Diapause termination and development of encysted Artemia embryos: roles for nitric oxide and hydrogen peroxide

Encysted embryos ( cysts) of the brine shrimp Artemia undergo diapause, a state of profound dormancy and enhanced stress tolerance. Upon exposure to the appropriate physical stimulus diapause terminates and embryos resume development. The regulation of diapause termination and post-diapause development is poorly understood at the molecular level, prompting this study on the capacity of hydrogen peroxide (H2O2) and nitric oxide ( NO) to control these processes. Exposure to H2O2 and NO, the latter generated by the use of three NO generators, promoted cyst development, emergence and hatching, effects nullified by catalase and the NO scavenger 2-phenyl-4,4,5,5,-tetramethylimidazoline-1-oxyl-3-oxide ( PTIO). The maximal effect of NO and H2O2 on cyst development was achieved by 4 h of exposure to either chemical. NO was effective at a lower concentration than H2O2 but more cysts developed in response to H2O2. Promotion of development varied with incubation conditions, indicating for the first time a population of Artemia cysts potentially arrested in post-diapause and whose development was activated by either H2O2 or NO. A second cyst sub-population, refractory to hatching after prolonged incubation, was considered to be in diapause, a condition broken by H2O2 but not NO. These observations provide clues to the molecular mechanisms of diapause termination and development in Artemia, while enhancing the organism's value in aquaculture by affording a greater understanding of its growth and physiology