Molecular evolution and subfunctionalization of alcohol dehydrogenase paralogues in cactophilic Drosophila. L.M. Matzkin , W.F. Eanes. Dept. Ecology & Evolution, State Univ New York, Stony Brook, NY.

   Comparison of paralogues genes offers a unique opportunity to study variation and molecular evolution. Classical models of paralogue evolution have focused on change in the coding region, thus are based on rapid amino acid evolution. More recent models have focused on change in the regulatory region of paralogues. Force et al. (1999) proposed that three possible fates exists for recently fixed duplications: neofunctionalization, nonfunctionalization and subfunctionalization. The first two deal with the creation of novel gene function via amino acid change and pseudogene formation, respectively, while subfunctionalization is the partitioning of original function between paralogues. Differential mutations in the regulatory region will tend to create paralogues with no or reduced overlap of expression. This will provide the possibility for natural selection to act independently on each paralogue.
   In Drosophila , alcohol dehydrogenase (Adh ) duplications have independently occurred in several lineages including the mojavensis species cluster. Two species within this cactophilic species cluster, D. mojavensis and D. arizonae , contain an adult expressed (Adh-2 ) and a larval and ovary expressed form (Adh-1 ). The cactus host of these species changes with geographic location and each cactus host contain different alcohol compositions. In D. mojavensis allozyme polymorphisms at Adh-2 have been associated with greater tolerance to certain alcohols. There is evidence suggesting an episode of adaptive amino acid evolution in Adh-1 , while this is not the case in Adh-2 . Additionally, there have been several amino acids fixed between the paralogues in both species, three of which occur in the active site of the enzyme. Hence, Adh in these species serves as an excellent system to study subfunctionalization and the molecular basis of adaptation to alternative environments.