Chromatid and Chromosome Type Breakage-Fusion-Bridge Cycles

During the development of disomic additions of rye (Secale cereak L.) chromosomes to wheat (Triticum aestivum L.), two reverse tandem duplications on wheat chromosomes 3D and 4A were isolated. By virtue of their meiotic pairing, the reverse tandem duplications initiated the chromatid type of the breakagefusion-bridge (BFB) cycle. This BFB cycle continued through pollen mitoses and in the early endosperm divisions, but no clear evidence of its presence in embryo mitoses was found. The chromosome type of BFB cycle was initiated by fusion of two broken chromosome ends resulting in a dicentric or a ring chromosome. Chromosome type BFB cycles were detected in embryo mitoses and in root tips, but they did not persist until the next meiosis and were not transmitted to the progeny. Active BFB cycles induced breakage of other wheat chromosomes that resulted in additional reverse tandem duplications and dicentric and ring chromosomes. Four loci, on chromosome arms ZBS, 3DS, 4AL, and most likely on 7DL, were particularly susceptible to breakage. The BFB cycles produced high frequency of variegation for pigmentation of the aleurone layer of kernels and somatic chimeras for a morphological marker. With the exception of low mutation rate, the observed phenomena are consistent with the activity of a Dslike element. However, it is not clear whether such an element, if indeed present, was of wheat or rye origin. M cCLINTOCK (1938a, 1941a,b, 1951) extensively studied the behavior of broken chromosome ends in maize (Zea mays L.) and described two types of breakage-fusion-bridge (BFB) cycles. In the chromatid type of BFB cycle, a newly broken end of a chromosome, following a fusion of sister chromatids at the point of breakage, produces an acentric fragment and a chromatid bridge in mitotic anaphase. The bridge is broken and each daughter nucleus receives one broken chromosome end. Upon chromosome replication, the sister chromatids fuse again at the new point of breakage, and the cycle continues until the broken ends are healed. In maize, the chromatid-type BFB cycle continues in the gametophyte and in the endosperm, but it almost always ceases in the zygote. When two chromosomes with broken ends are introduced into the zygote through both gametes, the broken ends fuse forming a dicentric chromosome. Depending on the orientation of the centromeres, a dicentric chromosome may produce a two-chromatid bridge in mitotic anaphase, thereby initiating a chromosome-type BFB cycle. Careful observations of the progenies of maize plants with active BFB cycles demonstrated that the chromatid type and the chromosome type BFB cycles may alter the structure of chromosomes other than the ones directly involved in the cycle. This led to the discovery of the Ac-Ds and other transposition systems (MCCLINTOCK 1951, 1978). It has been demonstrated in recent years that when present in the genetic background of wheat (Triticum Genetics 14