METALLOTHIONEIN MESSENGER-RNA ACCUMULATION IN EARLY DEVELOPMENTAL STAGES OF CRASSOSTREA-VIRGINICA FOLLOWING PREEXPOSURE AND CHALLENGE WITH CADMIUM

Embryos and larvae of the oyster Crassostrea virginica were preexposed to Cd concentrations ranging from the control to 0.5 mu M CdCl2, then challenged with the latter concentration for 24 h. The concentration dependence of MT mRNA accumulation during preexposure was related to t he developmental stage of individuals. Embryo/larval stages preexposed for 8 h following fertilization displayed only small increments over the control, whereas similar stages preexposed for the first 24 h afte r fertilization and 1 week old larvae exposed for 24 h exhibited subst antial levels of induced MT mRNA at the higher Cd concentrations. MT m RNA of preexposed individuals in each experimental group was induced t o a common level by challenge to 0.5 mu M CdCl2, regardless of the pre exposure concentration. Embryo/larval stages preexposed to Cd for the first 8 h following fertilization exhibited the full capacity for MT m RNA induction in response to the challenge, regardless of the preexpos ure Cd concentration. When the duration of the preexposure was extende d to 24 h for the embryo/larval and I week old larval stages, the incr eased accumulation of MT mRNA at the higher concentrations reduced the increment of the subsequent accumulation of MT mRNA reached during th e challenge. Preexposure established the concentrations of MT mRNA ini tially present during the challenge, but the effect of preexposure on subsequent MT mRNA accumulation appeared shortlived and not detectable after 24 h in the challenge. Any influence of preinduced MT mRNA in c onferring protection against Cd toxicity in these young stages is like ly related to a kinetic advantage associated with increasing rates of MT synthesis during the initial response to the Cd challenge. (C) 1997 Published by Elsevier Science B.V.