Tissue and sexually dimorphic expression of ovarian and brain aromatase mRNA in the Japanese medaka (Oryzias latipes): Implications for their preferential roles in ovarian and neural differentiation and development

Cytochrome P450 aromatase (CYP19) catalyzes conversion of testosterone to estrogen, and is thought to influence neural and reproductive development in vertebrates. Unlike higher vertebrates, many teleost fish, including the medaka (Oryzias latipes) have two aromatase genes, one expressed predominantly in the ovary (cyp19a) and the other in the brain (cyp19b). However, the exact roles of the two aromatase genes in neural or ovarian development in fish are unclear. The primary objective of this study was to determine the pattern of expression of each of the genes in developing and adult medaka. Real-time PCR analysis indicated that both isoforms are expressed in adult ovary and brain, with predominant expression of cyp19a in the ovary and cyp19b in the brain. cyp19a was expressed at significantly higher levels in ovaries than in testes, whereas cyp19b was expressed at higher levels in the adult brain of females than males. Ontogenic expression showed that neither of the aromatase transcripts is inherited maternally, with onset of zygotic expression of both isoforms occurring just prior to hatching (stage 39). Also the expression of the ovarian, but not the brain, isoform was significantly higher in genetically female individuals than in males of similar developmental stage. This coincided with the known increased proliferation of germ cells in XX genotypes, suggesting a possible role for cyp19a in ovarian differentiation. Differential expression of both isoforms in adults and during early larval development suggests that the genes have distinctly different roles: cyp19a contributing predominantly to ovarian differentiation and development; and cyp19b contributing towards neural development and perhaps sexual behavior in adults.