Plastic and evolved responses of larval tracheae and mass to varying atmospheric oxygen content in<i>Drosophila melanogaster</i>

Abstract

Structural changes in the tracheal system during development have the potential to allow insects to compensate for varying oxygen availability. Despite possible compensation, oxygen level during development may also affect insect body size. We investigated how atmospheric oxygen level affects the dimensions of the main dorsal tracheae (DT) and masses of larval Drosophila melanogaster (Meigen) reared for up to six generations in 10%, 21% or 40% O2 at 25 degrees C. Wandering-stage third-instar larvae were weighed every other generation, and the dimensions of the DT were measured. Hypoxia produced significantly lighter larvae after one generation of exposure, while hyperoxia did not affect larval mass. Atmospheric oxygen content did not significantly change the diameters of the anterior portions of the main tracheae; however, the posterior diameters were strongly affected. During the first generation of exposure, tracheal diameters were inversely proportional to rearing oxygen levels, demonstrating that developmental plasticity in DT diameters can partially (8-15%) compensate for variation in atmospheric oxygen level. After multiple generations in differing atmospheres and two further generations in 21% O2, larvae had tracheal diameters inversely related to their historical oxygen exposure, suggesting that atmospheric oxygen can produce heritable changes in insect tracheal morphology.