In vitro progesterone production by maternal and embryonic tissues during gestation in the southern snow skink (Niveoscincus microlepidotus)

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Abstract

The southern snow skink, Niveoscincus microlepidotus, has a protracted gestation, lasting approximately one year. Ovulation occurs in spring (November) and embryonic development is completed by early autumn (March); however, birth does not occur until the following spring. Previous studies have shown that plasma progesterone concentration peaks in preovulatory females (spring, October), remains high during early gestation, and decreases to basal by autumn. In vitro progesterone production by corpora lutea, non-luteal ovary, anterior oviduct, placental tissues, muscle, and embryonic adrenal-gonads from N. microlepidotus was assessed throughout gestation. Tissues were incubated with or without the precursor pregnenolone for 3 h at 24 °C; the resulting media were analysed for progesterone using radioimmunoassay. In vitro progesterone production by corpora lutea in media only was high during early gestation, dropping to basal by autumn. Maternal adrenal glands produced progesterone in vitro in media only throughout gestation; however, the pattern of production did not correlate with plasma concentrations and may represent steroid that is normally converted to corticosterone. Non-luteal ovary, anterior oviduct, placental tissues, muscle, and embryonic adrenal-gonads produced minimal progesterone in media only, but were able to convert pregnenolone to progesterone; this suggests steroid metabolic capability within these tissues. Further research is needed to address the possible endocrine role(s) of placental and embryonic tissues during gestation in viviparous squamates.

Introduction

Patterns of plasma progesterone concentrations associated with gestation vary widely among species of viviparous squamate (Jones and Baxter, 1991), with peaks occurring from periovulation (Girling et al., 2002), through mid (Bonnet et al., 2001; Edwards and Jones, 2001; Flemming, 1994; Kleis-San Francisco and Callard, 1986) to late gestation (Jones and Swain, 1996; Jones et al., 1997). In the red-sided garter snake (Thamnophis sirtalis parietalis), progesterone concentrations remained low or non-detectable throughout gestation (Whittier et al., 1987). The diversity of progesterone patterns amongst viviparous squamates reflects their diverse evolutionary history. Viviparity is believed to have evolved independently around 100 times in the squamate reptiles (Blackburn, 1982, Blackburn, 1985; Shine, 1985); it cannot be assumed that the mechanisms that maintain gestation are the same in each case.

The corpora lutea are believed to be the major source of progesterone in viviparous squamates (Callard et al., 1992; Xavier, 1987) and in the few species examined, patterns of plasma progesterone usually correlate with the activity of the corpora lutea (Bennett and Jones, 2002; Fergusson and Bradshaw, 1991; Flemming, 1994). In some species, however, plasma progesterone concentrations remain high even after regression of the corpora lutea (Sceloporus jarrovi, Guillette et al., 1981; Chalcides ocellatus and Chalcides chalcides, Angelini and Ghiara, 1991); therefore, an alternative source of this hormone must be postulated. Guillette et al. (1981) hypothesised that in S. jarrovi production of progesterone by corpora lutea may be replaced by production from corpora atretica or by placental tissues. In the viviparous skink C. chalcides, progesterone is produced in vitro by the placental tissues during the later stages of gestation (Guarino et al., 1998). It has also been hypothesised that adrenal glands may contribute to progesterone production during gestation (Bourne et al., 1986; Dauphin-Villemant and Xavier, 1985). However, a comprehensive analysis of progesterone production by different tissues over the course of gestation has not previously been undertaken in a viviparous squamate.

The southern snow skink, Niveoscincus microlepidotus, is a viviparous lizard found in alpine regions (above 1000 m elevation) of southern Tasmania, Australia (Hutchinson et al., 1989; Melville and Swain, 1999). These animals have an unusual biennial reproductive cycle that includes a protracted gestation period of approximately one year (Girling et al., 2002; Olsson and Shine, 1998, Olsson and Shine, 1999; Rawlinson, 1974; Swain, 1972). Ovulation occurs in the austral spring (November) and embryonic development is completed by early autumn (March), approximately one month prior to hibernation. Parturition, however, does not occur until the following spring (November), several weeks after animals emerge from the winter hibernation period. Although the extended nature of this reproductive cycle is attributed to the cool and variable alpine habitat experienced by these animals (Hutchinson et al., 1989; Olsson and Shine, 1998), the physiological mechanisms by which gestation is extended are unknown. We initially hypothesised that plasma progesterone concentrations would remain high until after hibernation in N. microlepidotus; however, although concentrations were high (15.4±5.9ngml−1) during the early stages of gestation (summer), they decreased to basal (1.2±2.0ngml−1) by early autumn (March), prior to hibernation, and several months before parturition in late spring (Girling et al., 2002). It appears, therefore, that elevated plasma progesterone concentrations are not necessary for the maintenance of this long gestation period.

In this experiment, we measure in vitro progesterone production by maternal and embryonic tissues (adrenal glands, corpora lutea, non-luteal ovary, oviduct, placental tissues, muscle, and embryonic adrenal/gonads) over the course of gestation in N. microlepidotus. We hypothesise that corpora lutea will show the greatest capacity for progesterone production during gestation. Secondly, we consider alternative sources of progesterone; we investigate whether localised progesterone production by, perhaps, placental tissues continues after plasma concentrations decrease in autumn. Based on the very limited information available from other reptiles, we hypothesise that localised production by placental tissues may occur.

Section snippets

Animal collection

Female N. microlepidotus in the following reproductive conditions were collected from January to November, 2000 (n=2–6, see figures for details of samples sizes for each tissue): postovulation (November), early pregnancy (January), mid pregnancy (February), late pregnancy prior to winter (March), hibernation (June and August), late pregnancy after winter (October), and postpartum (November). Skinks were captured from the summit (1270 m) of Mt Wellington, Hobart, Tasmania, Australia (43° 51S, 147°

Corpora lutea

In vitro progesterone production by corpora lutea varied significantly with month (F(7,45)=26.9,p=0.001) and in the presence or absence of pregnenolone (F(1,45)=442.6,p=0.001); there was a significant interaction effect (F(7,45)=12.3,p=0.001). In vitro progesterone production by corpora lutea incubated in media only peaked in early gestation (January: 3.7±0.29 ng CL−1), decreasing significantly by mid-gestation (p=0.001, February: 0.5±0.16 ng CL−1) (Fig. 1A). Production in media only was basal

Discussion

Various in vitro incubation experiments have indicated that corpora lutea of reptilian species are able to produce progesterone (for example: Ciarcia et al., 1993; Gobbetti et al., 1993; Xavier, 1982); activity of the enzyme 3β-hydoxy-Δ5-steroid dehydrogenase (3β-HSD), responsible for conversion of pregnenolone to progesterone, has also been reported (for example: Guarino et al., 1998). Results from our incubation experiments with tissues from the southern snow skink, N. microlepidotus, were

Acknowledgements

Thanks are due to all those who helped with field and laboratory work: V. Cartledge, A. Edwards, B. Gartrell, and the Jones’ family. The manuscript benefited from comments from A. Edwards, B. Gartrell, and E. Wapstra. Funding was provided by the New Zealand Foundation for Research, Science and Technology and a University of Tasmania Small ARC Research Grant.

References (37)

  • S.K. Kleis-San Francisco et al.

    Progesterone receptors in the oviduct of a viviparous snake (Nerodia): correlations with ovarian function and plasma steroid levels

    Gen. Comp. Endocrinol.

    (1986)
  • J.M. Whittier et al.

    Plasma steroid hormone levels of female red-sided garter snakes, Thamnophis sirtalis parietalis: relationship to mating and gestation

    Gen. Comp. Endocrinol.

    (1987)
  • F. Angelini et al.

    Viviparity in squamates

  • D.G. Blackburn

    Evolutionary origins of viviparity in the Reptilia. I. Sauria

    Amphib-Reptilia

    (1982)
  • D.G. Blackburn

    Evolutionary origins of viviparity in the Reptilia. II. Serpentes, Amphisbaenia and Ichthyosauria

    Amphib-Reptilia

    (1985)
  • I.P. Callard et al.

    Role of the corpus luteum and progesterone in the evolution of vertebrate viviparity

    Am. Zool.

    (1992)
  • J.R.G. Challis et al.

    Parturition

  • G. Ciarcia et al.

    Changes in ovarian follicles and in vitro sex hormone release in the lizard Podarcis sicula sicula

    Mol. Reprod. Dev.

    (1993)
  • Cited by (0)

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