The spatial order of chromosomes in root-tip metaphases ofAegilops umbellulata

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Abstract
Twelve root-tip metaphase cells of the diploid grass A. umbellulata Zhuk. (2n = 2x = 14) were reconstructed in 3-dimensions from electron micrographs of serial thin sections. In 10 of these cells each individual chromosome in the nuclei was identified by morphological criteria. No evdience for somatic association of centromeres of homologous chromosomes was found. Computer analysis suggested that the centromeres are not arranged randomly on the metaphase plate but are ordered in 2 haploid sets and that the order within each set is that predicted by a model based on spatial association of pairs of corresponding arms of most similar size on heterologous chromosomes.