The timing of chromosome elimination in hexaploid wheat × maize crosses

Early seed development in crosses between the hexaploid wheat genotype 'Chinese Spring' and the maize genotype 'Seneca 60' was studied to determine the timing of elimination of the maize chromosomes. Elimination of one or more maize chromosomes occurred in about 70% of zygotic mitoses. Metaphase nuclei from two-celled embryos had 5 to 10 maize chromosomes, most of which were lost during the second cell division. About half the metaphase nuclei from four-celled embryos had no maize chromosomes, and the remainder had one to five. Anaphase or telophase nuclei from four-celled embryos showed no maize chromosomes in about half the cells and one or more pairs of lagging maize daughter chromosomes in the remainder. No maize chromosomes were seen in metaphase preparations from embryos with eight or more cells. These data strongly suggest that all maize chromosomes were lost during the first three cell-division cycles in most embryos. All embryos with four or more cells had micronuclei, showing that embryo development was dependent on fertilization. The only primary endosperm metaphase obtained in the experiment had 42 wheat and 10 maize chromosomes and the presence of micronuclei in most developing endosperms showed that at least 85% were of hybrid origin. Few endosperm nuclei were formed in comparison with self-pollinated 'Chinese Spring' caryopses, and many were abnormal. The implications of the results for wheat haploid production and gene transfer from maize to wheat are discussed.Key words: wheat, maize, wide hibridization, chromosome elimination.