KEy WoRDS enhancer of rudimentary, CG15352, P element mutagenesis ABBREVIATIoNS e(r) enhancer of rudimentary r rudimentary AcKNoWlEDGEMENTSThis work was supported by grants from the National Institutes of Health (R15 GM64364), Saint Louis University, the Monsanto Company (Monsanto Scholars Program) and SUNY Brockport. ABSTRAcTThe enhancer of rudimentary gene, e(r), encodes a 104-amino-acid, highly conserved transcription cofactor. Hypomorphic mutations of e(r) show an enhancement of a hypomorphic rudimentary mutant wing phenotype. These mutants in a wild-type background are viable, fertile, and morphologically wild-type. Since the only mutant alleles were hypomorphic, it was important to isolate null mutations to determine if any other phenotypes might be associated with a loss-of-function of e(r). We utilized a marked P element, P{SUPor-P, y + }, located 895 bp upstream of the start of transcription of e(r) to generate nineteen deficiencies in the region. Deficiencies of e(r) enhance the mutant wing phenotype of a hypomorphic rudimentary allele, r hd1 . In a wild-type background, the deficiencies of e(r), unlike the hypomorphic alleles, have a low viability and females have low fertility. The expression of e(r) in the nurse cells of the ovary is consistent with the low fertility, and suggests an ovarian function for e(r). Deficiencies of CG15352, the gene directly upstream of e(r), are not associated with any obvious mutant phenotypes and present the possibility that it encodes a nonvital or redundant function.