Inter- and intragenomic chromosome pairing in haploids of durum wheat

To assess inter- and intragenomic chromosome pairing in durum wheat (Triticum turgidum L.), 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 5D. The Ph1- haploids extracted from seven durum cultivars had little pairing; on the average only 3.1% of the chromosome complement was paired with 0.23 chiasma per pollen mother cell (PMC). Variation in haploid chromosome pairing frequency was observed among the seven genotypes. Chromosomes of the A and B genomes in the ph1c-haploids showed increased pairing, with 38.6% of the complement paired and 3.0 chiasmata per cell. The potential of intergenomic pairing was more fully realized in the substitution haploids, which had 51.3% of the complement paired with a chiasma frequency of 4.1 per cell. Fluorescent GISH (genomic in situ hybridization) analysis of PMCs revealed that most of the pairing was intergenomic, that is, between the chromosomes of the A and B genomes. Up to six intergenomic bivalents were observed. Ring bivalents were common; a few showed interlocking. A low frequency of intragenomic pairing within the A genome and within the B genome was observed; the GISH analysis confirmed that this was not caused by intergenomic translocations. Bivalents within the A genome likely involved chromosomes 4A and 7A, and were more frequent than those within the B genome. Chromosome pairing and chiasma frequencies in the 5D(5B) substitution haploids were similar to those in amphihaploids obtained by hybridization between the putative progenitors of durum wheat. It is obvious that the homoeologous pairing control mechanism present In 5B exercises almost total pairing control that exists in durum wheat and that the effect of other pairing control genes, if any, is insignificant. Moreover, the A and B genomes have undergone little structural modification since the evolution of durum wheat several thousand years ago. Durum haploids, therefore, offer excellent tools for studying genomic relationships in durum wheat.