Temperature-oxygen interactions in developing eggs of a moth

Insect embryos support metabolism with oxygen obtained from the environment by diffusion across the eggshell. Thus the oxygen partial pressure (PO2) inside an egg depends on relative rates of oxygen supply and demand. Temperature affects these two processes differently: metabolism is relatively more temperature-sensitive than diffusion, suggesting that embryos should be relatively oxygen-limited at high temperatures. We tested whether survival, development, and metabolism of eggs of a sphingid moth, Manduca sexta, were sensitive to variation in ambient oxygen availability (5 – 50% O2), across a biologically realistic range of temperatures (22 – 37 ºC). Temperature-oxygen interactions were apparent in all experiments. Eggs tolerated hypoxia better at cool temperatures. CO2 emission was insensitive to variable oxygen at 22 ºC but strongly influenced by both hypo- and hyperoxia at 37ºC. We also measured radial profiles of egg oxygen content using an oxygen microelectrode. These data showed steep edge-to-center oxygen gradients, and in 3-d old eggs, broad central areas where PO2 was less than 2 kPa. In addition, high temperatures led to significantly lower internal PO2s. These data indicate that at realistically high temperatures (32 – 37 ºC), eggs of M. sexta were oxygen limited even in normoxia. This result has important implications for the evolution of eggshell structures and suggests a novel hypothesis about the co-occurrence of hyperoxia and insect gigantism during the Paleozoic