A new antimicrobial peptide SCY2 identified in Scylla Paramamosain exerting a potential role of reproductive immunity

A new antimicrobial peptide named SCY2 with 65.08% identity in amino acid sequence to the known scygonadin (SCY1) was first characterized in Scylla paramamosain based on its cloned full-length cDNA and genomic DNA sequences. The SCY2 gene was dominantly expressed in the ejaculatory duct of male crabs and its mRNA transcripts were discerned mainly in the glandular epithelium of the inner wall and the secretion inside the ejaculatory duct. Although the SCY2 gene could not be induced with the challenge of the bacteria and fungi tested, its induction reached the highest level at the peak period of mating in mature male crabs either in June or November, suggesting its induction was likely related to seasonal reproduction changes. Moreover, it was interesting to note that, from analysis of its transcripts and protein, SCY2 was significantly expressed only in the ejaculatory duct of pre-copulatory males before mating, however it was clearly detected in the spermatheca of post-copulatory females after mating accompanied by the decreased level of SCY2 expression in the ejaculatory duct. These results suggested that the SCY2 was probably transferred from the male during mating action with the female for the purpose of protecting fertilization. The recombinant SCY2 was more active against the Gram-positive than the Gram-negative bacteria tested. It was further observed that the SCY2 transcripts were significantly increased with addition of exogenous progesterone in tissue cultures whereas the several other hormones tested had no any effect on SCY2 expression, indicating that there might be a relationship between the SCY2 expression and the induction of hormones in vivo. In summary, this study demonstrated that one role of SCY2 was likely to be involved in crab reproduction and it exerted its reproductive immune function through the mating action and the maintenance of inner sterility in the spermatheca of the female, thus leading to successful fertilization of S. paramamosain.