Duplication, Dicistronic Transcription, and Subsequent Evolution of the Alcohol Dehydrogenase and Alcohol Dehydrogenase-related Genes in Drosophila

It has recently been discovered that the Alcohol dehydrogenase and Alcohol dehydrogenase-related genes of Drosophila melanogaster and closely related species constitute a single transcription unit and that the Alcohol dehydrogenase-related gene is exclusively expressed from a dicistronic mRNA. Here, we show that in Drosophila lebanonensis, subgenus Scaptodrosophila, Adhr: is also transcribed as a dicistronic transcript with Adh Using degenerate primers designed on the sequence of the known Adhr proteins, we have been able to amplify and sequence a partial sequence of Adhr: in species representative of the whole subgenus Drosophila. This has allowed the study of the organization and expression of Adhr: in Drosophila buzzatii. We find that in D. buzzatii Adhr is transcribed as a monocistronic transcript. Adh and Adhr are believed to originate by duplication, and our data suggest that the cotranscription of these two genes was the primitive state, and that their independent transcription in the subgenus Drosophila is derived. We can rationalize the D. buzzatii condition as being correlated with the two genes evolving independent transcriptional control. However, why these two genes with clear divergence in the functions of their proteins should remain cotranscribed in groups as divergent as the subgenus Sophophora and the subgenus Scaptodrosophila remains a mystery.