The role of steroids in reproduction in female elasmobranchs and reptiles.
Adequate evidence exists to suggest the importance of temporal changes in steroid hormone ratios in the normal reproductive/vitellogenin cycle in oviparous and viviparous elasmobranchs and reptiles. In oviparous species, where the cycle is relatively short, secretion of gonadal hormones is synchronous; thus inhibitory actions of progesterone ( P) on hepatic or reproductive tract functions would be offset by stimulatory actions of estradiol ( E), resulting in appropriate vitellogenin secretion and reproductive tract development. In viviparous species, temporal asynchrony of E and P secretion occurs, and the actions of the individual hormones can be more easily dissected out. Thus, during gestation, where P is the dominant hormone, antagonistic or stimulatory actions of E may be prevented, and the inhibitory action of P on vitellogenesis dominant. Hence vitellogenesis is limited to the follicular phase and eggs are retained. Although the elasmobranch and reptilian species discussed here do not form a continuum through phylogenesis, but rather are extant forms of a particular line of evolution, it is possible to extrapolate from these observations to the probable endocrine interactions in a species as viviparity evolves from oviparity. The theoretical intermediate stage would involve; (a) egg retention, (b) extension of the luteal phase and increased P secretion and (c) resulting in E/ P asynchrony and potential expression of "independent" P action, egg retention and yolk suppression.[1]References
- The role of steroids in reproduction in female elasmobranchs and reptiles. Callard, I.P., Etheridge, K., Giannoukos, G., Lamb, T., Perez, L. J. Steroid Biochem. Mol. Biol. (1991) [Pubmed]
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