MOLECULAR CYTOGENETIC ANALYSIS OF REPEATED SEQUENCES IN A LONG-TERM WHEAT SUSPENSION-CULTURE

A rapidly growing Triticum aestivum L. (wheat) derived long term suspe nsion culture (named TaKB1), that is probably not regenerable, was ana lysed for karyotype rearrangements, stability and changes in repetitiv e DNA. The cell line has an average chromosome number of 21 and the DN A amount of unreplicated cells of TaKB1 measured by flow cytometry is about 30% lower than an unreplicated (1C) bread wheat genome. In situ hybridization of a repetitive DNA sequence (pSc119.2), which occurs as tandemly repeated blocks (heterochromatin) in wheat, shows that chrom osomes from the TaKB1 line have fewer and weaker subtelomeric location s of the sequence than wheat, suggesting deletions of distal chromosom e segments and a reduction in the sites and copy number of the sequenc e. The in situ hybridization pattern and chromosome morphology allowed 27 chromosome types to be identified in the cell line. No two analyse d cells contained the same chromosome complement, although some chromo some types were present in every cell. Using Southern hybridization th e structure and copy number of a retroelement (Wis-2) and its flanking sequence was shown to be the same in the TaKB1 cell line and wheat. A n in situ analysis of rDNA in the TaKB1 cell line (using the, probe pT a71) showed a reduction in number of sites and rRNA genes in each cell from that in wheat. Interphase cells of the cell line showed disperse d signal throughout the nucleolus with no evidence for clusters of con densed and inactive rRNA genes.