Abstract: Using Daphnia pulex as the researching object, qPCR was used to analyze the changes of expression level of caspase-3 mRNA in different culture temperatures, population densities and food types; then the enzyme activities of Caspase-3 under three different culture conditions were measured using the specific substrate Ac-DEVD-AMC; in situ hybridization was used to detect the localizations and changes of caspase-3 mRNA in different reproductive states of D. pulex. The main results were as follows: expression levels of caspase-3 mRNA were up-regulated with the increase of temperature and population density, but there was no obvious changes of caspase-3 mRNA expression in the food type group. In addition, qPCR results were in good agreement with the result of enzyme activity assays. Whole-mount in situ hybridization showed that caspase-3 was expressed in living cells of D. pulex at different growth stages and was highly expressed in adult individuals, which was essentially consistent with the result of qPCR. The expression of caspase-3 gene was relevant to changes of environmental factors, while aging of D. pulex was related to the expression of caspase-3 mRNA and the enzyme activity of Caspase-3. Thus, further research is needed to explore how environmental factors affecting the expression of caspase-3 mRNA, which subsequently regulates cyclical parthenogenesis in Daphnia.

Key words: Daphnia pulex, caspase-3, mRNA expression, enzyme activity, in situ hybridization