The daughterless (da) gene in Drosophila encodes a broadly expressed 
transcriptional regulator whose specific functions in the control of sex 
determination and neurogenesis have been extensively examined. We describe here 
a third major developmental role for this regulatory gene: follicle formation 
during oogenesis. A survey of da RNA and protein distribution during oogenesis 
reveals a multiphasic expression pattern that includes both germline and soma. 
Whereas the germline expression reflects da's role in progeny sex determination, 
the somatic ovary expression of da correlates with the gene's role during egg 
chamber morphogenesis. Severe, but viable, hypomorphic da mutant genotypes 
exhibit dramatic defects during oogenesis, including aberrantly defined 
follicles and loss of interfollicular stalks. The follicular defects observed in 
da mutant ovaries are qualitatively very similar to those described in Notch (N) 
or Delta (Dl) mutant ovaries. Moreover, in the ovary da- alleles exhibit 
dominant synergistic interactions with N or Dl mutations. We propose that all 
three of these genes function in the same regulatory pathway to control follicle 
formation.