SEXUAL-BEHAVIOR IN DROSOPHILA IS IRREVERSIBLY PROGRAMMED DURING A CRITICAL PERIOD

Sexual differentiation in Drosophila is controlled by a short cascade of regulatory genes, the expression pattern of which determines all as pects of maleness and femaleness, including complex behaviours display ed by males and females [1-3], One sex-determining gene is transformer (tra), the activity of which is needed for female development. Flies with a female karyotype (XX) but which are mutant for tra develop and behave as males. In such flies, a female phenotype can be restored by a transgene that carries the female specific cDNA of tra under the con trol of a heat-shock promoter. This transgene, called hs[tra-fem], als o transforms XY animals into sterile females [4], When we raised these XX and XY 'females' at 25 degrees C, however, they displayed vigorous male courtship while at the same time, as a result of their female ph eromone pattern, they were attractive to males. Intriguingly, their ma le courtship behaviour was indiscriminately directed towards both fema les and males. When we forced expression of tra by heat shock, applied during a limited period around puparium formation, male behaviour was abolished and replaced by female behaviour. We conclude that sexual b ehaviour is irreversibly programmed during a critical period as a resu lt of the activity or inactivity of a single control gene.