Is oxygen limitation a cue for the initiation of molting in Drosophila? Viviane Callier¹, Colin Brent², Jinkyu Kim¹, Shampa M. Ghosh³, Alexander W. Shingleton³, Jon F. Harrison¹. 1) School of Life Sciences, Arizona State University, Tempe, AZ; 2) USDA-ARS Arid-Land Agricultural Research Center, Maricopa, AZ; 3) Department of Zoology, Michigan State University, East Lansing, MI.

   Body size profoundly affects many aspects of animal biology, yet it remains one of the fundamental unsolved problems of developmental biology. Attainment of a critical weight causes Drosophila larvae to terminate growth and initiate metamorphosis, processes regulated by a hormonal cascade. Although the endocrinology of molting is well understood, the sensory mechanisms used by a larva to determine the timing of the hormonal cascade is largely unknown. The result is a conspicuous gap in our understanding of the mechanisms that regulate body size. A putative developmental pacer is oxygen availability. As larvae grow through an instar, oxygen supply structures are largely fixed, but metabolizing tissues increase in mass. As the ratio of demand to supply grows, internal hypoxia may occur, serving as a physiological cue to initiate a molt. To test for late-instar oxygen limitation, we measured respiration rates of Drosophila larvae across the third instar. Our data indicate that respiration rates level off at the critical weight, consistent with the hypothesis of oxygen-limitation. To test whether larvae experience internal hypoxia late in the instar, we used a GFP reporter to assess the activity of Hypoxia-Inducible Factor (HIF). We found that HIF activity is increased in late-instar larvae relative to early-instar larvae. To further test the role of internal hypoxia in pacing development, early instar larvae were reared in either hypoxic (10%) or normoxic conditions. Hypoxia induced earlier metamorphosis and a reduced critical weight. Oxygen deprivation also caused an elevation in ecdysteroid levels, the hormones driving the molting process. Collectively, these data support oxygen limitation as a cue used by Drosophila to regulate developmental pacing. The research was supported by NSF IOS 1122157 to JFH, and IOS 0845847 to AWS.