EARLY GONADAL DEVELOPMENT AND SEXUAL-DIFFERENTIATION IN MUSKELLUNGE (ESOX MASQUINONGY)

Primordial germ cells (PGCs) were first identified in muskellunge (Eso x masquinogy) of 14 mm total length (TL) 3 weeks post fertilization. A t 32 mm TL, gonad strings were complete and formed a typical gonad sha pe in cross section. Blood vessels were first found in the gonads with Crossmon staining at 46 mm TL. Some of the PGCs underwent mitotic div ision at this stage. The ovarian sac started to develop in a fish of 8 2 mm TL, while the germ cells were still considered to be undifferenti ated. In a fish of 138 mm TL, female gonads could be clearly identifie d from the ovarian sac and groups of oogonia, whereas in another type of gonad, the morphology of undifferentiated gonads was maintained. Ge rm cells became numerous in both sexes at 211 mm TL. Female gonads con tained lobes with germ cells, including oogonial early-prophase oocyte s, and large oocytes. Spermatogonia and cells undergoing mitosis were observed in the testis. Ovaries in a fish of 250 mm TL were at the ear ly stage of perinucleolus (early diplotene). Our observations indicate that in muskellunge (i) the PGCs remained in a resting state for up t o 8 weeks post fertilization. (ii) gametogenesis occurred earlier in f emales than in males, (iii) the gonads developed from an undifferentia ted stage directly into an ovary or testis, and (iv) the somatic eleme nts in the gonads differentiated prior to the germ cells.