Corticosteroid and neurohypophyseal hormone control of parturition in the guppy,Poecilia reticulata

Both increased day length and immersion in distilled water elicited parturition in the pregnant guppy, Poecilia reticulata. Injections of deoxycortisol, oxytocin, vasotocin, and an extract of carp neurohypophyses also induced premature parturition. In vitro, deoxycortisol promoted the breakdown of follicular membranes, thus expediting explusion of young by contractions of ovarian smooth muscle induced by neurohypophyseal hormones.     

Prostaglandins and teleost neurohypophyseal hormones induce premature parturition in the guppy, Poecilia reticulata.

Administration of PGE2, PGF2 alpha, isotocin, and vasotocin to pregnant guppies during late gestation induced premature parturition. Indomethacin (10 micrograms/g body wt), the cyclooxygenase inhibitor, did not inhibit parturition at term, suggesting that some lipoxygenase metabolites may be involved in the induction of parturition. Pituitary extract, as well as LH-RH, did not induce premature parturition. It may be that in the guppy, in which ovulation occurs at the end of gestation just prior to parturition, gonadotropin is not involved in the induction of ovulation and parturition.     

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.     

Effect of methallibure on gestation in the guppy, Poecilia reticulata

Methallibure treatment (1 mg fed on alternate days to a female and a male kept in 1 liter of 25% seawater) started from 1 or 7 days postpartum (early gestation) inhibited gestation in the wild guppy, Poecilia reticulata. A few of the fish treated from 7 days postpartum (2/9) did give birth, but the young either were dead at birth or did not survive long after birth. When treatment was started 2 weeks postpartum (midgestation), gestation was not inhibited, but 25% of the young were born dead while the rest showed retardation of postnatal growth. Several possible interpretations of the results are discussed.     

Female mating preference for bold males in the guppy, Poecilia reticulata

Although females prefer to mate with brightly colored males in numerous species, the benefits accruing to such females are virtually unknown. According to one hypothesis of sexual selection theory, if the expression of costly preferred traits in males (such as conspicuous colors) is proportional to the male's overall quality or reveals his quality, a well-developed trait should indicate good condition and/or viability for example. A female choosing such a male would therefore stand to gain direct or indirect fitness benefits, or both. Among potential phenotypic indicators of an individual's quality are the amount and brightness of its carotenoid-based colors and its boldness, as measured by its willingness to risk approaching predators without being killed. Here, we show experimentally that in the Trinidadian guppy (Poecilia reticulata) the visual conspicuousness of the color pattern of males correlates positively with boldness toward, and with escape distance from, a cichlid fish predator. Bold individuals are thus more informed about nearby predators and more likely to survive encounters with them. Mate-choice experiments showed that females prefer colorful males as mates, but prefer bolder males irrespective of their coloration when given the opportunity to observe their behavior toward a potential fish predator. By preferentially mating with colorful males, female guppies are thus choosing on average, relatively bold, and perhaps more viable, individuals. In doing so, and to the extent that viability is heritable, they potentially gain indirect fitness benefits by producing more viable offspring than otherwise.     

Effect of L-thyroxine on ovarian development and gestation in the viviparous guppy, Poecilia reticulata

Treatment of female guppies from 1 day postpartum with L-thyroxine-sodium salt (Eltroxin, Glaxo) by immersion (0.5 ppm in 25% seawater renewed daily) significantly shortened the brood interval without affecting the brood size or the weight of the brood. This suggests an acceleration of early development of the fish by thyroxine. The thyroxine treatment also accelerated the development of the next batch of oocytes, particularly exogenous vitellogenesis (yolk-globule stages). The evidence suggests a role of the thyroid in vitellogenesis and gestation in the guppy.     

Morphometrical studies of the ultrastructure of the epidermis of the guppy, Poecilia reticulata Peters, following adaptation to seawater and treatment with prolactin.

Adult male guppies adapted to seawater or fresh water were treated for a period of 4 weeks with different doses of prolactin. The epidermis of scales from the midlateral body region was studied by light and electron microscopy. Prolactin treatment of guppies adapted to fresh water changes the pattern of superficial ridges and increases the numbers of cell layers, mucous cells, and subepidermal capillaries. Adaptation to seawater increases the body surface, the surface extent of superficial cells, the number of small electron-dense vesicles, and the height of the glycocalyx. The most striking finding is the occurrence of chloride cells in the epidermis of all guppies adapted to seawater. The number of chloride cells is reduced after low and medium doses of prolactin, and increased following high doses.     

Steroid metabolism by ovarian follicles and extrafollicular tissue of the guppy (Poecilia reticulata) during oocyte growth and gestation.

In the viviparous guppy, fertilization and gestation are intrafollicular. Fully developed embryos are ovulated at the end of gestation just prior to parturition. The metabolism in vitro of various radiolabeled steroid precursors by isolated ovarian follicles at various stages of the reproductive cycle and extrafollicular (EF) tissue of the guppy was investigated. While estradiol-17 beta was one of the end products of metabolism in vitellogenic follicles, 17 alpha, 20 beta-P and several 5-reduced metabolites were synthesized by postvitellogenic follicles. The yield of 17 alpha, 20 beta-P, however, was much lower than some 5 beta-reduced metabolites synthesized by postvitellogenic follicles. Gestation stage follicles rapidly converted the precursors into 5-reduced and polar 7-hydroxylated steroids, and their glucuronides. Although postpartum follicles showed very poor potential for steroid metabolism, they synthesized estradiol-17 beta from testosterone. These results demonstrate distinct changes occurring in the steroidogenic potential of the follicles during the reproductive cycle. Unlike in other viviparous vertebrates, no particular steroid seems to be involved in maintaining gestation in the guppy; all the steroid precursors are converted into highly polar metabolites and their conjugates during gestation, thereby facilitating their excretion. The EF ovarian tissue also synthesized 7-hydroxylated steroids and their glucuronides, providing evidence for the first time that the teleost ovarian EF tissue plays a role in steroidogenesis. The possible physiological significance of the synthesis of the novel polar steroids by the follicles and the EF tissue is discussed.     

Effects of neurohypophyseal and adenohypophyseal hormones, steroids, eicosanoids, and extrafollicular tissue on ovulation in vitro of guppy (Poecilia reticulata) embryos.

In the viviparous guppy, oocyte maturation is followed by intrafollicular fertilization and gestation. The fully developed embryos are ovulated at term just prior to parturition. Various agents were tested in vitro for their effects on ovulation of embryos in isolated follicles of the guppy. Arachidonic acid (10 and 100 microM), PGE2, PGF2 alpha, and 6-keto-PGF1 alpha (0.1 microgram/ml) induced ovulation, while PGE1, 15-keto-PGF2 alpha, leukotriene B4, 5-, 12-, and 15-HETEs (0.01 to 0.1 microgram/ml), cortisol, 11-deoxycortisol (25 and 250 ng/ml), estradiol-17 beta, testosterone, 17 alpha,20 beta-P, progesterone (5 and 50 ng/ml), isotocin, vasotocin (0.02 to 2 microgram/ml), and guppy pituitary extract (one and two glands per fish) did not. Extrafollicular (EF) ovarian tissue cocultured with isolated follicles induced ovulation, and the medium levels of PGE and PGF in such incubations were higher than those in the control. Indomethacin, the cyclooxygenase inhibitor, did not inhibit ovulation induced by arachidonic acid and EF tissue, although it inhibited PGE and PGF production. NDGA, the lipoxygenase inhibitor, did not inhibit ovulation induced by arachidonic acid or EF tissue. A combination of eicosanoids synthesized by follicles and EF tissue may be involved in the induction of ovulation. Dibutyryl cAMP inhibited ovulation induced by PGE2, PGF2 alpha, 6-keto-PGF1 alpha, and EF tissue suggesting that a low level of cAMP may be associated with ovulation in the guppy.     

Glucosidation of estradiol-17 beta in the cultured ovaries of the silkworm, Bombyx mori

Metabolic pathways of estradiol-17 beta and other vertebrate steroid hormones of cultured silkworm pupal ovaries were examined using 14C-labeled steroids. The isolated ovaries showed significant uptake and metabolic activity of the 14C-labeled estradiol-17 beta added to the medium. Analysis of the metabolized compounds by thin-layer chromatography (TLC) and high-performance liquid chromatography (HPLC) revealed extensive metabolic conversion of [14C]estradiol-17 beta and estrone; i.e., estrone was reduced to estradiol-17 beta and estradiol-17 beta was metabolized to conjugates, including estradiol-3-beta-D-glucoside and estradiol-17-alpha-D-glucoside. [14C]Testosterone was not transformed appreciably by the ovaries. Metabolic activity and physiological significance of the vertebrate steroid hormones in the silkworm ovaries are discussed.     

Short-term profiles of plasma gonadotropin and estradiol-17 beta levels in the female rainbow trout, from early ovarian recrudescence and throughout vitellogenesis

At various stages throughout the annual reproductive cycle, female rainbow trout (Salmo gairdneri) were fitted with a catheter in the dorsal aorta. They were bled via the catheter at frequencies of once every 30, 60, or 240 min over periods of 5 to 24 hr. Gonadotropin (GtH), estradiol-17 beta (E(2)17 beta). and estrone levels were measured in the plasma samples. At early ovarian recrudescence (March), short-term (1-2 hr), high-amplitude (delta GtH = up to 100 ng/ml), episodic pulses of GtH were recorded in the plasma of 12 of the 26 studied fish. In the others, GtH levels remained low and constant. No synchronization was found among the individual GtH profiles. E(2)17 beta levels in the same fish were low and constant while estrone was not detectable. At early stages of exogenous vitellogenesis (June), plasma GtH (1-3 ng/ml) and E(2)17 beta (0.5-1.5 ng/ml) levels were low and constant. At advanced stages of exogenous vitellogenesis (September-October), fluctuating GtH levels were found again in most of the females; basal GtH concentrations were only slightly higher than those recorded in June. The fluctuations consisted of short-term (1-3 hr) random GtH pulses of moderate amplitude (delta GtH = up to 5 ng/ml), occurring at a relatively high frequency (up to 5 per 12 hr). Although no regular synchronous daily pattern of GtH was noted, most of the GtH pulses were observed during the photophase and early scotophase. The appearance of GtH pulsatility during exogenous vitellogenesis was accompanied by a large increase in plasma E(2)17 beta up to levels ranging from 6 to 30 ng/ml. In contrast to the GtH profiles, the E(2)17 beta profiles showed continuous and progressive variations, superimposed the abrupt GtH pulses, and a high degree of synchronization. E(2)17 beta levels increased during the photophase and reached maxima toward and during early scotophase.     

Inhibition of cytochrome p450 brain aromatase reduces two male specific sexual behaviours in the male Endler guppy (Poecilia reticulata)

In mammalian and avian vertebrate groups, androgens act as controlling agents on male aggression and courtship behaviour by their conversion to oestrogens by cytochrome P450 aromatase in well-defined brain regions. Despite the fact that bony fishes have exceptionally high brain aromatase activity, little is known about it's possible regulatory effects on the reproductive behaviours of teleosts. In this study, Endler guppy males (Poecilia reticulata) were subjected to 26-29 days of 24-h exposure to two different concentrations (15 and 100 microg/L) of the aromatase inhibitor fadrozole in the water. Compared with the control males, two of three courtship activities in males exposed to the higher concentration were reduced when they were paired with receptive stimulus females. Reduction in brain aromatase activity was confirmed in both exposed groups with the use of the tritiated water assay.