Differential Adaptation to Plant Toxins: The role of Chemically-mediated Selection in Reproductive Isolation between Mammalian Herbivores.

Board Of Regents, Nshe, Obo University Of Nevada, Reno, Reno NV

Investigators

Abstract

One of the most fundamental ways that an animal interacts with its environment is through its diet. Woodrats (or packrats) are known to specialize on particularly toxic food plants in the wild. This project tests the hypothesis that woodrats overcome certain plant toxins in their diet through a combination of liver detoxification enzymes (biochemical catalysts) as well as through the activity of their gut microbes. The hypothesis will be tested in a natural setting where two species of woodrats come into contact with each other at a sharp habitat transition. Each species is primarily found in one habitat or the other, and appears to specialize on habitat-specific toxic plants. The project will examine whether each species is uniquely adapted to the plant toxins in their native habitat such that they would not be able to sustain themselves on the diet available in the adjacent habitat. As such, these chemically-mediated ecological adaptations may limit genetic exchange, or hybridization, between the species and play an important role in maintaining their distinction. These results will therefore expand understanding of biodiversity and its management in nature. For herbivores, plant chemicals found in most of their food items affect growth, health, and response to changes in diet. Thus, this research will have relevance to production, improvement, and safety of mammalian animals used as food by humans. This research will provide training for numerous students and will allow expansion of a current high school education program wherein teachers and their students participate in a week-long, hands on program focused on the genetic basis of sensitivity in humans to the taste of particular chemicals. This research aims to identify the mechanisms underlying genotype-phenotype-environment interactions across a mammalian hybrid zone that spans a sharp ecological boundary. The system entails two species of woodrat (Neotoma) that hybridize across a sharp ecological transition but occupy distinct habitats and maintain habitat-specific diets with distinct toxin profiles. Woodrats often specialize on highly toxic food plants, but exposure to novel plant toxins generates severe metabolic costs. The hypothesis tested is that diet specialization and the cost of habitat/diet switching is the primary selective force underlying the strong genotype-phenotype-environment relationship in the woodrat system. Further, because strong selection against hybrids during early stages of life, when individuals are transitioning to a plant-based diet, has been demonstrated previously, another hypothesis to be tested is that many hybrids may not be capable of metabolizing either parental diet. To test these hypotheses, field studies and laboratory experiments will be integrated to quantify the pattern (metabolomics, Aim 1), mechanisms (gene expression and gut microbiome, Aim 2), and ecologically-relevant outcome (performance, Aim 3) of diet-mediated selection. Exposure of various genotypic classes (i.e. pure and hybrid woodrats) to habitat-specific diets will be experimentally manipulated to establish patterns of metabolic processing of these diets, identify the underlying liver gene expression and gut microbial community profiles associated with exposure to these diets, and quantify the energetic costs associated with consumption of these diets. The ecological and evolutionary principles associated with this research in diet-based ecological adaptation will be translated to augment current high school outreach programs that focus on the genetic basis of sensitivity in humans to the taste of particular chemicals.