Population Genetics and Geographic Variation of Alcohol Dehydrogenase (Adh) Paralogs and Glucose-6-Phosphate Dehydrogenase (G6pd) in Drosophila mojavensis

Populations of Drosophila mojavensis from the deserts of the Baja California peninsula and mainland Mexico utilize different cactus hosts with different alcohol contents. The enzyme alcohol dehydrogenase (ADH) has been proposed to play an important role in the adaptation of Drosophila species to their environment. This study investigates the role of ADH in the adaptation of the cactophilic D. mojavensis to its cactus host. In D. mojavensis and its sibling species, D. arizonae, the Adh gene has duplicated, giving rise to a larval/ovarian form (Adh-1) and an adult form (Adh-2). Studies of sequence variation presented here indicate that the Adh paralogs have followed different evolutionary trajectories. Adh-1 exhibits an excess of fixed amino acid replacements, suggesting adaptive evolution, which could have been a result of several host shifts that occurred during the divergence of D. mojavensis. A 17-bp intron haplotype polymorphism segregates in Adh-2 and has markedly different frequencies in the Baja and mainland populations. The presence of the intron polymorphism suggests possible selection for the maintenance of pre-mRNA structure. Finally, this study supports the proposed Baja California origination of D. mojavensis and subsequent colonization of the mainland accompanied by a host shift.