| Abstract: | The effects of 20-hydroxyecdysone on ovary maturation in the firebrat Thermobia domestica were investigated by in vivo injections of different doses of the hormone on various days of the reproductive cycle. Several females were neck-ligated or treated with precocene II before injection. Experiments were also performed on ovarioles incubated in vitro. It was demonstrated that 20-hydroxyecdysone has direct and dose-dependent effects on the ovary, inducing growth of all the previtellogenic follicles whatever the day of the reproductive cycle, except at times when the ovaries contain follicles undergoing choriogenesis. The major effect of the hormone is the stimulation of young-follicle formation in the anterior part of the previtellarium and the accelerated growth of a set of basal previtellogenic follicles, which reach the critical size required for yolk deposition. However, 20-hydroxyecdysone did not induce cellular differentiation, in particular, the enlargement of the perioocyte space and the development of oocyte microvilli, which normally occur before yolk precursor incorporation. The present results give a better understanding of the temporal relationships between molting and reproductive cycles and also explain the periodicity of ovarian maturation. |
