Uniparental Chromosome Elimination at Mitosis and Interphase in Wheat and Pearl Millet Crosses Involves Micronucleus Formation, Progressive Heterochromatinization, and DNA Fragmentation

Complete uniparental chromosome elimination occurs in several interspecific hybrids of plants. We studied the mechanisms underlying selective elimination of the paternal chromosomes during the development of wheat (Triticum aestivum) x pearl millet (Pennisetum glaucum) hybrid embryos. All pearl millet chromosomes were eliminated in a random sequence between 6 and 23 d after pollination. Parental genomes were spatially separated within the hybrid nucleus, and pearl millet chromatin destined for elimination occupied peripheral interphase positions. Structural reorganization of the paternal chromosomes occurred, and mitotic behavior differed between the parental chromosomes. We provide evidence for a novel chromosome elimination pathway that involves the formation of nuclear extrusions during interphase in addition to postmitotically formed micronuclei. The chromatin structure of nuclei and micronuclei is different, and heterochromatinization and DNA fragmentation of micronucleated pearl millet chromatin is the final step during haploidization.