DESCRIPTION: Genetic and molecular studies have shown that the Drosophila ovarian tumor gene functions at several key points during oogenesis. Mutations in this gene can be roughly grouped into three classes on the basis of the severity of the phenotype. The most severe mutants fall into the quiescent class (QUI). The ovaries of these mutants appear to lack germ cells altogether, and consist of small empty sheaths of somatic tissue.The second phenotypic class, for which locus was originally named, are the tumorous ovary alleles (ONC). In these mutants germ line cells are present; however, oogenesis is arrested at an early step and the ovary becomes populated with many, often hundreds, of small undifferentiated germ cells. The third phenotypic class, DIF, is least severe. The early steps in egg chamber formation often proceed almost normally in this class of mutants; however some chambers are formed which lack the oocyte or have abnormal numbers of nurse cells. Other chambers appear almost wild type, but at late stages of oogenesis the nurse cells in these chambers fail to dump their contents into the oocyte. The range of phenotypic defects in the different classes is most easily explained by a model which postulates that increasing levels otu function are required as oogenesis proceeds. Other hints as to the possible function of the otu product come from the observation that several of the ONC alleles show defects in the expression of Sex-lethal protein and the processing of Sxl mRNAs. Consistent with the idea that otu is required for the proper expression of Sxl in the female germ line is the finding that ONC mutants can be partially rescued by a constitutive Sxl allele. However, facilitating Sxl expression is not only function of otu since the consitutive Sxl allele has no effect on the phenotypes of the QUI or DIF otu mutants.