Meiotic instability of tandemly iterated plasmid sequences in the yeast chromosome

The stability of tandemly arrayed plasmid sequences in the yeast genome has been studied. Haploid strains carrying 75 copies of a 25 kb plasmid repeat did not undergo significant loss of copy number during mitosis. Diploids in which only one chromosome contained the multiple iterated sequences displayed a similarly low level of mitotic instability. In contrast, 60-90% of the meiotic diploid progeny showed significant decreases in the number of iterated sequences. Segregation for the integrated sequences was generally 2:2, indicating that interchromosomal recombination was not the cause of meiotic instability. Meiotic progeny from diploids in which both chromosomes contained multiple integrated sequences also showed the same high rate of loss of iterated sequences. These results suggest that the meiotic instability is a result of intrachromosomal recombination, a process which is enhanced during meiosis.