Patterns of Enzyme Ontogeny in Developing Sunfish

Quantitative changes in the activities of twelve different enzymes have been investigated during early development in both the warmouth and the green sunfish. The overall patterns of enzyme ontogeny are quite similar for all enzymes studied in the two species in spite of the fact that the precise quantitative and temporal aspects of enzyme changes are often somewhat different. Thus, enzymes involved in glycolysis, ATP metabolism, and mitochondrial shuttle systems are initially present at relatively low levels at the time of fertilization. The enzymes increase to a maximum activity (per embryo) after hatching when the fry exhibit rhythmic jaw movements, but before they become free-swimming, and subsequently decrease during the latest developmental stages which have been studied. In contrast, the initial changes in enzyme activity, characteristic of embryonic development, represent three distinct patterns. Some enzymes, such as NADP-dependent isocitrate dehydrogenase, are at nearly undetectable levels at fertilization and begin to increase in activity within hours. This pattern suggests a minimal maternal contribution of enzyme activity to the developing zygote, combined with the initiation of synthesis of new enzyme molecules during the earliest embryonic stages. The activity of pyruvate kinase and of several other enzymes remains essentially constant throughout embryonic development and only begins to increase at about the time of hatching. This pattern is consistent with the persistence of significant amounts of stable enzyme of maternal origin during embryogenesis coupled with the synthesis of new enzyme molecules at hatching. Finally, enzymes such as creatine kinase exhibit marked decreases in activity during early embryonic development and then subsequently increase in activity. Initial rapid turnover of maternally derived enzyme followed by embryonic synthesis of new enzyme molecules appears responsible for this pattern. Studies of malate and lactate dehydrogenase in single embryos reveal that the variance in activity among individuals is much less during the period of early embryonic development than it is after hatching. The elevated heterogeneity coincident with increasing enzyme activity is probably due to asynchrony in the timing of gene activation and/of unequal rates of enzyme realization during post-hatching development.