Distribution of Nonstructural Variation along Three Chromosome Arms between Wheat Cultivars Chinese Spring and Cheyenne

Metaphase I (MI) pairing of wheat homologous chromosomes is usually reduced in hybrids between cultivars relative to the parental inbred lines. Previous work suggested that this phenomenon is caused by polymorphism in nucleotide sequences (nonstructural chromosome variation) among wheat cultivars. The present work investigated the distribution of this variation along three selected chromosome arms between cultivars Chinese Spring and Cheyenne. Chinese Spring ditelosomics 3Aq, 6Ap and 6Bp were crossed with disomic substitutions of Cheyenne chromosomes 3A, 6A and 6B in Chinese Spring, respectively. The resulting F₁ plants, called substituted monotelodisomics, were crossed with the respective Chinese Spring monosomics, producing potentially "recombinant" substituted monosomics. When these "recombinant" chromosomes were combined with the parental Chinese Spring telosomes, marked reductions in mean telosome-pairing frequency were found compared with the corresponding Chinese Spring monotelodisomics. The mean pairing frequencies of the "recombinant" chromosomes showed a continuous distribution between those of the substituted and Chinese Spring monotelodisomics. The results suggest that the nonstructural variation that reduces MI pairing between chromosomes of different wheat cultivars is not localized in a specific site but distributed along each chromosome arm. Little variation was found among monotelodisomics for either the number of ring bivalents per cell or the number of univalents other than those constituting the heteromorphic pair. This implies that the reductions in MI pairing between the Cheyenne and Chinese Spring chromosomes are caused by something residing within these specific chromosomes that does not affect the pairing of the remaining Chinese Spring chromosomes in the same cell. Furthermore, the absence of parental types among the "recombinant"-substituted monotelodisomics suggests that the sequences involved in the variation studied here are capable of converting heterohomologous chromosomes to something intermediate in nature in the span of only a single generation.