CHROMOSOME-PAIRING RELATIONSHIPS AMONG THE A-GENOME, B-GENOME, AND D-GENOME OF BREAD WHEAT

Chromosome pairing and chiasma frequency were studied in bread wheat euhaploids (2n = 3x = 21; ABD genomes) with and without the major pairing regulator Ph1. This constitutes the first report of chromosome pairing relationships among the A, B, and D genomes of wheat without the influence of an alien genome. All Ph1 euhaploids had very little pairing, with 0.62-1.05 rod bivalents per cell; ring bivalents were virtually absent and mean arm-binding frequency (c) values ranged from 0.050 to 0.086. In contrast, the ph1b euhaploids had extensive homoeologous pairing, with chiasma frequency 7.5-11.6 times higher than that in the Ph1 euhaploids. They had 0.53-1.16 trivalents, 1.53-1.74 ring bivalents, and 2.90-3.57 rod bivalents, with c from 0.580 to 0.629. N-banding of meiotic chromosomes showed strongly preferential pairing between chromosomes of the A and D genomes; 80% of the pairing was between these genomes, especially in the presence of the ph1b allele. The application of mathematical models to unmarked chromosomes also supported a 2:1 genomic structure of the ph1b euhaploids. Numerical modeling suggested that about 80% of the metaphase I association was between the two most related genomes in the presence of ph1b, but that pairing under Ph1 was considerably more random. The data demonstrate that the A and D genomes are much more closely related to each other than either is to B. These results may have phylogenetic significance and hence breeding implications.