HENRY, J.R.*; HARRISON, J.F.; Arizona State University, Tempe; Arizona State University, Tempe: Plastic and evolved responses of tracheal dimensions to varying atmospheric oxygen content in Drosophila melanogaster.

Structural changes in the tracheal system during development have the potential to allow insects to compensate for varying atmospheric oxygen levels. We investigated how oxygen availability affects the dimensions of the main dorsal tracheae of fruitfly larvae. Larvae, which do not possess air sacs, may make compensatory enlargements in tracheal diameter to maintain oxygen delivery to cells during hypoxia and conversely may reduce tracheal diameters to prevent oxidative stress during hyperoxia. In this study, we raised a strain of D. melanogaster for up to 6 generations in 10%, 21%, or 40% O₂ at 25&degC. In an oxygen selection experiment, flies were randomly mated, while in an artificial selection experiment, females were selected for large adult body mass upon eclosion. Third instar larvae were weighed every other generation and tracheal dimensions were analyzed under a dissecting microscope. After 5-6 generations, all lines were placed in 21% O₂ to determine if evolution or plasticity was responsible for the observed changes. Regardless of selection parameters, tracheal diameters increased with larval mass. Atmospheric oxygen content did not noticeably change the diameters of the anterior portions of the main tracheae; however, the posterior diameters near the functional spiracles did differ significantly. Within one generation, tracheal diameter was inversely proportional to rearing oxygen levels, demonstrating that tracheal morphology can at least partially compensate for oxygen availability. This linear effect of oxygen on tracheal diameter was maintained even after the lines had been returned to normoxia for two final generations indicating that oxygen acted as a selective pressure driving the evolution of cross-sectional tracheal areas.