Ecological and evolutionary physiology of Manduca sexta
My lab focuses on the physiological ecology of insects. Physiological ecology studies how organisms and environments interact, with emphasis on physiological aspects of the interaction--including how organisms use physiology to respond to environmental change, whether in new environments plasticity or evolution is more important, and how physiological tradeoffs and constraints shape ecological interactions. Our work stems from our basic interest in insect metabolism. Metabolism is interesting because it couples physiological events in an insect's body to the traffic of gases, materials, and energy between the insect and its environment--that is, metabolism is the central integrator of physiology and ecology. Over the past few years, my lab has come to understand several gas-exchange processes in detail. We are now in the process of moving our discoveries back into an ecological context--in particular, focusing on how insects and host plants interact.
Complex life cycles and model insects. Insects have life cycles characterized by multiple, distinct life stages. The majority, the holometabolous insects, have 4-stage life cycles--egg, larva, pupa, and adult--that follow one another in sequence (if all goes well). For a physiological ecologist, such life-cycle complexity is dazzling because each stage interacts with a different environment. That is, before reproducing an insect must perform well in each stage, in the process reinventing itself multiple times. Yet each reinvention stems from the same genome. Over the next decade, I intend to work out the physiological ecology of a complex life cycle for at least one species (Manduca sexta), with explicit focus on cross-stage constraints. We will ask questions like: How strong are the physiological legacies of one stage in the next? Does evolution of physiology in one stage affect the physiological ecology of other stages?
Studying the whole-lifecycle will be non-trivial, and we need some way of breaking it into manageable chunks. We're doing this by working on life stages that we perceive to be neglected--eggs and pupae. In the past few decades, most work on insect physiology and ecology has focused on the obvious stages--larvae and adults. Studying larvae and adults makes sense because they behave, eat, mate, oviposit, and other activities of obvious basic or economic interest. By contrast, work on eggs and pupae is virtually nonexistent, with a few notable exceptions. We think that studying eggs and pupae should be enormously interesting and productive for three reasons. First, both stages are nonfeeding; they enter the stage with all materials and energy they will have until the next stage. Second, eggs and pupae are immobile. Immobility means that they have to deal with environmental change using only physiology; they can't move to a more suitable place. Third, eggs and pupae both support important developmental events, and environmental differences during that development likely lead to developmental plasticity.
The physiology of gas exchange in the egg stage. For the past few years, we have studied oxygen, carbon dioxide, and water vapor exchange by eggs of M. sexta. This work has focused on two themes--temperature-oxygen interactions and oxygen-water tradeoffs. By temperature-oxygen interactions, we mean that how eggs obtain and use oxygen depends on how hot they are. In particular, we have shown that hot eggs are oxygen limited even in normoxia. This result is surprising because eggs are so small--only about a millimeter in diamater. It turns out that they eggs live on the edge of oxygen starvation because their eggshells are so impermeable to gases. Why so impermeable? We think impermeability is a mechanism for saving water. That is, permeable eggshells that let in lots of oxygen also let out lots of water, which can desiccate eggs. Eggs deal with this oxygen-water tradeoff by controlling eggshell permeability over development--in effect, they make the eggshell permeable only toward the end of the development, when the embryo's metabolic demand is rising sharply. We have shown experimentally that embryos can increase permeability in response to ambient hypoxia.
Insect eggs on host leaves. Coming soon.
The physiology of an extended phenotype. Coming soon.
