Three types of mutants that exhibit defective mitotic chromosome behavior are being used to analyze the events of the chromosome cycle in Drosophila. 1) Meiotic mutants: To determine whether meiotic mutants (mutants in which meiotic recombination and/or chromosome segregation is abnormal) are utilized in somatic cells their effects on mitotic chromosome stability have been examined genetically; many disrupt mitotic chromosome integrity or segregation. Of particular interest, several disjunction-defective meiotic mutants may be leaky mutants in processes common to meiotic and mitotic chromosome segregation. The mitotic effects of these mutants will be examined in detail, 2) Mutagen-sensitive mutants: We have shown genetically that many mutagen-sensitive mutants (mutants hypersensitive to killing by mutagens; Boyd et al. 1976, Smith 1976) also specify functions used in non-mutagenized cells to maintain chromosome integrity. Cytological studies of recombination-defective and mutagen-sensitive mutants reveal high frequencies of spontaneous chromosome aberrations. Mutants at different loci exhibit characteristic patterns (chromosome vs chromatid) and distributions (euchromatic vs heterochromatic) of aberrations. Experiments are planned to determine the cause of the differences. 3) Mitotic mutants: We have recently isolated 15 temperature-sensitive lethal mitotic mutants (mutants in essential loci that disrupt mitotic chromosome segregation and/or the maintenance of mitotic chromosome integrity). The initial genetic and cytological characterizations of these mutants will identify the aspect of the chromosome cycle (segregation, condensation, DNA synthesis, etc) in which they are defective. An examination of these zeste-white region of the X chromosome suggests that it is a reverse repeat. This notion will be tested. Mutants defective in sex determination are being studied to sequence the steps they specify and the developmental time at which these loci act. The time at which the number of X chromosomes vs autosomes functions in sex determination is being determined.