WOODS, HA; HILL, RI; Univ. of Texas, Austin; Univ. of Texas, Austin: Temperature-oxygen interactions in developing eggs of a moth, Manduca sexta

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 �C). Temperature-oxygen interactions were apparent in all experiments. Eggs tolerated hypoxia better at cool temperatures. CO₂ emission was insensitive to variable oxygen at 22 �C but strongly influenced by both hypo- and hyperoxia at 37�C. 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 �C), 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.