Progestogen is a potential trigger initiating sex inversion in rice field eel (Monopterus albus).

The rice field eel or swamp eel (Monopterus albus) is a hermaphroditic freshwater teleost fish that transitions from a functional female to a functional male. However, the biological mechanism of sex inversion in this species remains unclear. Here we clone the 5' proximal flanking region of the rice field eel dmrt1 gene and find that the progesterone response element (PRE) and androgen response element (ARE) uniquely exist in this region. More importantly, based on reporter gene analysis, in vitro tissue culture, and progestogen hormone level assays, we reveal that progestogen is the prerequisite for the sex inversion of the rice field eel, and androgen plays an important role in the positive regulation of the sex inversion process. Collectively, our results suggest that progestogen is the trigger that initiates the rice field eel's sex trans-differentiation during final oocyte maturation and clarifies the biological mechanism of sex inversion in the rice field eel.