In vertebrates, the growth and maturation of the ovarian follicle is dependent on the appropriate dynamics of sex steroid secretion, which is dictated by gene expression of the steroidogenic enzymes. The molecular aspects of steroid regulation are poorly understood in fishes, so as a first step we determined the pattern of expression of four key steroidogenic genes throughout the ovarian cycle in an annually spawning teleost, the channel catfish (Ictalurus punctatus). The abundance of transcripts encoding 3β-hydroxysteroid dehydrogenase (3β-HSD) and cholesterol side chain cleavage (P450scc), 17α-hydroxylase/lyase (P450c17), and aromatase (P450arom) were determined by rtqRT-PCR or ribonuclease protection assay and correlated to ovarian growth and plasma titers of estradiol (E2) and testosterone (T) in two populations of catfish. Elevations in transcript abundance for P450c17, P450scc, and P450arom were observed at the onset of ovarian recrudescence and during early vitellogenic growth of the oocytes; however, all three decreased precipitously with the completion of vitellogenesis. Changes in the expression of these genes strongly suggest a direct correlation to E2 and T titers. Alternatively 3β-HSD transcript abundance was relatively stable throughout the year. This study suggests that the genes encoding the three steroidogenic cytochrome P450s have a similar regulatory mechanism.
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