Graduate Thesis Or Dissertation
 

Sex steroid profiles of coho salmon, Oncorhynchus Kisutch, during early development and sexual differentiation

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  • Whole body sex steroid levels of coho salmon, Oncorhynchus kisutch, were measured via radioimmunoassay during early development and sexual differentiation. Steroid profiles of testosterone (T), 11-ketotestosterone (KT) , androstenedione (A), progesterone (P4) , 17β, -hydroxy- 20β -dihydroprogesterone (DHP) , and 17β -estradiol (E2) were generated beginning with unfertilized eggs and ending at 87 days post-fertilization (DPF). Ovarian fluid was also examined for steroid content. At hatching (42 DPF) an androstenedione assay was substituted for the P4 assay. Onset of exogenous feeding occurred at 60 DPF. Steroid profiles of unfertilized eggs essentially paralleled that found for ovarian fluid. Following fertilization, steroid levels in developing embryos declined precipitously until 30 DPF. At hatch, levels of all steroids increased slightly and then declined during yolk sac absorption. Levels of the androgens (T, KT, and A) followed similar patterns during this time while DHP and E2 content also showed similar trends to one another. During yolk sac absorption yolk and embryos were separated from each other and assayed individually. Steroid content of yolk declined while it generally increased or remained constant in the embryos. Varying degrees of bimodality appeared in the data regarding steroid levels within samples of fish collected between 42 and 56 DPF. The bimodal distributions of steroids disappeared at the onset of feeding and did not return again until 87 DPF. The hormone levels generally decreased or remained constant following the onset of exogenous feeding, except for E2, whose levels increased during this time. Histological analyses showed the presence of undifferentiated gonads between hatching and 70 DPF. By 77 DPF ovarian development could be easily discerned. Fish born the following brood year were presumptively sexed at 101 DPF and steroid levels were determined; a sexual dimorphism was apparent in levels of T, KT, and A, but not for DHP or E2. The increase in steroid content of developing coho at hatch, coupled with their bimodal distributions during yolk sac absorption and the increase in E2 levels during ovarian development may suggest a possible role of sex steroids in the process of sexual differentiation apparent later in development. Changes in whole body steroid levels at hatch may also be indicative of the onset of sexual differentiation even though no signs of gonadal differentiation were histologically discernable at that time.
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