Analysis of karyotypic stability of homoeologous-pairing (ph) mutants in allopolyploid wheats

Karyotypic analysis of wheat lines with different genotypes for the homoeol ogous-pairing loci Ph1 and Ph2 was carried out by means of a genomic in sit u hybridization method that allowed unequivocal identification of the A, B and D genomes. Chromosomal rearrangements mainly affecting the A and D geno mes were found in all plants of allohexaploid wheat (AABBDD) lacking Ph1 ac tivity. The frequency of intergenomic exchanges per plant in ph1b mutant an d nulli-5B lines was 4.31 and 3.40, respectively. In addition, an unbalance d genomic constitution was found in a few plants, some even showing a euplo id chromosomal number. By contrast, rearranged karyotypes were detected nei ther in the ph1 mutant line (ph1c) of allotetraploid wheat (AABB) nor in th e allohexaploid wheat lines lacking Ph2 activity, namely ph2b mutant and nu lli-3D lines. These results were compared with the chromosomal pairing beha viour displayed by mutant lines ph1c, ph1b and ph2b at first meiotic metaph ase. Despite the finding of standard, nonrearranged karyotypes in the ph1c tetraploid mutant, the frequency of A-B homoeologous metaphase I associatio n was similar to that observed in the ph1b hexaploid mutant. The results pr esented clearly demonstrate that inactivity of the Ph1 locus induces karyot ypic instability in wheat. Intergenomic exchanges have probably been accumu lating since the original phl mutant and aneuploid lines were obtained, whi ch should be taken into account when it is planned to use these lines for b asic research on Ph1 function or in applied wheat breeding programmes.