Inhibition of alcohol dehydrogenase after 2-propanol exposure in different geographic races ofDrosophila mojavensis: Lack of evidence for selection at theAdh-2 Locus

Abstract

High frequencies of the fast allele of alcohol dehydrogenase-2 (Adh-2F) are found in populations of Drosophila mojavensis that inhabit the Baja California peninsula (race BII) whereas the slow allele (Adh-2S) predominates at most other localities within the species' geographic range. Race BII flies utilize necrotic tissue of pitaya agria cactus (Stenocereus gummosus) which contains high levels of 2-propanol, whereas flies from most other localities utilize different cactus hosts in which 2-propanol levels are low. To test if 2-propanol acts as a selective force on Adh-2 genotype, or whether some other yet undetermined genetic factor is responsible, mature males of D. mojavensis lines derived from the Grand Canyon (race A) and Santa Catalina Island (race C), each with individuals homozygous for Adh-2F and Adh-2S, were exposed to 2-propanol for 24 h and ADH-2 specific activity was then determined on each genotype. Flies from five other localities homozygous for either the fast or slow allele also were examined. Results for all reported races of D. mojavensis were obtained. 2-propanol exposure inhibited ADH-2 specific activity in both genotypes from all localities, but inhibition was significantly less in two populations of race BII flies homozygous for Adh-2F. When F/F and S/S genotypes in flies from the same locality were compared, both genotypes showed high 2-propanol inhibition that was not statistically different, indicating that the F/F genotype alone does not provide a benefit against the inhibitory effects of 2-propanol. ADH-1 activity in female ovaries was inhibited less by 2-propanol than ADH-2. These results do not support the hypothesis that 2-propanol acts as a selective factor favoring the Adh-2F allele.

Microsatellite variation among diverging populations of Drosophila mojavensis

“…The evolution of host races in D. mojavensis Drosophila mojavensis populations show striking genetic differentiation across the geographical distribution of the species, even when only four microsatellite loci are surveyed. Specifically, four major groups emerge from our genetic survey: one is consistent with what has been called race B1 [designated D. mojavensis sonora by Hocutt (2000)] in Sonora and southern Arizona, which breeds in organ pipe cactus, another with race B2 in Baja (D. mojavensis baja) breeding in agria, a third matches race A breeding in barrel cactus in the Mojave Desert and the Grand Canyon (D. mojavensis mojavensis), and a fourth distinct group, using prickly pear, is found exclusively on Catalina Island [designated D. mojavensis wrigleyi by Hocutt (2000) or race C by Pfeiler et al (2005)]. Because of the large number of individuals sampled and the concordance of pattern among all microsatellite loci even though they are unlinked, it is unlikely that additional microsatellite loci would provide a different picture.…”

Genetic, ecological and morphological differences among populations of the cactophilicDrosophila mojavensisfrom southwestern USA and northwestern Mexico, with descriptions of two new subspecies

“…In addition to D. m. mojavensis and D. m. baja, the two additional subspecies proposed by Hocutt (2000) were D. m. sonora (here designated D. m. sonorensis) for the populations of D. m. baja (sensu Mettler 1963) from mainland Mexico and southern Arizona (subrace BI of Zouros 1973) and D. m. wrigleyi for the population of D. m. mojavensis on Santa Catalina Island (Ruiz et al 1990; also called race C in Pfeiler et al 2005).…”

“…In addition to differences in host plant use, individuals of D. mojavensis from different geographic areas vary in several characteristics, including size, colour, chromosome polymorphisms, allele frequencies and DNA sequences (Mettler 1963;Zouros 1973;Johnson 1973Johnson , 1980Etges and Heed 1987;Ruiz et al 1990;Krebs 1991;Etges 1993;Hocutt 2000;Matzkin and Eanes 2003;Matzkin 2005;Ross and Markow 2006;Machado et al 2007;Reed et al 2007;Matzkin 2008). They also exhibit differences in biochemical, physiological and behavioural traits (Starmer et al 1977;Batterham et al 1982;Etges and Klassen 1989;Etges and Ahrens 2001;Krebs and Thompson 2005;Pfeiler et al 2005).…”

“…Population genetic studies using allozymes and DNA sequence data (reviewed later) have supported the existence of additional subspecies of D. mojavensis, one from the Sonoran Desert of mainland Mexico and southern Arizona, USA, and the other from Santa Catalina Island, off the southern California coast (Hocutt 2000;Ross and Markow 2006;Machado et al 2007;Reed et al 2007;Matzkin 2008). Subspecies names have been proposed for these two populations in an unpublished doctoral dissertation (Hocutt 2000), and although these names have appeared in the literature (Pfeiler et al 2005;Ross and Markow 2006), they have never been formally described. There is a need, therefore, to update and stabilize the nomenclature of the genetically differentiated populations of D. mojavensis.…”

“…The geographically isolated and genetically differentiated populations of D. mojavensis have been given a variety of names, including races, subraces and strains (Zouros 1973;Ruiz et al 1990; Krebs and Thompson 2005;Pfeiler et al 2005), with two subspecies, D. m. mojavensis from the Mojave Desert and D. m. baja from the Sonoran Desert, having been described (Mettler 1963). Population genetic studies using allozymes and DNA sequence data (reviewed later) have supported the existence of additional subspecies of D. mojavensis, one from the Sonoran Desert of mainland Mexico and southern Arizona, USA, and the other from Santa Catalina Island, off the southern California coast (Hocutt 2000;Ross and Markow 2006;Machado et al 2007;Reed et al 2007;Matzkin 2008).…”