Steroid hormone profile during gestation and parturition of the guppy (Poecilia reticulata)

The guppy is an ovoviviparous teleost with fertilization and gestation occurring intrafollicularly, and ovulation and hatching at around parturition. The levels of estradiol-17 beta (E), testosterone (T), cortisol (F), 17 alpha-hydroxyprogesterone (17-OHP), 17 alpha-20 beta-dihydroxy-4-pregnen-3-one (17,20 beta-P), and progesterone (P) in the guppy at different stages of gestation were measured in 60 microliters of serum with a highly specific, sensitive, precise, and accurate procedure involving high-performance liquid chromatography and radioimmunoassay. Levels of E and T increased during vitellogenic growth of oocytes and declined sharply after fertilization, increasing again gradually during late gestation to reach prefertilization levels at periparturition. F levels decreased significantly during fertilization followed by an increase during gestation and a sharp decline again at periparturition. The level of 17-OHP was significantly lower at periparturition than at other stages, while that of 17,20 beta-P was significantly higher in periparturition than in late gestation. There was no significant change in the levels of P at all stages of gestation, suggesting that, unlike mammals, P may not be involved in maintaining gestation in this fish. Changes in the levels of E and T seem to be related to oocyte growth rather than gestation. The role of F in gestation is uncertain since the effect of stress could not be discounted or defined in this study. Thus, on the present results, the role of any particular steroid in maintaining gestation of the guppy is not obvious. The elevated levels of E at term may facilitate parturition in addition to its role in vitellogenesis during oocyte growth.