During the life history of cladocera (e.g. Daphnia), their growth and molting are alternately [1]. The molting action runs through their whole life cycle, and it is a necessary step before they grow and reproduce [2]. Molting is a result of long-term evolution in arthropod, which are regulated by many factors [3]. In crustacean ecdysis, ecdysteroid is the most important regulatory factor, which the expression levels vary among species [4]. Among ecdysones, 20-hydroxyecdysone (20E) is one of the more active hormones in insects [5]. Moreover, ecdysone can not only regulate the molting physiology in arthropods, but also play important roles in their growth, reproduction and phenotypic plasticity [6, 7, 8, 9, 10].
The synthetic pathway of ecdysone has been extensively studied in insects [11, 12]. Usually, the synthesis of insect ecdysone is divided into two stages. Firstly, the cholesterol in food was digested and absorbed through the intestine, and then transported to the prothymus (PG) by hemolymph. The cholesterol was transformed to 5β-diketol (3D2, 22, 25dE) under the catalysis of both Neverland and CYP307A1 gene [13, 14, 15]. Secondly, the 5β-diketol was converted to inactive ecdysterone catalyzed by various cytochrome P450s (CYP306Al, CYP302Al, and CYP315Al) [16, 17, 18, 19]. The inactive ecdysone could be also converted to 20-hydroxyecdysone (20E) under the catalysis of the CYP314A1 gene [20]. Among them, theCYP307A1 (Spook, Spo), CYP306A1 (Phantom, Phm), CYP302A1 (Disembodied, DIB), CYP315A1 (Shadow, Sad) and CYP314A1 (Shade, Shd) gene are referred to as the Halloween genes. The 20E mediates its biological activities through the ecdysone receptor (EcR) complex, a heterodimer consisting of two nuclear hormone receptors, EcR and the retinoid X receptor homologue Ultraspiracle (USP) [21]. It can regulate the downstream primary genes (E75, Br-C, E74 and E93) [22] and secondary response genes (HR3, HR4, HR38 and E78), and then regulate the expressions of terminal genes through FTZ-f1 gene [23]. In Drosophila, the transcript levels of Phm and DIB dropped significantly with the loss of FTZ-f1 function in PG cells [24]. In Daphnia magna, the Neverland, CYP314A1 and CYP307A1 genes had been identified, and their functions had been analyzed [25, 26, 27]. However, the gene expression and functional analysis of CYP302A1, CYP306A1 and CYP315A1 in cladocera (including Daphnia) have not been explored.
As one of the key genes in insect ecdysone synthesis, CYP302A1 that can catalyze the carbon-22 hydroxylase is a member of the mitochondrial cytochrome P450 family [28, 29, 30, 31]. Chavez et al. (2000) found that inactive ecdysone and 20-hydroxyecdysone (20E) hadlower titers in the CYP302A1 (DIB) mutant embryos of Drosophila, and two 20E-inducible genes (IMP-E1 and L1) failed to express in some tissues, resulting in anaphase abnormality in morphology. After RNAi in Sogatella furcifera and Laodelphax striatellus, the expression levels of both CYP302A1 gene and ecdysone receptor gene (EcR) decreased significantly, and the development and death time of nymphs delayed [31]. In spatio-temporal expression profiling of Bombyx mori, CYP302A1 gene showed a higher expression in the ovary, testis and head of the larvae [32]. A few investigations have reported on the genes related to the ecdysone synthesis pathway of cladocera [33, 25, 26, 27], but the molecular mechanisms of ecdysone synthesis pathway and ecdysone signal transduction pathway need still to be further revealed.
The study on molecular biology of Daphnia species has become a hot spot, with the successive reports on the genome of Daphnia pulex and D. magna [34, 35]. In this study, based on the transcriptome, real-time PCR and RNAi technologies, the CYP302A1 gene cloning, and the changes of downstream response gene expressionsand individual phenotypic characteristics after know-down of the gene were analyzed in D. sinensis. Meanwhile, the function of the CYP302A1 gene was discussed. Moreover, the expression sites of the CYP302A1 gene in D. sinensis was also detected by whole mount in situ hybridization technique. Our results will help to clarify the ecdysone synthesis pathway of Daphnia species, and provide a reference for the future study of ecdysis-related signaling pathways.