Cytogenetic effects of methylmercury in embryos of the killifish,Fundulus heteroclitus

Fertilized eggs of the killifish,Fundulus heteroclitus, were exposed experimentally to methylmercury (MeHg) to evaluate whether this compound induced cytogenetic effects expressed in the embryos. An additional objective was to assess cytogenetically whetherFundulus embryos from groups that were determined to be more susceptible to the teratogenic effects of MeHg were also more susceptible to its mutagenic effects. Embryos exposed to 0.05 mg/L MeHg for 1 and 7 days post-fertilization were preserved in 10% formalin for cytological examination, while unfixed embryos from the same clutches were evaluated for teratogenic effects. Untreated embyros from each clutch served as controls. Cytogenetic-cytological analyses of fixed treated susceptible embryos that exhibited teratogenic effects revealed decreased mitotic counts (5.0-fold), and increased chromosomal aberrations (2.5-fold) when compared to their controls. There were also decreased mitotic counts (1.5-fold) and increased chromosomal aberrations (2-fold) in embryos from resistant groups when compared to their controls. Exposure to MeHg also retarded development. Forty to fifty percent of the susceptible and resistant control embryos reached the tail-bud stage, but only 28% of the treated resistant and none of the treated susceptible embryos reached this stage. There was also a significant correlation (P < 0.05) between teratogenic and mutagenic effects of MeHg in early stages ofFundulus.