CHROMOSOME BEHAVIOUR AND FERTILITY IN DIPLOID WHEAT WITH TRANSLOCATION COMPLEXES OF FOUR AND SIX CHROMOSOMES

Abstract

Seven different reciprocal chromosome translocations involving six of the seven haploid chromosomes of Triticum monococcum have been studied in all possible heterozygous combinations. Plants with one complex of four chromosomes show only 5 to 10% sterility; those with two complexes of four, 10 to 20%; those with one complex of six, 20 to 30%. Completely random segregation in such types should cause 66.6, 88.8, and 90% sterility, respectively, while segregation directed only by the necessity that homologous centromeres go to opposite poles should cause 50, 75, and 75%, respectively. The very low sterility of translocation heterozygotes in wheat, as compared with these expectations, and with results reported in maize and other plants, is due to the fact that segregation is usually alternate (disjunctional) in complexes both of four and six.The forces of repulsion operating at metaphase are not restricted to the centromere but involve the whole body of the chromosome. In the absence of complicating features this naturally results in alternate segregation in complexes. Semisterility is due, not to basically random segregation, but to special conditions such as interstitial chiasmata, early opening of the complex, and crossing over between the centromere and the point of interchange. The latter is favoured by a non-median position of the centromere, great length of chromosomes, and shortness of at least one interchanged segment. The 50% sterility usually reported for translocations has no special significance; no particular percentage is characteristic of translocations in general.In complexes of six, double-cross configurations are more numerous than stars, and present many variations in form depending on the length and position of the segments exchanged. Additional factors producing sterility in large complexes are unwieldiness and crossing over in the segment that joins the two crosses.