Expression of biotransformation genes in woodrat (Neotoma) herbivores on novel and ancestral diets: identification of candidate genes responsible for dietary shifts

Abstract

The ability of herbivores to switch diets is thought to be governed by biotransformation enzymes. To identify potential biotransformation enzymes, we conducted a large-scale study on the expression of biotransformation enzymes in herbivorous woodrats (Neotoma lepida). We compared gene expression in a woodrat population from the Great Basin that feeds on the ancestral diet of juniper to one from the Mojave Desert that putatively switched from feeding on juniper to feeding on creosote. Juniper and creosote have notable differences in secondary chemistry, and thus, should require different biotransformation enzymes for detoxification. Individuals from each population were fed juniper and creosote diets separately. After the feeding trials, hepatic mRNA was extracted and hybridized to laboratory rat microarrays. Hybridization of woodrat samples to biotransformation probes on the array was 87%, resulting in a total of 224 biotransformation genes that met quality control standards. Overall, we found large differences in expression of biotransformation genes when woodrats were fed juniper vs. creosote. Mojave woodrats had greater expression of 10x as many biotransformation genes as did Great Basin woodrats on a creosote diet. We identified 24 candidate genes that may be critical in the biotransformation of creosote toxins. Superoxide dismutase, a free radical scavenger, was also expressed to a greater extent by the Mojave woodrats and may be important in controlling oxidative damage during biotransformation. The results are consistent with the hypothesis that biotransformation enzymes limit diet switching and that woodrats in the Mojave have evolved a unique strategy for the biotransformation of creosote toxins.

Evidence for functional convergence in genes upregulated by herbivores ingesting plant secondary compounds

“…One treatment was fed powdered rabbit chow amended with resin from creosote bush; the other group was fed a control diet of the same powdered rabbit chow with no additions. Resin was extracted and creosote diets prepared as in [34]. …”

“…Comparative data on enzyme activities and gene expression of populations of N. lepida support this contention. Neotoma lepida that feed on juniper have different enzyme activity and gene expression profiles compared to those that feed on creosote even when they are fed the same diet [32-34]. Indeed, populations of N. lepida in the Mojave desert have adapted to a diet of creosote as evidenced by their ability to ingest greater quantities of creosote compared to those from the Great Basin desert that feed on juniper and have no prior exposure to creosote [35].…”

Inter-population differences in the tolerance of a marsupial folivore to plant secondary metabolites

“…First, evolution may shape the responses of herbivores to PSMs by producing genetic diVerences between separated populations. Indeed, Magnanou et al (2009) demonstrated diVerences in the expression of biotransformation genes between two populations of woodrats when fed either juniper or creosote bush. Perhaps the best example, in the case of brushtail possums, is the diVering ability of possums in diVerent parts of Australia to metabolise Xuoroacetate.…”

A Pharm-Ecological Perspective of Terrestrial and Aquatic Plant-Herbivore Interactions

“…Although the general concepts of pharmacology were introduced to ecologists to explain plant-herbivore interactions 38 years ago (Freeland and Janzen, 1974), the empirical use of pharmacology to understand mechanisms driving foraging patterns in any system is limited (Haley et al, 2008;Magnanou et al, 2009;Marsh et al, 2006b;McLean and Duncan, 2006;Sotka et al, 2009). In contrast to the broad knowledge base that exists on the mechanisms required to process nutrients as well as mechanisms used by domestic and laboratory species to metabolize drugs, there is a general lack of understanding of how PSMs are processed by herbivores, especially in the context of a variable nutrient environment (Appel, 1993;Casarett et al, 2008;Gross and Bakker, 2012;Karasov and Hume, 1997).…”