Changes in the expression of genes encoding steroidogenic enzymes in the channel catfish (Ictalurus punctatus) ovary throughout a reproductive cycle

In vertebrates, the growth and maturation of the ovarian follicle is depend ent 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 d etermined the pattern of expression of four key steroidogenic genes through out the ovarian cycle in an annually spawning teleost, the channel catfish (Ictalurus punctatus). The abundance of transcripts encoding 3 beta -hydrox ysteroid dehydrogenase (3 beta -HSD) and cholesterol side chain cleavage (P 450(scc)), 17 alpha -hydroxylase/lyase (P450(c17)), and aromatase (P450(aro m)) were determined by rtqRT-PCR or ribonuclease protection assay and corre lated to ovarian growth and plasma titers of estradiol (E-2) and testostero ne (T) in two populations of catfish. Elevations in transcript abundance fo r P450(c17), P450(scc), and P450(arom) were observed at the onset of ovaria n recrudescence and during early vitellogenic growth of the oocytes; howeve r, all three decreased precipitously with the completion of vitellogenesis. Changes in the expression of these genes strongly suggest a direct correla tion to E-2 and T titers. Alternatively 3 beta -HSD transcript abundance wa s relatively stable throughout the year. This study suggests that the genes encoding the three steroidogenic cytochrome P450s have a similar regulator y mechanism.