Background
My training is in environmental physiology, particularly the physiology of lipid-based systems such as cell membranes and insect cuticular lipids. It has long been believed that the function of these systems is dependent on the physical properties of the component lipids, but it is not always clear which physical properties are most important. I have therefore used biophysical techniques (fluorescence polarization, FTIR spectroscopy) to perform mechanistic studies of how lipid systems work, then used this functional information to understand the consequences of lipid variation at the organismal, population, or inter-specific level.
My other major research interest is evolutionary physiology. 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. 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.
Study Systems
Most of my recent research efforts have been directed to the physiology and evolution of insect water balance, using grasshoppers (Melanoplus sanguinipes) and Drosophila as my study systems. Individual M. sanguinipes vary greatly in the composition and physical properties of their cuticular lipids, which provide the primary barrier to evaporative water loss. In the field, grasshoppers thermoregulate at temperatures high enough to partially melt the surface lipids, and we have demonstrated that this melting causes a rapid increase in water loss. Populations from lower latitudes and altitudes have higher lipid Tm values, suggesting that there has been local adaptation to warmer habitats, and Tm values increase at higher rearing temperatures. Interestingly, genetic variation affects different steps of lipid biosynthesis than thermal acclimation, so that organismal and evolutionary responses to the environment occur via different mechanisms. We have thus developed an integrated understanding of lipid variation in M. sanguinipes, from population-level differences in lipid properties, to acclimatory changes within individuals, to the organismal effects of lipid melting on rates of water loss, to the biosynthetic basis for differences in lipid composition.
In Drosophila melanogaster, I have performed physiological analyses of replicate populations subjected to laboratory selection under two types of water stress: desiccation stress in the adult stage or osmotic stress as larvae. Comparative studies suggest how these populations should adapt; my interest has been to determine whether they evolve as expected. They do not. For example, desiccation-selected D. melanogaster accumulate large stores of water, but desert Drosophila species do not. Instead, desert flies are more tolerant of low water content than mesic species, a difference we do not find under laboratory selection. Laboratory populations have also evolved in unexpected directions, in some cases providing novel insights into mechanisms of stress resistance.
Current Research Projects
I have two major research goals. First, I am attempting to integrate laboratory and comparative studies of water balance in Drosophila more directly. Our differing results in the two types of studies indicate that the selective forces acting in each situation are not as clear as one might think. Our major immediate goal is to understand how stress affects gene expression, using DNA microarrays to analyze expression patterns in desiccation-selected populations of Drosophila melanogaster. We are performing simultaneous physiological measurements to try to link changes in gene expression to processes at the organismal level. Ultimately we want to know whether desert and mesic species exhibit similar responses to environmental stress, and use microarrays or other molecular approaches to identify which types of stresses are ecologically relevant.
My other major interest is the use of experimental evolution to test functional hypotheses about cell membranes, using E. coli as a model system. I have remained interested in this subject since my dissertation work on membrane enzymes in deep-sea fishes. Several biophysical models for membrane adaptation to temperature have been proposed, emphasizing different aspects of membrane behavior (viscosity, phase state, etc.). However, decades of comparative studies have been unable to distinguish which is most correct. I have begun to study membrane properties in replicated lines of E. coli, that have been selected at temperatures ranging from 20 to 42 oC for 2000 generations or more. I also plan to develop my own lines, selected under additional membrane-perturbing conditions. Each model makes different, quantitative predictions about the outcome of membrane evolution, so these lines provide a strong test of the competing models.