Submitted to: Journal of Heredity
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/18/1999
Publication Date: N/A
Interpretive Summary: Common bread wheat has three progenitors which contributed three genomes (sets of chromosomes), called AA, BB, and DD, to wheat. Pairing relationships among chromosomes of these genomes have been assessed from the study of haploids (plants with half the chromosome number) of bread wheat. We wanted to know how the chromosomes of the A and B genomes would pair in the absence of the D genome. Because the A and B genomes are present in durum wheat we extracted haploids of durum to assess the pairing relationships between the A and B genomes as well as within the A and within the B genome. Haploids with and without the pairing regulator, Ph1, were produced by crossing appropriate durum genotypes with maize the technique standardized earlier in our laboratory. We studied the specificity of chromosome pairing using a specialized technique, fluorescent in situ hybridization (FISH). We have shown that chromosomes of the A and B genomes pair very well in the absence of Ph1. This pattern of pairing is similar to that in synthetic hybrids between the A- and the B-genome donors. Thus, we have demonstrated that the two genomes have undergone very little change since the evolution of durum wheat over 10,000 years ago.
Technical Abstract: To assess inter- and intragenomic chromosome pairing relationships in durum wheat, chromosome pairing and chiasma frequency were studied in durum haploids (2n=2x=14; AB genomes) with the Ph1 allele, haploids with the ph1c allele, and substitution haploids with chromosome 5B replaced by chromosome 5D. The Ph1-haploids extracted from seven durum cultivars, Durox, Langdon, Lloyd, Medora, Monroe, Renville and Vic, had very little pairing, with only 3.14% of the chromosome complement paired and 0.23 chiasma per cell. Genotypic variation in chromosome pairing was observed among these haploids. Chromosomes of the A and B genomes in the ph1c-haploids showed an increase in pairing, with 38.57% of the complement paired and 3.0 chiasmata per cell. The potential of intergenomic pairing was much better realized in the substitution haploids, which had 51.33% of the complement paired with chiasma frequency of 4.1 per cell. FISH (fluorescent in situ hybridization) analysis of meiocytes revealed that most of the pairing was intergenomic, i.e. between the chromosomes of the A and B genomes. Up to 6 intergenomic bivalents were observed, and the highest association was 1 III + 5 II + 1 I. Intergenomic ring bivalents were frequently observed, a few showing interlocking. A low frequency of intragenomic pairing within the A genome and within the B genome was observed but the FISH analysis indicated that this was not caused by intergenomic translocations. Intragenomic bivalents within the A genome were more frequent than within the B genome. These might have formed because of the presence of residual homology with a genome or perhaps due to intragenomic translocations.