Origin and evolution of a chimeric fusion gene in Drosophila subobscura, D. madeirensis and D. guanche.

An understanding of the mutational and evolutionary mechanisms underlying the emergence of novel genes is critical to studies of phenotypic and genomic evolution. Here we describe a new example of a recently formed chimeric fusion gene that occurs in Drosophila guanche, D. madeirensis, and D. subobscura. This new gene, which we name Adh-Twain, resulted from an Adh mRNA that retrotransposed into the Gapdh-like gene, CG9010. Adh-Twain is transcribed; its 5' promoters and transcription patterns appear similar to those of CG9010. Population genetic and phylogenetic analyses suggest that the amino acid sequence of Adh-Twain evolved rapidly via directional selection shortly after it arose. Its more recent history, however, is characterized by slower evolution consistent with increasing functional constraints. We present a model for the origin of this new gene and discuss genetic and evolutionary factors affecting the evolution of new genes and functions.