REYNOLDS, J.A.^*; HAND, S.C.: Depression of aerobic metabolism during diapause: insights from mitochondria of Artemia franciscana embryos

In response to cues that signify onset of winter, female brine shrimp, A. franciscana, switch from ovoviviparous reproduction to oviparous production of encysted embryos that enter diapause. Diapause is a pre-programmed state of developmental arrest that is often accompanied by metabolic depression and provides organisms a means of tolerating environmental insult. Fully hydrated A. franciscana embryos that are in diapause are remarkable in their ability to reduce oxygen consumption to barely detectable levels under conditions of normoxia (Clegg, Drinkwater and Sorgeloos, 1996, Physiol. Zool., 69: 49-66). The mechanisms associated with this extreme metabolic depression are unclear. Here we show that mitochondria isolated from metabolically depressed, diapause embryos exhibit a respiratory control ratio (RCR) that is statistically the same as that of mitochondria from 8 h post-diapause embryos with active metabolism [5.23 � 0.09 (n = 20) for diapause and 5.17 � 0.11 (n = 24) for active, mean � SEM]. Rates of state 3 and state 4 respiration of mitochondria isolated from diapausing embryos are only about 30% lower than those of mitochondria from actively developing embryos. Electron micrographs of embryos in diapause reveal mitochondria that are structurally intact and well differentiated, very similar to those of active embryos. These data suggest that metabolic depression during diapause cannot be fully explained by mitochondrial degradation or loss of oxidative capacity, but rather they indicate that a mechanism for active depression of mitochondrial metabolism may be operative in vivo. [NSF Grant IBN – 9723746]