Homologous recombination and transposition generate chromosome I neopolymorphism during meiosis in Saccharomyces cerevisiae

We have studied the meiotic segregation of a chromosome length polymorphism (CLP) in the yeast Saccharomyces cerevisiae. The neopolymorphism frequentl y observed within the smallest chromosomes (I, VI. III and IX) is not compl etely understood. We focused on the analysis of the structure of chromosome I in 88 segregants from a cross between YNN295 and FL100trp. Strain FL100t rp is known to carry a reciprocal translocation between the left arm of chr omosome III and the right arm of chromosome I. PCR and Southern hybridizati on analyses were performed and a method for the rapid detection of chromoso me I rearrangements was developed. Seven chromosome 1 types were identified among the 88 segregants. We detected 32 recombination events between homol ogous chromosomes I and seven ectopic recombination events between FL 100tr p chromosome III and YNN295 chromosome I. These recombination events occurr ed in 20 of the 22 tetrads studied (91%). Nine tetrads (41%) showed two rec ombination events. This showed that homologous recombination involving poly morphic homologues or heterologous chromosomes is the main source of neopol ymorphism. Only one of the seven chromosome I variants resulted from a tran sposition event rather than a recombination event. We demonstrated that a T y1 element had transposed within the translocated region of chromosome I, g enerating mutations in the 3' LTR, at the border between U5 and PBS.