Expression pattern of the zona pellucida 3 (ZP3) gene during ovarian development and the location of ZP3 protein in oocytes in a natural, wild triploid crucian carp mutant, Carassius auratus var. Pingxiangnensis

Carassius auratus var. Pingxiangnensis (designated CaP), distributed in the Pingxiang region of Jiangxi Province, China, is a natural, wild triploid crucian carp mutant that has two reproductive development modes: gynogenesis and bisexual reproduction. Little information is available about the expression pattern of the zona pellucida 3 (ZP3) gene during ovarian development and the location of the ZP3 protein in oocytes of this fish. In this study, we obtained the full-length cDNA of ZP3 (CaP_ZP3). CaP_ZP3 contains an open reading frame of 1305 bp that encodes 435 amino acid residues. Real-time polymerase chain reaction (PCR) was used to determine the CaP_ZP3 mRNA expression levels in the ovary at different stages of maturation. Results revealed high levels of CaP_ZP3 expression in 4- to 8-month-old ovaries (stage II-stage III), with a significant decline in 9- to 12-month-old ovaries (stages IV-stage V). The high levels of CaP_ZP3 transcripts during the early growth period suggest an important role for CaP_ZP3 in early oocyte development. In addition, a polyclonal antibody was prepared against CaP_ZP3, and the immunofluorescence localization was determined. CaP_ZP3 protein was detected close to the oocyte plasma membrane. The results also showed that no fluorescent signal was detected in stage I and II oocytes. CaP_ZP3 protein is primarily detected in stage III oocytes, and the protein accumulates as oocytes develop into stage IV oocytes. These results suggested that the transcription and translation of the CaP_ZP3 gene is asynchronous and that the transcription of the CaP_ZP3 protein occurs prior to its translation in this triploid fish.