Meeting Abstract
Herbivores confront the possibility of being poisoned by their food, which contains plant secondary compounds (PSCs). The mechanisms used by mammalian herbivores to metabolize PSCs are poorly understood, particularly with respect to specialization on toxic diets. We are investigating the evolution of dietary specialization in herbivores by studying dramatic diet changes in woodrats (Neotoma spp.). Starting approximately 18,000 years ago, populations of two woodrat species switched from feeding on juniper and cactus to diets rich in creosote bush (Larrea tridentata), a shrub containing high levels of toxic compounds. These different food sources have radically disparate PSC profiles and thus require different hepatic enzymes for biotransformation. We assayed the ability to tolerate creosote toxins in 12 populations of two species of woodrats (N. lepida, N. bryanti, and their hybrids). Some of these populations are obligate creosote feeders, while others feed on the ancestral diet. In laboratory feeding trails, we determined the maximum tolerable dose (MTD) of creosote resin. MTD varied between species, with N. lepida having the highest MTD, ~1.5 times that of N. bryanti. Hybrids were intermediate. This pattern was evident across collection sites, and within a site where both species occurred and creosote dominated the landscape. Surprisingly, access to creosote bush did not explain tolerance variation: we found no differences in MTD between populations within a species with or without access to creosote bush. Instead, MTD was negatively correlated with increasing distance from the proposed origin of creosote bush invasion in southwestern North America (R2=0.69, p=0.002). We are currently conducting genomic and transcriptomic analyses to identify the genomic basis for dietary specialization on creosote bush.
