The probability with which different regions of a genome come in contact with one another is a question of general interest. The current study addresses this subject for vegetatively growing diploid cells of fission yeast Schizosaccharomyces pombe by application of the Cre/loxP site-specific recombination assay. High levels of allelic interactions imply a tendency for chromosomes to be colocalized along their lengths. Significant homology-dependent pairing at telomere proximal loci and robust nonspecific clustering of centromeres appear to be the primary determinants of this feature. Preference for direct homolog-directed interactions at interstitial chromosomal regions was ambiguous, perhaps as a consequence of chromosome flexibility and the constraints and dynamic nature of the nucleus. Additional features of the data provide evidence for chromosome territories and reveal an intriguing phenomenon in which interaction frequencies are favored for nonhomologous loci that are located at corresponding relative (rather than absolute) positions within their respective chromosome arms. The latter feature, and others, can be understood as manifestations of transient, variable, and/or occasional nonspecific telomeric associations. We discuss the factors whose interplay sets the probabilities of chromosomal interactions in this organism and implications of the inferred organization for ectopic recombination.
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