Frequent meiotic recombination between the ends of truncated chromosome fragments of Saccharomyces cerevisiae

A single truncated chromosome fragment (TCF) in diploid cells undergoes fre quent ectopic recombination during meiosis between markers located near the ends of the fragment. Tetrads produced by diploids with a single TCF show frequent loss of one of the two markers. This marker loss could result eith er from recombination of the TCF with one of the two copies of the chromoso me from which it was derived or from ectopic recombination between the ends of the TCF. The former would result in shortening of a normal chromosome a nd lethality in one of the four spores. The high frequency of marker loss i n tetrads with four viable spores supports recombination between the TCF en ds as the main source of marker loss. Most of the spore colonies that displ ay TCF marker loss contained a TCF with the same marker on both ends. Delet ion of most of the pBR322 sequences distal to the marker at one of the subt elomeric regions of the TCF did not reduce the overall frequency of recombi nation between the ends, but affected the loss of one marker significantly more than the other. We suggest that the mechanism by which the duplication of one end marker and loss of the other occurs is based on association and recombination between the ends of the TCF.