NATURAL SEX CHANGE OF THE “VIRGIN” FEMALE RICEFIELD EEL MONOPTERUS ALBUS

Ricefield eel (Monopterus albus Zuiew) is a protogynous hermaphrodite teleost that naturally undergoes a sex change from a functional female to a functional male through on intersexual phase, and it is becoming an important aquaculture species in China. It is generally believed that the sexual change of ricefield eels occurs after spawning, which may be one of the major obstacles in the massive artificial breeding of this species. In order to investigate whether spawning is necessary for the process of female to male sexual change, we performed routine hematoxylin-eosin staining on gonads of ricefield eels of different ages raised in laboratory from fertilized eggs or larvae, traced the gonadal developmental changes of 34-month-old female ricefield eels in a one-month period through biopsy examination, and examined the proliferation of gonadal cells with Proliferating cell nuclear antigen (PCNA) immunohistochemistry in gonads of ricefield eels at different sexual stages. No spawning activities or reproductive behaviors of experimental ricefield eels were observed during the study. The results showed that the intersexual and/or male fish in 5-month-old, 12-month-old (1-year-old), and 34-month-old (3-year-old) ricefield eels, were 4 (12.5%) of intersexual and 6 (20.0%) of male observed in total 30 of 5-month-old fish, 0 of intersexual and 3 (17.6%) of male in total 17 of 12-month-old (1-year-old) fish, and 12 (12.8%) of intersexual and 8 (8.5%) of male in total 94 of 34-month-old (3-year-old) fish, respectively. In addition, in 36 female ricefield eels of 34 months old, 6 females (16.7%) started sexual change and developed into intersexual stages within one month. Obvious PCNA immunostaining were observed in the connective tissues of gonadal lamellaes at the initial intersexual stage, and PCNA positive cells are presumably the interstitial cells, spermatogonia, and primary spermatocytes. Collectively, the present study suggest that spawning is not a necessary process for female to male sexual change in ricefield eels, and the earliest event for gonadal sex change seems to involve the proliferation of interstitial cells and spermatogonia in connective tissues of thickened gonadal lamellaes. This study helps to further understand and unravel mechanisms underlying the natural sex change of ricefield eels.