Origin and Evolution of a Chimeric Fusion Gene in Drosophila subobscura, D. madeirensis and D. guancheSequence data from this article have been deposited with the EMBL/GenBank Data Libraries under accession nos. AY874360, AY874361, AY874362, AY874363, AY874364, AY874365, AY874366, AY874367, AY874368, AY874369, AY874370, AY874371, AY874372, AY874373, AY874374, AY874375, AY874376, AY874377, AY874378.

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Abstract
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.