Recombination in an isochromosome preferentially occurs between cis isochromatids

An isochromosome has identical arms attached to the same centromere. At the pachytene stage of meiosis, it has four isochromatids and recombination ca n occur either between cis isochromatids (attached to the same half-centrom ere) or trans isochromatids (attached to different half-centromeres). Norma lly such recombination cannot be detected because all four chromatids are h omogenetic (arose from misdivision of a centromere to which genetically ide ntical sister chromatids were attached). We isolated an isochromosome of wh eat that is heterogenetic for the distal 64% of the long arm. The heterogen etic isochromosome was recovered from the progeny of a cross between Tritic um aestivum cv. Chinese Spring containing an isochromosome for the long arm of chromosome 5B (i5BL) and a disomic substitution line of Triticum turgid um sap. dicoccoides chromosome 5B in Chinese Spring wheat. New recombinants were produced when the two arms of i5BL(rec) paired at metaphase I of meio sis. Only trans isochromatid exchanges led to some homozygous loci in i5BL( rec), whereas exchanges between cis isochromatids resulted in heterozygosit y at an loci similar to the parental type. There was an average frequency o f 0.87 chiasmata per pollen mother cell for the heterogenetic i5BL, which w ill result in 0.44 cis and 0.44 trans isochromatid exchanges, assuming that both are occurring at the same frequency. The average crossover frequency based on recombination between trans isochromatid exchange detected by rest riction fragment length polymorphism analysis in 98 plants was 0.29. This o bserved value is significantly lower (P < 0.01) than the value of 0.44 as e xpected from chiasmata counts. Our study provides the first experimental ev idence that crossovers preferentially occur between cis isochromatids rathe r than trans isochromatids.