Paternal chromosome incorporation into the zygote nucleus is controlled bymaternal haploid in Drosophila

maternal haploid (mh) is a strict maternal effect mutation that causes the production of haploid gynogenetic embryos (eggs are fertilized but only mat ernal chromosomes participate in development). We conducted a cytological a nalysis of fertilization and early development in mh eggs to elucidate the mechanism of paternal chromosome elimination. In mb eggs, as in wild-type e ggs, male and female pronuclei migrate and appose, the first mitotic spindl e forms, and both parental sets of chromosomes congress on the metaphase pl ate. In contrast to control eggs, mb paternal sister chromatids fail to sep arate in anaphase of the first division. As a consequence the paternal chro matin stretches and forms a bridge in telophase. During the first three emb ryonic divisions, damaged paternal chromosomes are progressively eliminated from the spindles that organize around maternal chromosomes. A majority of mb embryos do not survive the deleterious presence of aneuploid nuclei and rapidly arrest their development. The rest of mb embryos develop as haploi d gynogenetic embryos and die before hatching. The mb phenotype is highly r eminiscent of the early developmental defects observed in eggs fertilized b y ms(3)K81 mutant males and in eggs produced in incompatible crosses of Dro sophila harboring the endosymbiont bacteria Wolbachia. (C) 2001 Academic Pr ess.