Sex determination mechanisms and techniques to control sex differentiation in selected species of sunfishes (Centrarchidae)

Control of sexual differentiation through steroid treatments and sex determination mechanisms in three economically important centrarchids, bluegill Lepomis macrochirus, black crappie Pomoxis nigromaculatus , and largemouth bass Micropterus salmoides were studied. Effective timing, duration and mode of administration of steroid treatments were investigated using two synthetic androgens, 17alpha-methyltestosterone and trenbolone acetate, and a natural estrogen, estradiol-17beta. Dietary administration of androgen to bluegill resulted in variable level of masculinization. Immersion method was an effective mode of androgen administration to bluegill to achieve masculinization. Inclusion of a permeation enhancer, dimethyl sulfoxide, to the solutions can increase the masculinization potency of androgens. Unlike androgens, both oral and immersion modes of estrogen administration were effective to induce a high level of feminization in bluegill. Timing of gonadal differentiation in black crappie depends on growth rate, and both size and age were important in the selection of appropriate size black crappie fry for steroid treatments. Dietary administration of androgens to black crappie might give inconsistent results because factors such as feed training or water temperatures might interfere with feed consumption and delivery of steroids. More consistent results were obtained by administering androgens through the rearing water. Feminization in black crappie was achieved through periodic estrogen immersions, but optimum timing, frequency and intensity of estrogen immersions remains unclear. Successful and functional alteration of phenotypic sex in largemouth bass requires initiation of steroid treatments before 40 days of age and 34 mm total length. An estradiol-17beta concentration lower than 200 mg/kg diet and probably a 17alpha-methyltestosterone concentration lower than 60 mg/kg diet should be used to prevent the adverse effects of steroids on gonadal development of largemouth bass.; Sex determination mechanisms were studied by comparing the sex ratios of progeny derived from pair spawning of normal (not hormone treated) brooders and backcrossing of hormone treated brooders to normal brooders. Effect of prevailing water temperatures on sex ratios of bluegill were also evaluated by rearing young fry in two different temperature extremes. Sex determination mechanism in largemouth bass has a dual chromosomal basis. Sex determination mechanism of black crappie is predominantly monofactorial and male is the heterogametic sex. Sex determination mechanism of bluegill is polyfactorial, involving multiple autosomal sex modifying genes over the underlying chromosomal system that is yet to be elucidated.