Molecular cytogenetic analysis of tetraploid and hexaploid Aegilops crassa

The distribution of highly repetitive DNA sequences on chromosomes of tetra ploid and hexaploid cytotypes of Aegilops crassa ((DXcr)-X-cr1 and (DXDcr2) -X-cr1-D-cr genomes) was studied using C-banding and in situ hybridization analyses with the pSc119, pAs1 and pTa794 DNA clones. In total, 14 tetraplo id and five hexaploid accessions were examined. All chromosomes can be iden tified by their C-banding and ISH pattern with the pAs1 DNA clone. Only a f ew pSc119 hybridization sites were observed in the telomeric regions of sev eral chromosomes. We found a high level of C-banding polymorphism and only minor Variations in labeling patterns. The position of C-bands generally co incided with the location of the pAs1 sequence. Three 5S rDNA loci were det ected in tetraploid Ae. crassa, whereas five pTa794 ISH sites were observed in 6x Ae. crassa. Ail the hexaploid accessions differed from the tetraploi ds by a reciprocal non-centromeric translocation involving chromosomes A an d N. Three additional translocations were detected in the accessions analyz ed. The D-cr1 genome of 4x Ae. crassa is highly modified compared with the D genome of the progenitor species Ae. tauschii. Because of the large amoun t of chromosomal rearrangements, the origin of the X-cr genome remains unkn own. The second D-cr2 genome of 6x Ae. crassa is different from the D-cr1 g enome but is similar to the D-genome chromosomes of Ae. tauschii, indicatin g that no additional large rearrangements occurred at the hexaploid level.