Differential sensitivity of zonagenesis and vitellogenesis in Atlantic salmon (Salmo salar L) to DDT pesticides

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Abstract
In Atlantic salmon (Salmo salar L) female sexual maturation entails both zonagenesis and vitellogenesis, both of which are controlled by increasing levels of estradiol‐17β (E2). Antibodies against salmon zona radiata proteins (eggshell zr‐proteins) and vitellogenin were used to monitor induction of oogenesis in juvenile salmon. Molecular weights of zr‐monomers were estimated to about 66, 61, and 55 kDa, and to about 180 kDa for vitellogenin. Xenobiotics such as the pesticide DDT impair biological reproduction. The o,p′‐DDT (1,1,1‐trichloro‐2‐[2‐ chlorophenyl]‐2‐[4‐chlorophenyl]ethane) isomer seems to be a xenoestrogen. Serum levels of zr‐proteins and vitellogenin, and hepatocytic biosynthesis of these components, were determined after in vivo treatment of salmon with DDT (technical, p,p′‐(1,1,1‐trichloro‐2,2‐bis[4‐chlorophenyl]ethane) or o,p′‐DDT) or E2. Exposing fish to frequent doses of o,p′‐DDT (25 mg/kg b.w. (Body weight) twice a week, six times totally) resulted in induction of all three zr‐protein monomers, but not of vitellogenin. In contrast, three weekly injection of 10 mg/kg b.w. of either of the three DDT preparations did not induce typical zr‐proteins or vitellogenin in serum. In vivo studies with combined DDT + E2 injections showed that none of the DDT preparations influenced E2‐induced biosynthesis of zr‐proteins or vitellogenin. E2 induction of these oogenetic processes was not blocked even by a high concentration (125 mg/kg b.w.) Of o,p′‐DDT. Furthermore, pretreatment of salmon with o,p′‐DDT for 2 weeks, followed by one injection of E2, did not antagonize biosynthesis of zr‐proteins, but serum concentration of vitellogenin was decreased. The data indicate that in juvenile salmon o,p′‐DDT may be xenoestrogenic with regard to zonagenesis, but weakly anti‐(xeno)estrogenic with regard to vitellogenesis. These findings suggest new complexities in fish reproductive toxicology of xenoestrogens. Compared to vitellogenesis, zonagenesis is a more sensitive parameter for monitoring reproductive effects of xenoestrogens. J. Exp. Zool. 281:346–353, 1998.