The maternal effect mutation sesame affects the formation of the male pronucleus in Drosophila melanogaster

After entering the oocyte and before the formation of the diploid zygote, t he sperm nucleus is transformed into a male pronucleus, a process that invo lves a series of conserved steps in sexually reproducing animals. Notably, a major modification of the male gamete. lies in the decondensation of the highly compact sperm chromatin. We present here the phenotype of sesame (ss m), a maternal effect mutation which affects the formation of the male pron ucleus in Drosophila melanogaster. Homozygous ssm(185b) females produce hap loid embryos which develop with only the maternally derived chromosomes. Th ese haploid embryos die at the end of embryogenesis. Cytological analyses o f the fertilization in eggs laid by ssm(185b) mutant females showed that bo th pronuclear migration and pronuclear apposition occurred normally. Howeve r, a dramatic alteration of the male pronucleus by which its chromatin fail ed to fully decondense was systematically observed, Consequently, the affec ted male pronucleus bees not enter the first mitotic spindle, which is orga nized around only the maternally derived chromosomes. Immunodetection of la mina antigens indicates that a male pronuclear envelope is able to form aro und the partially decondensed paternal chromatin, This suggests that the ma ternally provided sesame(+) function is required for a late stage of sperm chromatin remodeling. (C) 2000 Academic Press.