Esterase isozyme patterns in developing embryos of Brachydanio rerio (zebra danio), Brachydanio abolineatus (pearl danio), and their hybrids

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Abstract
The ontogeny of esterase isozymes in Brachydanio rerio (zebra danio), Brachydanio albolineatus (pearl danio), and hybrids formed by their reciprocal crosses was investigated using polyacrylamide disc electrophoresis. Seven esterase isozymes were identified in each species from the unfertilized egg stage to nine days posthatch. Electrophoretic analysis of qualitative changes in enzyme pattern indicated that some esterases were present at all stages of development while other esterases abruptly appeared at a specific stage of morphological differentiation. The esterases of both species were classified on the basis of differential substrate and inhibitor specificities.In developing hybrids formed by B. rerio eggs inseminated with B. albolineatus sperm, the maternal isozyme pattern persisted until Stage 17 (gastrulation). Embryonic extracts from Stage 17 onward showed a slow‐moving, DFP‐sensitive carboxylesterase of paternal origin. In developing hybrids fromed by B. albolineatus eggs inseminated with B. rerio sperm, a paternal contribution to the esterase pattern was probably present by the end of gastrulation; esterase activity of distinctively paternal origin was present by Stage 22 (retinal pigmentation). The maternal contribution to the total esterase profile appeared to remain high through hatching.Additional evidence for gene activity at gastrulation was obtained in experiments utilizing actinomycin‐D and cycloheximide. Results of exposing embryos of B. rerio to 15μg/ml of actinomycin‐D indicated that transcription of the template RNA coding for cholinesterase occurred during gastrulation or some 20–30 hours prior to the appearance of the isozyme at Stage 22. This template RNA was translated sometime during the 10‐hour interval immediately preceding Stage 22.

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