fruitless Gene products truncated of their male-like qualities promote neural and behavioral maleness in Drosophila if these proteins are produced in the right places at the right times.

To bring GAL4 production under the control of the sex promoter (P1) contained within Drosophila's fruitless gene, a gal4 cassette was previously inserted downstream of P1. This insert should eliminate male-specific FRU(M) proteins, which normally contain 101 amino acids (aa's) at their N termini. Thus males homozygous for the P1-gal4 insert should be courtless, as was briefly stated to be so in the initial report of this transgenic type. But XY flies whose only fru form is P1-gal4 have now been found to court vigorously. P1-gal4 females displayed no appreciable male-like actions except courtship rejection behaviors; yet, they developed a male-specific abdominal muscle. No immunoreactivity against the male-specific aa's was detectable in P1-gal4 flies. But male-like neural signals were observed in XY or XX P1-gal4 pupae and adults after applying an antibody that detects all FRU isoforms; transgenic females displayed reduced expression of such proteins. RT-PCR's rationalized these findings: P1 transcripts include anomalous splice forms from which gal4 was removed, allowing FRU's lacking M aa's to be produced in male-like patterns in both sexes. Within males, such defective proteins promote neural differentiation and function that is sufficient to support spirited P1-gal4 courtship. But dispensability of the male-specific FRU N-terminus is tempered by the finding that intra-fru sequences encoding these 101 aa's are highly conserved among interspecific relatives of D. melanogaster.