MUTATIONAL ANALYSES OF FS(1)YA, AN ESSENTIAL, DEVELOPMENTALLY-REGULATED, NUCLEAR-ENVELOPE PROTEIN IN DROSOPHILA

The fs(1)Ya protein (YA) is an essential, maternally encoded, nuclear lamina protein that is under both developmental and cell cycle control . A strong Ya mutation results in early arrest of embryos. To define t he function of YA in the nuclear envelope during early embryonic devel opment, we characterized the phenotypes of four Ya mutant alleles and determined their molecular lesions. Ya mutant embryos arrest with abno rmal nuclear envelopes prior to the first mitotic division; a proporti on of embryos from two leaky Ya mutants proceed beyond this but arrest after several abnormal divisions. Ya unfertilized eggs contain nuclei of different sizes and condensation states, apparently due to abnorma l fusion of the meiotic products immediately after meiosis. Lamin is l ocalized at the periphery of the uncondensed nuclei in these eggs. The se results suggest that YA function is required during and after egg m aturation to facilitate proper chromatin condensation, rather than to allow a lamin-containing nuclear envelope to form. Two leaky Ya allele s that partially complement have lesions at opposite ends of the YA, p rotein, suggesting that the N- and C-termini are important for YA func tion and that YA might interact with itself either directly or indirec tly.