Aromatase activity in the hindbrain vocal control region of a teleost fish: divergence among males with alternative reproductive tactics

Type I male midshipman fish acoustically court females, whereas type II mal es do not but instead sneak or satellite spawn to compete with type I males for fertilizations. 'Singing' type I males diverge from type II males and females in the organization of an expansive hindbrain pacemaker-motoneuron circuit that establishes the physical attributes of vocalizations. Here, le vels of aromatase activity were determined in homogenates of brain by measu ring the conversion of H-3-androstenedione (AE) to H-3-oestrone (E-1) and H -3-oestradiol (E-2). Levels were highest in the telencephalon-preoptic area and similar for all morphs. Lower levels were in a region including the di encephalon, midbrain and cerebellum, although levels were significantly hig her in females compared with type I males. In the vocal hindbrain region, a romatase levels were three- to five-fold higher in type II males and female s than in type I males, and in castrated type II males than in castrated ty pe I males. Conversion of testosterone to oestrogen in type II males and fe males may effectively prevent testosterone-induced maturation of the vocal system that characterizes type I males. Aromatase may thus be a key enzyme regulating the expression of individual-specific brain circuitry and behavi ours among members of one sex.