Abnormalities of male-specific FRU protein and serotonin expression in theCNS of fruitless mutants in Drosophila

The fruitless gene in Drosophila produces male-specific protein (FRU (M)) i nvolved in the control of courtship. FRUM spatial and temporal patterns wer e examined in fru mutants that exhibit aberrant male courtship. Chromosome breakpoints at the locus eliminated FRUM. Homozygous viable mutants exhibit ed an intriguing array of defects. In fru (1) males, there were absences of FRUM-expressing neuronal clusters or stained cells within certain clusters , reductions of signal intensities in others, and ectopic FRUM expression i n novel cells. fru(2) males exhibited an overall decrement of FRUM expressi on in all neurons normally expressing the gene. fru(4) and fru(sat) mutants only produced FRUM in small numbers of neurons at extremely low levels, an d no FRUM signals were detected in fru(3) males. This array of abnormalitie s was inferred to correlate with the varying behavioral defects exhibited b y these mutants. Such abnormalities include courtship among males, which ha s been hypothesized to involve anomalies of serotonin (5-HT) function in th e brain. However, double-labeling uncovered no coexpression of FRUM and 5-H T in brain neurons. Yet, a newly identified set of sexually dimorphic FRUM/ 5-HT-positive neurons was identified in the abdominal ganglion of adult mal es. These sexually dimorphic neurons (s-Abg) project toward regions of the abdomen involved in male reproduction. The s-Abg neurons and the proximal e xtents of their axons were unstained or absent in wild-type females and exh ibited subnormal or no 5-HT immunoreactivity in certain fru-mutant males, i ndicating that fruitless controls the formation of these cells or 5-HT prod uction in them.