Since the first transcriptional study in D. mojavensis (Matzkin et al., 2006), my laboratory and others have found significant effects of cactus host utilization on gene expression (Matzkin, 2012; Smith et al., 2013). Furthermore, significant transcriptional changes associated with other environmental factors such as water stress (Matzkin, Markow, 2009; Rajpurohit et al., 2013) and between the cactus host populations (Matzkin, 2012) has also been observed. Cactus host utilization in the larval stage induces a series of genes associated with the metabolism of xenobiotics, as well as genes involved in general metabolism and development (Matzkin et al., 2006; Matzkin, 2012; Smith et al., 2013). The induction of these enzymes is likely in response to the chemicals associated with the cactus necroses (Kircher, 1982). Among the detoxification gene families observed to be differentially expressed were Glutathione S-transferases (GST), Cytochrome P450 (P450), Esterases (EST), UDP-Glycosyltransferases (UGT).
Although induction of genes associated with cactus use is suggestive of their role in adaptation, it is predicted that the constant exposure to the differential local environmental conditions across the four host populations would have created differences in the interpopulation transcriptional pattern in addition to environment-independent transcriptional differences. When larvae from all four host populations are reared in a cactus-free common media, clustering of their transcriptome profile recapitulates population identity (Matzkin, 2012). The distinct transcriptome profiles of the host populations suggest that among other factors (e.g. drift, sexual selection, etc), the adaptation to the local environmental conditions influenced the interpopulation transcriptional divergence. Approximately 3,000 genes (21% of the genome) differ in expression among the four host populations, many involved in detoxification and metabolism as well as chemosensory perception such odorant receptors (Or) and gustatory receptors (Gr). Compared to adults, larvae have a reduced set of receptors but are able to respond to chemosensory stimuli (Vosshall, Stocker, 2007). In fact, D. mojavensis larvae actively seek out yeast and microhabitats (Fogleman et al., 1981), presumably using many of the same chemosensory genes that were observed to have fixed expression differences across the host populations.