ONTOGENY OF LOW-MOLECULAR-WEIGHT STRESS PROTEIN P26 DURING EARLY DEVELOPMENT OF THE BRINE SHRIMP, ARTEMIA-FRANCISCANA

Embryogenesis in the brine shrimp, Artemia sp., occurs by one of two p athways: (i) the direct, uninterrupted development of nauplius larvae within the female or (ii) the production of embryos that arrest develo pment at the gastrula stage and enter diapause. Diapause embryos are r eleased from females into the aqueous environment where they remain in diapause until activated by appropriate environmental cues and resume development. These encysted embryos possess at least one low molecula r weight stress protein, which we refer to as p26 and which has been i mplicated previously in the stress response of activated embryos. We i nvestigated the appearance of p26 in developing diapause embryos in ut ero and looked for its presence in embryos developing directly into na uplii. We found p26 to be specific to diapause-destined embryos; it wa s not detected in direct-developing embryos. We conclude that p26 is n ot required for the basic developmental program that produces the naup lius. In diapause-destined embryos, p26 was first detectable after 3 d ays of development, at which time the embryos were late gastrulae. Thi s protein continues to increase in amount until the encysted embryos a re released, approximately 5 days after fertilization. At the time of release almost all p26 is located in the low speed supernatant fractio n, but as released embryos continue diapause, p26 transfers to the pel leted nuclear fraction in increasing amounts. Our working hypothesis v iews p26 as a molecular chaperone preventing protein denaturation and aggregation under conditions associated with metabolic arrest and othe r stressful states, which these encysted embryos encounter.