young duplicate gene copies.