Changes in plasma progesterone in relation to vitellogenesis and gestationin the viviparous snake Vipera aspis

Viviparity has arisen from oviparity on more than 100 independent occasions in squamate reptiles, providing an unique opportunity to investigate the w ays in which endocrine control of gestation length has been modified by nat ural selection during this major transition in reproductive modes. Intuitiv ely, the evolution of viviparity might be expected to involve an increasing ly important role for the steroid hormone progesterone, rather than estradi ol. Unfortunately, published data on this topic in snakes are scarce and of ten contradictory. Females of the viviparous snake Vipera aspis reproduce w ith a lower than annual frequency, providing the opportunity to examine ste roid profiles simultaneously in vitellogenic, pregnant, and postparturient versus nonreproductive females. From 1990 to 1994, more than 500 blood samp les were collected from more than 100 females. Progesterone, estradiol-17 b eta, and several plasma metabolites were assayed by radioimmunoassay and sp ectrophotometry. In contrast to earlier studies, we found significant diffe rences between plasma progesterone levels in reproducing (10.5 +/- 9.1 ng m l(-1), N = 168) and nonreproducing (5.1 +/- 4.2 ng ml(-1), N = 121) females . Estradiol reached high levels during vitellogenesis (4.8 +/- 4.0 ng ml(-1 ), N = 16), and progesterone levels increased during gestation (from 3.7 ng ml(-1) before gestation to 18.7 ng ml(-1) at midgestation). However, exper imental elevation of plasma progesterone levels with implants (up to 44.4 n g ml(-1)) did not modify plasma metabolite levels or delay parturition. (C) 2001 Academic Press.