MALE AND FEMALE GENOMES ASSOCIATED IN A SINGLE PRONUCLEUS IN HUMAN ZYGOTES

The ploidy of single-pronucleated human zygotes obtained after convent ional in vitro fertilization was determined by fluorescent in situ hyb ridization (FISH) using multiple simultaneous probes for gonosomes and autosomes. After zona removal the single-pronucleated zygotes were ex posed to cytochalasin B, and the pronucleus, surrounded by scant cytop lasm and the plasma membrane (karyoplast), was divided from the rest o f the egg (cytoplast). The karyoplasts and the corresponding cytoplast s were analyzed separately by FISH. Of the 16 zygotes analyzed, 10 had haploid pronuclei and 6 were diploid. Four diploid pronuclei containe d XY chromosomes, and 2 contained XX chromosomes. These results sugges t that during the course of their interaction, human gamete nuclei can associate together and form diploid, single-pronucleated zygotes. The se findings confirm a newly recognized variation of human pronuclear i nteraction during syngamy.