CONSTRUCTION OF MIDGET CHROMOSOMES IN WHEAT

To test the usefulness of breakage-fusion-bridge (BFB) cycles in gener ating new chromosome aberrations in bread wheat (Triticum aestivum L.) and to extend the range of aberrations available, a series of midget chromosomes was produced from the long arm of chromosome 1B. Using a r everse tandem duplication initiated chromatid type BFB cycle, the 1BL arm was broken and fused with centromeres of either chromosome 5BL or 1RS to form dicentric chromosomes. The 1R and 5B centromeres were brok en by centric misdivision. Among the progenies of plants with dicentri c chromosomes, two classes of monocentric chromosomes were selected: d eficient chromosomes 1B acid chromosomes that had IRS or 5BL for one a rm and various fragments of 1BL for the other arm. Following centric m isdivision of these monocentrics, midget chromosomes 1BL were isolated : deficient and deletion telocentrics and telocentrics derived from in terstitial regions of 1BL. By chance, one deficient chromosome IBS and one deletion chromosome 1BS were identified in unrelated lines of the same wheat. Following centric misdivision of these chromosomes, two m idget chromosomes covering the whole of 1BS were added to the set.