fruitless tunes functional flexibility of courtship circuitry during development

Drosophila male courtship is controlled by the male-specific products of the fruitless (fru(M)) gene and its expressing neuronal circuitry. fru(M) is considered a master gene that controls all aspects of male courtship. By temporally and spatially manipulating fru(M) expression, we found that fru(M) is required during a critical developmental period for innate courtship toward females, while its function during adulthood is involved in inhibiting male–male courtship. By altering or eliminating fru(M) expression, we generated males that are innately heterosexual, homosexual, bisexual, or without innate courtship but could acquire such behavior in an experience-dependent manner. These findings show that fru(M) is not absolutely necessary for courtship but is critical during development to build a sex circuitry with reduced flexibility and enhanced efficiency, and provide a new view about how fru(M) tunes functional flexibility of a sex circuitry instead of switching on its function as conventionally viewed.