PHYSICAL DISTRIBUTION OF TRANSLOCATION BREAKPOINTS IN HOMOEOLOGOUS RECOMBINANTS INDUCED BY THE ABSENCE OF THE PH1 GENE IN WHEAT AND TRITICALE

The physical distribution of translocation breakpoints was analyzed in homoeologous recombinants involving chromosomes 1A, 1B, 1D of wheat a nd 1R of rye, and the long arms of chromosome 7S of Aegilops speltoide s and 7A of wheat. Recombination between homoeologues was induced by r emoval of the Phl gene. In all instances, translocation breakpoints we re concentrated in the distal ends of the chromosome arms and were abs ent in the proximal halves of the arms. The relationship between the r elative distance from the centromere and the relative homoeologous rec ombination frequency was best explained by the function f(x)=0.0091e(0 .0592x). The pattern of recombination in homoeologous chromosomes was essentially the same as in homologues except that there were practical ly no double exchanges. Among 313 recombinant chromosomes, only one re sulted from a double crossing-over. The distribution of translocation breakpoints in translocated arms indicated that positive chiasma inter ference operated in homoeologous recombination. This implies that the reduction of the length of alien chromosome segments present in transl ocations with wheat chromosomes may be more difficult than the product ion of the original recombinants.