Regulatory gene evolution: adaptive differences in expression of alcohol dehydrogenase in Drosophila melanogaster and Drosophila simulans.

In Drosophila melanogaster X D. simulans hybrids, the alcohol dehydrogenase (ADH) electromorphs characteristic of the two parents display tissue- and stage-specific differences in relative level of expression. This implies distinct cis-acting regulatory elements associated with the respective Adh alleles. These cis-acting elements account in part, but not completely, for markedly different overall patterns of ADH expression in the two species. The regulatory patterns seem to be adaptively significant since they correlate with species-specific patterns of ethanol tolerance. The activity differences are accounted for by different levels of enzyme protein, but the underlying mechanisms have not been fully analysed and may be complex. Independent evolution of various aspects of the ADH developmental programme may relate to use of different promoters for transcription of the Adh locus in different developmental contexts. This system illustrates the potential importance of regulatory genes in evolution and provides a model for investigating the molecular basis of evolved regulatory differences.