Hamster origin of metaphases with multiple chromosome rearrangements in first cleavage human-hamster embryos

Laboratories using the human sperm-hamster egg fertilization system to analyse sperm chromosomes obtain, sporadically, metaphases with multiple aberrations. Due to the high number of aberrations, these metaphases cannot be fully karyotyped. In some of them, one or several human chromosomes can be identified, guaranteeing the human origin of the whole metaphase. However, in others, none of the chromosomes can be recognized as human. This latter type of grossly rearranged metaphases is characterized by complex chromatid exchanges, multifragmented chromosomes and pulverized chromosome material. Using fluorescent in-situ hybridization techniques (FISH) with either human or hamster genomic DNA probes, we examined the origin of this second type of metaphase with multiple chromatid exchanges and fragmented chromosomes. Our study demonstrates that all of them hybridize with hamster genomic DNA probes and not with human DNA, proving their hamster origin. Since some of these metaphases seem to be diploid, we suggest that they may arise from hamster eggs that have failed to complete meiosis and have not extruded the second polar body.