Mapping Chromosome Landmarks in the Centromere I Region ofNeurospora crassa

Abstract Chromosome translocation breakpoints, RFLP heterozygosity in partial chromosome duplications, and RFLP-marked crossover events have been used as chromosomal landmarks to find the position and orientation of cloned regions flanking centromere I of Neurospora crassa. Determination of physical:genetic ratios in genomic regions flanking the loci mei-3, un-2, and his-2 supports previous evidence indicating that recombinational activity is lower in regions flanking centromere I than in the general N. crassa genome. The homogeneous distribution of crossover events found in these regions suggests that there is not a gradient of crossover inhibition in the vicinity of centromere I. Thus, a largely extended centromeric effect and/or a general crossover inhibitory effect operating on linkage group I (LGI) could constitute the basis of these abnormal physical:genetic ratios. A DNA element containing about 76% A + T was isolated from the centromeric end of a cloned region on LGIR. The fragment includes a previously undescribed DNA sequence, highly repeated in the Neurospora genome, which may correspond to centromeric DNA.