An ordered arrangement of chromosomes in the somatic nucleus of common wheat Triticum aestivum L. - II. Spatial relationships between chromosomes of different genomes

Spatial relationships between chromosomes of different genomes, both homoeologues (genetically related) and non-homoeologues, were studied in root-tip cells of common wheat, Triticum aestivum L. (2n = 6x = 42) by measuring the distance between cytologically marked chromosomes in cold arrested metaphases. No significant differences were found between the means distances of the different homoeologues. Comparisons of distances between homoeologues with those between their corresponding homologues showed clearly that, in every homoeologous group, the homologous chromosomes were always closer to one another than the homoeologues. Such a pattern of homologous association and relative homoeologous separation in premeiotic cells is believed to facilitate the exclusive pairing of homologues which characterizes the meiosis of common wheat. - No significant differences were found for distances of homoeologues compared with distances of non-homoeologues of different genomes. On the other hand, mean distance between chromosomes of different genomes was significantly greater than that between non-homologues of the same genome. This difference, of course, was much greater if distances between homologues were included in the comparison. Hence, the chromosomes of different genomes are relatively separated from one another. This implies that the three wheat genomes are not intermixed but, rather, tend to occupy different areas of the somatic nucleus. The significance of these intergenomic spatial relationships, their genetic control and cellular mechanism is discussed.