Meiotic recombination and flanking marker exchange at the highly unstable human minisatellite CEB1 (D2S90)

Unequal crossover has long been suspected to play a role in the germline-sp ecific instability of tandem-repeat DNA, but little information exists on t he dynamics and processes of unequal exchange. We have therefore characteri zed new length alleles associated with flanking-marker exchange at the high ly unstable human minisatellite CEB1, which mutates in the male germline by a complex process often resulting in the gene conversion-like transfer of repeats between alleles. DNA flanking CEB1 is rich in single-nucleotide pol ymorphisms (SNPs) and shows extensive haplotype diversity, consistent with elevated recombinational activity near the minisatellite. These SNPs were u sed to recover mutant CEB1 molecules associated with flanking-marker exchan ge, directly from sperm DNA. Mutants with both proximal and distal flanking -marker exchange were shown to contribute significantly to CEB1 turnover an d suggest that the 5' end of the array is very active in meiotic unequal cr ossover. Coconversions involving the interallelic transfer of repeats plus immediate flanking DNA were also common, were also polarized at the 5' end of CEB1, and appeared to define a conversion gradient extending from the re peat array into adjacent DNA. Whereas many mutants associated with complete exchange resulted in simple recombinant-repeat arrays that show reciprocit y, coconversions were highly gain-biassed and were, on average, more comple x, with allele rearrangements similar to those seen in the bulk of sperm mu tants. This suggests distinct recombination-processing pathways producing, on the one hand, simple crossovers in CEB1 and, on the other hand, complex conversions that sometimes extend into flanking DNA.