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%. This research will test this hypothesis with a series of experiments that examine the effect of single- and multi-generation exposure to different atmospheric O2 levels on insect size, developmental rate, tracheal structure and function. Most of these experiments use fruitflies (Drosophila melanogaster), but one broad comparative study of the developmental plasticity of 16 insect species in response to variation in atmospheric O2 levels is included.
The first goal of this research is to test whether D. melanogaster evolve different body sizes in response to variation in atmospheric O2 level (10, 20, 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. A third goal is to test for O2 delivery limitations, and potential compensatory responses of the respiratory system to variation in atmospheric oxygen level in fruitflies. Specifically these experiments will examine respiratory responses to rearing flies in 10, 20 and 40% O2 for one generation, multiple generations, or when selecting for large size. Atmospheric O2 effects on the capacity of the tracheal system to deliver oxygen will be measured morphologically with electron microscopy, and physiologically by measuring the lowest O2 level that permits normal metabolic rate. A fourth goal is to test three non-alternative hypotheses for why rearing O2 level affects individual fruitfly size: 1) The Direct O2 Limitation (DOL) Hypothesis that increasing O2 availability increases larval growth rates by increasing nutrient intake rates, 2) The O2 Cue (OC) Hypothesis, that increasing oxygen levels extend development rate by delaying the initiation of molting, and 3) The Cell Size (CS) hypothesis, that higher O2 levels increase fly size by increasing cell size at constant cell number. Finally, we will test for the generality of rearing O2 level on insect size and development rate. This comparative study of 16 species will also test whether O2 effects on these variables are influenced by insect size, developmental pattern, or habitat.
This project involves a unique system in which we can quantify the degree of physiological constraint (O2 availability) on the evolution of major life history traits (body size, developmental rate), and thus will be of interest to a wide array of evolutionary biologists, physiologists and ecologists. In addition, the possible control of atmospheric O2 on insect size (and historical insect gigantism) is of substantial interest to many non-biologists including paleogeologists, environmental scientists and the general public. Results will be widely disseminated through reviewed scientific papers in physiological and evolutionary journals as well as more general outlets such as Scientific American or Natural History. A web site on insect respiratory physiology and oxygen effects on insect size will be created and linked to the Insect Physiology On-line web site (http://lamar.colostate.edu/%7Einsects/index.html). Finally, this award will also fund postdoctoral, graduate, and undergraduate training programs for individuals from groups currently under-represented among biology professions.
