Ethanol tolerance and the Adh polymorphism in a natural population of Drosophila melanogaster

Abstract

A DETAILED understanding of the balanced selective forces maintaining the alcohol dehydrogenase (Adh) polymorphism of Drosophila melanogaster will come about when the properties of the enzymatic products of the genotypes can be directly related to selective forces acting on the phenotypes. This approach has been discussed by Clarke¹, but it was Gibson² who first probed the Adh locus in this way when he successfully selected for a specific allele using high levels of ethanol. Enzyme activity of Adh^F / Adh^F, Adh^F / Adh^s and Adh^s / Adh^s genotypes on specific alcohol substrates can generally be related to differential survival and changes in gene frequency in population cages^1–8. Briscoe, Robertson and Malpica³ showed a higher Adh^F frequency in a Spanish wine cellar population than at a neighbouring site and explained this by the relative ethanol resistance of adult Adh phenotypes. In a study of an Australian winery population McKenzie and Parsons⁹ also observed differences in ethanol tolerance between the cellar and peripheral sections of the population but they found no correlation between tolerance and Adh frequencies. We now report a more intensive study of this population where samples have been taken from inside and outside the cellar during both vintage and non-vintage periods over two years. In this population a different adaptive system is operating. Unlike Briscoe et al., we found no relationship between the distributions of tolerance and of Adh phenotypes although tolerance can be directly related to the selective effect of alcohol in the adult environment.