Academic journal article Genetics

The Molecular Basis of Host Adaptation in Cactophilic Drosophila: Molecular Evolution of a Glutathione S-Transferase Gene (GstD1) in Drosophila Mojavensis

Academic journal article Genetics

The Molecular Basis of Host Adaptation in Cactophilic Drosophila: Molecular Evolution of a Glutathione S-Transferase Gene (GstD1) in Drosophila Mojavensis

Article excerpt

ABSTRACT

Drosophila mojavensis is a cactophilic fly endemic to the northwestern deserts of North America. This species includes four genetically isolated cactus host races each individually specializing on the necrotic tissues of a different cactus species. The necrosis of each cactus species provides the resident D. mojavensis populations with a distinct chemical environment. A previous investigation of the role of transcriptional variation in the adaptation of D. mojavensis to its hosts produced a set of candidate loci that are differentially expressed in response to host shifts, and among them was glutathione S-transferase D1 (GstD1). In both D. melanogaster and Anopheles gambiae, GstD1 has been implicated in the resistance of these species to the insecticide dichloro-diphenyl-trichloroethane (DDT). The pattern of sequence variation of the GstD1 locus from all four D. mojavensis populations, D. arizonae (sister species), and D. navojoa (outgroup) has been examined. The data suggest that in two populations of D. mojavensis GstD1 has gone through a period of adaptive amino acid evolution. Further analyses indicate that of the seven amino acid fixations that occurred in the D. mojavensis lineage, two of them occur in the active site pocket, potentially having a significant effect on substrate specificity and in the adaptation to alternative cactus hosts.

THE concept of ''evolution at two levels'' proposed by King and Wilson (1975) predicts that evolutionary change is a function of both coding sequence and transcriptional variation. There are myriads of examples highlighting the role of coding sequence variation in evolution and recently this pattern has been observed at the genomic level (Fay et al. 2002; Bierne and Eyre-Walker 2004; Bustamante et al. 2005; Voight et al. 2006). As well, it is becoming apparent that natural selection plays a large role in interspecific transcriptional variation (Rifkin et al. 2003; Nuzhdin et al. 2004; Gilad et al. 2006). What is sometimes lacking in many studies, and often the least tractable, is an understanding of how the transcriptional and sequence variation relates to the ecology of the organism.

Drosophila mojavensis offers a unique opportunity to incorporate knowledge of its ecology with its transcriptional and sequence variation. D. mojavensis and its sister species D. arizonae are cactophilic flies that diverged ~1.5 million years ago (Matzkin and Eanes 2003; Matzkin 2004; Reed et al. 2007). These species utilize the necrotic tissues of several cactus species as their host. The range of D. mojavensis is composed of four geographically and genetically isolated host races (Ross and Markow 2006; Machado et al. 2007; Reed et al. 2007). Each host race, mainland Sonora Desert, Baja California, Catalina Island, and Mojave Desert, utilizes a different species of cactus: organpipe (Stenocereus thur-beri), agria (S. gummosus), prickly pear (Opuntia spp.), and barrel (Ferocactus cylindraceus), respectively (Fellows and Heed 1972; Ruiz and Heed 1988). D. mojavensis has been proposed (Ruiz et al. 1990) to have originated in Baja California, utilizing a Stenocereus cactus (possibly agria), and then migrated up the peninsula and colonized Catalina Island and the Mojave Desert, shifting cactus hosts in the process. A subsequent colonization (and host shift) from Baja to Sonora established the present-day mainland Sonora Desert population.

The differences in the chemical composition of the cacti, in addition to the microflora associated with the necrosis (Starmer 1982; Starmer et al. 1986), produce very distinct chemical environments to which each population of D. mojavensis must adapt (Heed 1978; Vacek 1979; Kircher 1982). Some of the chemical differences include such compounds as alcohols, alkaloids (in Opuntia), triterpenes, and glycosides. Previous studies in D. mojavensis have shown that this chemical variation can drive the molecular and functional evolution of metabolic genes (Matzkin and Eanes 2003;Matzkin 2004, 2005). …

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