Melatonin, a chronobiotic molecule, is known to modulate several physiological functions in crustaceans including reproduction, molting and
glucose homeostasis. In our earlier studies (Sainath and Reddy, 2010a), we observed
hyperglycemia in crabs after
melatonin administration and concluded that
melatonin is another
crustacean hyperglycemic hormone. In the current study, we have further examined the role of
melatonin in regulating the levels of
methyl farnesoate and
ecdysteroid in the giant mud crab Scylla serrata and determined that
melatonin indeed is a reproductive
hormone. Further, we have determined partial nucleotide sequences of
retinoid X receptor (RXR) and
ecdysone receptor (EcR) in S. serrata and also studied the effect of
melatonin on expression of these genes. Cloned RXR and EcR possess high sequence similarity with other Brachyuran genes. Administration of
melatonin elevated circulatory
methyl farnesoate (MF) and
ecdysteroid levels in crabs. Since MF and
ecdysteroid act through RXR and EcR respectively and these receptors are involved in the regulation of reproduction in crustaceans, we measured the expression levels of RXR and EcR in hepatopancreas and ovary after
melatonin administration. The expression levels of both RXR and EcR increased significantly in the hepatopancreas and ovary of
melatonin injected crabs when compared to the controls. In vitro culture of mandibular organ (MO) and Y-organ (YO) in the presence of
melatonin resulted in a significant increase in the secretion of
methyl farnesoate and
ecdysteroid respectively. From the above studies it is clear that
melatonin stimulates YO and MO, resulting in increased synthesis of
ecdysteroids and
methyl farnesoate, and thereby inducing reproduction in S. serrata.