Research

Research

Background

I am an evolutionary physiologist. I am interested in how organisms interact with and adapt to their surroundings, on time scales from minutes to millenia. I have worked in a diversity of systems and habitats, from deep-sea fish to Drosophila, but the fundamental questions are the same: How do environmental variables affect organismal function, and how do populations and species evolve in response? I take a vertically integrated approach, drawing on techniques and ideas from evolution, physiological ecology, organismal biology, functional genomics, genetics and biophysics. My primary research program concerns water balance and energetics in insects, but this interest has led me into several different areas.

A fundamental physiological problem for insects as terrestrial organisms is that they are relatively small; thus, they have a relatively large surface area:volume ratio. They contain very little water, but have a relatively large body surface area through which water can be lost. Despite this limitation, insects and other arthropods are the most abundant animals (in numbers and biomass) in the driest deserts on earth. My interest in this area began in my post-doctoral years, when I used Fourier transform infrared (FTIR) spectroscopy to study the physical properties of epicuticular waxes, the primary barrier to water loss through the insect cuticle. In a series of studies, my lab linked changes in lipid composition to differences in lipid physical properties, and lipid phase behavior to cuticular water-proofing.

My research on the biophysics of cuticular waterproofing led to an interest in the evolutionary significance of variation in surface lipids. Physiologists have generally assumed that the traits they study are the product of natural selection, but rigorous testing of adaptive hypotheses is rare. I use a variety of approaches to study how physiological processes evolve in nature and in the lab. These include phylogenetic analyses of different species, comparisons of different geographic populations within a species, and field studies of the environmental conditions actually experienced by organisms in nature. My major approach, however, has been the use of experimental evolution. In the lab, I study how physiological processes evolve in model organisms subjected to selection for stress resistance under well-defined conditions. These conditions are designed to imitate those environmental stresses we think are important in nature.

Current Research Projects

In my early work, I demonstrated that laboratory selection for desiccation resistance in Drosophila melanogaster results in flies that have evolved some of the expected physiological differences (e.g. lower water-loss rates), but not others (e.g. tolerance of low body-water content). We have since found that desiccation selection also results in flies that are less active and store very large quantities of glycogen. Most importantly, glycogen storage begins in the larval stage, with an extended third-instar feeding period. Desiccation selection is imposed upon the adults; our work demonstrates that selection acts indirectly on other life-history stages. Transcriptome analysis of the desiccation-selected lines is in progress.

We recently founded a new suite of starvation-selected lines. After ten generations they had already evolved to survive twice as long without food than control lines. They contain very large amounts of lipids and carbohydrates, all of which they accumulate as larvae. The primary energy storage site in insects is the fat body, and larval fat body is one of the few larval tissues that persists into the adult after metamorphosis. We have shown that, even in wild-type flies, larval energy storage is important for adult starvation resistance. In the starvation-selected lines, the larval stage is much longer than in unselected control populations, and tissue remodeling of the larval fat body appears to be disrupted. This has significant life history consequences, with fecundity being reduced by nearly 50% in the selected populations.

Although experimental evolution is a powerful tool to understand the process of adaptation, the ultimate goal of my research program is to understand how insects deal with environmental stress in nature. To that end, I am working with Dr. William Etges (University of Arkansas) to study the desert fruitfly, Drosophila mojavensis. This is the only desert organism for which a whole-genome sequence and microarrays are available. It is also highly resistant to a variety of stresses (heat, cold, desiccation, starvation, ethanol, etc.), making it the “bionic” fly. This provides us a unique opportunity to investigate the physiological and molecular mechanisms of adaptation to an extreme environment. We have completed several large experiments (>100 microarrays each) concerning the effects of environmental stress on gene expression. Ultimately, we want to use this information to gain insight into what flies actually do in nature, by analyzing the transcriptomes of wild-caught animals. We recently established a field site near Las Vegas, and we have begun establishing laboratory populations and collecting climate (temperature and humidity) data from this site.