Ornithine Decarboxylase Exhibits Negative Thermal Modulation in the Sea Star Asterias vulgaris: Potential Regulatory Role During Temperature-Dependent Testicular Growth

Effect of estradiol and progesterone on c-myc expression in the sea star testis and the seasonal regulation of spermatogenesis

Analysis of the expression of cell cycle control genes provides a sensitive assay for mitotic and meiotic progression under experimental conditions. Transcription of the c-myc proto-oncogene and thymidine incorporation into DNA were both used to assay the mitogenic effects of estradiol and progesterone on the testicular germinal epithelium of the starfish Asterias vulgaris. Experimental in vitro studies during the annual spermatogenic cycle show that testes are not sensitive to mitotic stimulation by estradiol alone, whereas progesterone alone is mitotically inhibitory. The germinal epithelium can only be mitotically stimulated by estradiol after pretreatment with progesterone. The net increase in mitotic activity following a progesterone to estradiol transfer is dependent on the length of progesterone pretreatment. Overall, the seasonal pattern of testicular estradiol and progesterone levels evident in field populations of A. vulgaris (Hines et al., 1992a: Endocrinology 100:1468-1471) do not appear to be responsible for initiating spermatogonial mitosis at the onset of the annual spermatogenic cycle. Expression rates of the c-myc proto-oncogene are substantially more sensitive than thymidine incorporation in quantifying mitotic activity. Measuring proto-oncogene expression provides an ideal experimental approach to investigations of the regulatory mechanisms that control growth and reproduction in marine organisms.

Sex steroid levels in the testes, ovaries, and pyloric caeca during gametogenesis in the sea star Asterias vulgaris

The concentrations of progesterone, testosterone, and estradiol were determined via radioimmunoassay in testes, ovaries, and pyloric caeca of the sea star Asterias vulgaris during one complete and two partial gametogenic cycles. These compounds were found in all tissues examined and were present in quantities similar to those reported previously in other echinoderms and in vertebrates. Testes and ovaries exhibited annual growth cycles during which testicular and ovarian mass increased up to 100-fold as gametes were produced and stored until spawning. Pyloric caecal mass varied during the annual reproductive season; however, no seasonal trends were apparent. In the testes, sex steroid levels were highest at the onset of spermatogenesis. Transient increases in the levels of estradiol coincided with spermatogonial mitotic proliferation. Transient increases in the levels of testosterone and progesterone in the testes coincided with spermatogenic column formation and with spermiogenesis, respectively. In the ovaries, estradiol and testosterone levels were highest at the onset of oogenesis while progesterone levels did not change significantly throughout the annual gametogenic cycle. Male and female pyloric caeca exhibited similar seasonal variations in levels of sex steroids as compared with the gonads. It is hypothesized that transient increases in the levels of sex steroids during gametogenesis may serve as endogenous modulators of reproduction.