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Insulin-Like Growth Factor-I Induces Oocyte Maturational Competence but Not Meiotic Resumption in White Bass (Morone chrysops) Follicles In Vitro: Evidence for Rapid Evolution of Insulin-Like Growth Factor Action¹

National Center for Cool and Cold Water Aquaculture,³ ARS, USDA, Kearneysville, West Virginia 25430 Department of Zoology,⁴ North Carolina State University, Raleigh, North Carolina 27695

A combination of recombinant human (rh) insulin-like growth factor-I (IGF-I) (25 nM) and the maturation-inducing hormone (MIH), 17,20ß,21-trihydroxy-4-pregnen-3-one (20ß-S; 72.5 nM), induced germinal vesicle breakdown (GVBD) in ovarian follicles of white bass incubated in vitro, whereas a four times greater concentration of each hormone was ineffective alone. These results indicate that IGF-I induces oocyte maturational competence (OMC) but not meiotic resumption in white bass. Culture medium concentrations of 20ß-S remained below detection limits for ovarian fragments incubated with rhIGF-I. Actinomycin D blocked GVBD in response to hCG but not to rhIGF-I plus 20ß-S, suggesting that IGF-I requires de novo translation but not transcription to induce OMC. Gap junction uncouplers, 1-octanol and 1-heptanol, and the phosphatidylinositiol 3-kinase (PI 3-K) inhibitors, wortmannin and LY 294002, attenuated hCG-, 20ß-S-, and rhIGF-I plus 20ß-S-induced GVBD. Although these inhibitors reduced hCG-induced progestin release, PI 3-K inhibitors did not alter MIH synthesis in some incubations and addition of 20ß-S to the incubations did not fully overcome the effects of either class of inhibitors, suggesting that decreasing MIH production is not their only inhibitory effect on gonadotropin (GtH) action. Our data suggest that gap junctions and PI 3-K activity are necessary for GtH and IGF-I to induce and maintain OMC in white bass. The induction of OMC but not meiotic resumption by IGF-I in white bass, compared with the induction of meiotic resumption but not OMC by IGF-I discovered in the congeneric striped bass suggests rapid evolution of the reproductive actions of IGF-I among temperate basses (genus Morone).

follicle, growth factors, mechanisms of hormone action, meiosis, signal transduction

² Correspondence: Gregory M. Weber, National Center for Cool and Cold Water Aquaculture, ARS, USDA, 11861 Leetown Road, Kearneysville, WV 25430-1861. FAX: 304 725 0351; gweber{at}ncccwa.ars.usda.gov