Abstract
Sex steroids are essential hormones for reproduction in vertebrates. The existence of a sex steroidogenic pathway in invertebrates is controversial because cytochrome P450 (CYP) genes have not been detected in the genomes of an echinoderm and a urochordate. However, cloning and gene expressions of sex steroid‐metabolizing enzymes have been reported in a cephalochordate. In this study, in vitro conversion of sex steroids from pregnenolone (P5) to progesterone (P4), from 17α‐hydroxyprogesterone (17α‐P4) to 17α, 20β‐dihydroxy‐4‐pregnen‐3‐one (17α, 20β‐P) and 17α, 20α‐P, and from androstenedione (AD) to estrone (E1), estradiol‐17β (E2), and testosterone (T) were confirmed by an incubation experiment performed using ^14^C‐labeled precursors and mature ovarian homogenates of the amphioxus Branchiostoma belcheri. In amphioxus, the ability of immature ovaries to synthesize sex steroids was much lower than that of mature ovaries. Post‐spawning, the mRNA of CYP11A significantly decreased in females. Transcripts of the CYP19 gene also declined in one‐fourth of the females after spawning, although this trend was not supported statistically. The mRNAs of CYP17 and 17β‐HSD did not change before and after spawning. Our results indicate the presence of Δ^4^ and another derivative pathway in the amphioxus ovary and suggest that the synthesis of sex steroids, particularly estrogen synthesis, may be low in females after spawning behavior. J. Exp. Zool. 309A:83–93, 2008. © 2008 Wiley‐Liss, Inc.