Localization and expression of aromatase mRNA in adult zebrafish

Aromatase (CYP19), the enzyme responsible for the conversion of testosterone to estradiol, plays a key role in the control of many behavioural and physiological aspects of reproduction. The zebrafish and many other teleosts have two aromatase genes (CYP19A1 and CYP19A2) which are expressed predominantly in the ovary or brain, respectively. In order to determine the site-specific expression of these two genes, whole mount and section in situ hybridizations were conducted with adult zebrafish brains, pituitaries, and gonads. In addition, transcript abundance was determined by real-time quantitative RT-PCR analysis (TaqMan) in the pituitary, 4 regions of the brain (olfactory bulb, telencephalon, hypothalamus and the remainder of the brain), and 4 size-classes of ovarian follicles [large (>680 m), medium (>500 m), small (>350 m) and oogonia embedded in connective tissue]. In situ hybridizations showed that CYP19A2 was strongly expressed in the olfactory bulb (OB), ventral telencephalon (TEL), preoptic area (POA) and ventral/caudal hypothalamic zone (HT) of the brain, and in the anterior and posterior lobes of the pituitary. The regional distribution of the CYP19A2 mRNA did not vary with sex however transcript abundance varied within and between sexes. For example, in males with a "high expression" pattern, mRNA levels were higher in the OB, TEL and HT than in the rest of the brain and pituitary. Furthermore, CYP19A2 mRNA levels in the OB, TEL and HT tended to be higher in males than in females. In situ hybridizations of CYP19A1 failed to develop a signal in the brain or pituitary but were detectable by PCR. CYP19A1 was highly expressed in the medium-sized follicles with significantly lower levels in the large follicles, small follicles, and oogonia. The differential expression of the aromatase genes, particularly CYP19A2 in the brain, suggests that the two aromatase genes play important and different roles in the reproductive behaviour and/or physiology of bony fish, as has been demonstrated for the single gene in mammals.