It has been hypothesized that the giant insects of the late Paleozoic were made possible by high atmospheric oxygen levels, and that current insect body sizes are constrained by our atmospheric oxygen level of 21%. We are testing this hypothesis with multiple approaches including laboratory selection, physiological studies of the mechanisms of oxygen effects on insects, and cross-species comparative studies of tracheal system structure and function.
In our laboratory selection studies, we examine the effect of single- and multi-generation exposure to different atmospheric O2 levels on insect size, developmental rate, tracheal structure and function. We have previously shown that fruitfly (Drosophila melanogaster) body size is reduced in hypoxia (10%) and increased in hyperoxia (40%) when reared for a single generation in these conditions. We are testing whether D. melanogaster evolve different body sizes in response to variation in atmospheric O2 level (10%, 21%, 40% O2) in the lab. A second goal is to determine whether atmospheric O2 level can serve as a constraint on the evolution of large body size in D. melanogaster by selecting for large size in different O2 atmospheres.
Our physiological studies have two main questions: (1) how does oxygen level in the atmosphere affect tracheal system structure and function? And (2) what is the mechanism of oxygen effects on insects? To address (1), we examine the effect of rearing oxygen level on tracheal system structure using x-ray synchrotron imaging, light microscopy and electron microscopy. We test tracheal system function using flow-through respirometry, and measurement of the critical PO2 that limits function. To begin to address (2), we have been examining the effect of oxygen on fruitfly cell sizes, feeding behavior and determining critical developmental periods for oxygen effects on size. Also, we are examining the links between atmospheric oxygen level, oxidative damage and longevity to begin to address how oxygen affects fly fitness.
Insect species differ in their developmental responses to oxygen, with some species (eg. grasshoppers) showing no oxygen effect on size. To try to understand why, we are conducting a broad comparative study of the growth responses of different insects (beetles, caterpillars, cockroaches, etc.) to oxygen. We are also using comparative approaches to test whether larger insects really have more problems with oxygen delivery (does critical PO2 decrease with size?) Finally, we are comparing across- and within-species to test whether larger insects must invest proportionally more in the tracheal system. All of these studies have been partially supported by grants from the National Science Foundation.