Differentiation and morphogenesis of the ovary and expression of gonadal development-related genes in the protogynous hermaphroditic ricefield eel Monopterus albus.

The ovarian differentiation, morphogenesis and expression of some putative gonadal development-related genes were analysed in the ricefield eel Monopterus albus, a protogynous hermaphroditic teleost with a single elongate ovary. At c. 1 day post-hatching (dph), the gonadal ridge was colonized with primordial germ cells (PGCs) at the periphery and transformed into the gonadal primordium, which appeared to contain two germinal epithelia. At c. 7 dph, four ovarian cavities appeared in the gonadal tissue with two in each germinal epithelial compartment, and the indifferent gonad might have begun to differentiate into the ovary. The oocytes at the leptotene stage in meiosis I appeared at c. 14 dph, and oocytes at the diplotene stage at c. 30 dph. As development proceeded, the connective tissue separating the two germinal epithelia disappeared, and two of the four ovarian cavities collapsed into one. At 60 dph, the gonad had already taken the shape as observed in the adults. One outer and two inner ovarian cavities could be easily recognized, with slightly basophilic primary growth oocytes usually residing close to the outer ovarian cavity. The expression of cyp19a1a and erb in the early gonad was detected at 6 dph. The abundant expression of foxl2 coincided with the up-regulation of cyp19a1a at 8 dph onwards. The expression of dmrt1 isoforms was not detectable until 8 dph for dmrt1a and dmrt1b and until 33 dph for dmrt1d. The earlier appearance of cyp19a1a compared to dmrt1 transcripts in the indifferent gonad may contribute to the initial differentiation of the gonad towards the ovary in M. albus.