Forty-six stable (21II) hexaploid plants were cytologically screened in the F5 generation of a cross between a hexaploid triticale cv. Armadillo (2D/2R substitution type) and a bread wheat cv. Chinese Spring. In order to determine the constitution of R- and D-genome chromosomes of the stable F5 plants, F6 progeny of each F5 plant was analyzed by C-banding. Of the 46 plants, 43 had no translocation, while one was homozygous and two were heterozygous for translocation. Of the theoretically possible 26=64 kinds of chromosome constitutions, only 12 kinds were obtained in the stable plants without translocation. They had zero to six pairs of R-genome chromosomes and appeared with different frequencies. Frequently observed chromosome constitutions, which independently originated from many F2 progenitors, had one, four, five or six pairs of R-genome chromosomes. Chromosomes 1R, 3R and 6R were independently replaced by their respective homoeologous D-genome chromosomes. Chromosomes 4R, 5R and 7R always behaved together except in two infrequent chromosome constitutions in which 5R was separated from 4R and 7R. From the information so far reported about the homoeologous relationship between rye and wheat chromosomes, we inferred that the incomplete homoeology of 4R, 5R and 7R to the corresponding homoeologous D-genome chromosomes was responsible for the concurrent presence or absence of these three R-genome chromosomes.
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