F. heteroclitus embryos were treated from 1 to 7 dpf with DMSO (0.01%, vehicle), TBT (Medium: 50 nM and High: 100 nM) and/or PCB126 (61 pM) in seawater and then moved to a filter paper moistened with clean water until 10 dpf. mRNA expression was measured by RT-qPCR in pooled embryonic killifish. (a) Basal cyp1a expression in SC and NBH embryos. (b) Chemical-induced cyp1a expression. Two Factor ANOVA: Population p = 0.015, Treatment p < 0.001. Interaction: p = 0.190. Each pool consisted of 5 embryos. N=6 pools per chemical treatment, per population from three experimental replicates from SC and NBH. Data points of the same color are from biological replicates produced in the same experiment. * Significantly different from DMSO in the same population (p<0.05, ANOVA, Dunnett’s)