The expression pattern of scygonadin during the ontogenesis of Scylla paramamosain predicting its potential role in reproductive immunity.

The antimicrobial peptide scygonadin (Scy) was first isolated from the gonad of Scylla serrata and its gene is predominantly expressed in the ejaculatory duct of adult males. Thus, its function was predicted to be associated with reproductive immunity, but this is still unclear and needs further investigation. In our study, the expression pattern of Scy at different developmental stages of both male and female S. paramamosain was investigated, so that the potential function of this peptide could be examined. Using real-time quantitative PCR, Scy mRNA transcripts were demonstrated obviously in the vulnerable embryos and larvae-zoea I but very weakly detected in the larvae-zoea III, megalops and juveniles. The gene expression pattern showed a decreasing trend during the early developmental stages. The Scy gene had low expression in the ejaculatory duct of small and medium crabs (100g and 200g in weight) whose gonads were underdeveloped. However, the level of Scy expression was significantly increased in large crabs (300g in weight), which had normally become sexually mature at this size. It was further observed that the numbers of Scy mRNA transcripts in sexually mature crabs were significantly more abundant than in immature ones. In addition, the Scy gene was significantly expressed in the ejaculatory duct of mature male crabs during the mating period (April and May) and reached their highest expression in May. Using immunohistochemistry, the Scy protein was strongly detected in the testis and seminal vesicle of small crabs. However, in large crabs, Scy protein was intensively present in more tissues than in small crabs, including the ejaculatory duct, posterior ejaculatory duct, gill and muscle of males, and also in the spermatheca, gill and muscle of females. It is also interesting to note that Scy mRNA transcripts were detected in other crab species and showed similar expression pattern to those in S. paramamosain. This study extended our knowledge concerning the antimicrobial peptide scygonadin, which has its function principally in the ejaculatory duct of males but which may also play a role at different developmental stages of S. paramamosain from embryogenesis to maturation, and is also widely distributed in other crabs.