| The effect of the deficiency of the long arm of chromosome
5B on meiotic pairing in Triticum aestivum R. RILEY and V. CHAPMAN Plant Breeding Institute, Cambridge, England The original hypothesis of RILEY and CHAPMAN (1958) and of SEARS and OKAMOTO (1958), that the deficiency of chromosome 5B from the chromosome complement of Triticum aestivum results in the meiotic pairing of homoeologous as well as of homologous chromosomes, has now been confirmed (RILEY and KEMPANNA 1963). At the time of the first investigation of the activity of this chromosome, in 20 - chromosome nullisomic - 5B haploids, evidence was available from 21 - chromosome haploids carrying the long arm of 5B as an isochromosome that this arm was independently capable of preventing homoeologous pairing. This was confirmed by observation of the entirely homologous pairing behaviour of 41 - and 42 - chromosome plants respectively monotelocentric and ditelocentric for the long arm but deficient for the short arm. However, it was difficult to ascertain whether the short arm produced any effect on homoeologous association since plants of T. aestivum that carried the short but lacked the long arm could not be obtained. This obstacle arose because although the telocentric for the short arm had been isolated it could only be maintained in combination with a complete chromosome. When a plant with one complete and one short telocentric chromosome 5B was self - pollinated the progeny obtained never contained individuals in which the short telocentric was disomic, presumably due to certation. Ultimately by crossing a range of forms of T. aestivum, that had different conditions of 5B, with Aegilops cylidrica, hybrids were obtained that were either deficient for the complete chromosome, or for its long or short arm separately, or in which the complete chromosome was present (RILEY 1960). The meiotic behaviour of hybrids carrying the short arm telocentric was similar to that of those lacking the complete chromosome, whereas the behavior of the hybrids carrying the long arm telocentric resembled that of the euploid hybrids. Consequently it was reasonable to conclude that the activity that resulted in the suppression of homoeologous affinity was confined to the long arm alone. However, in an attempt to check this result work was continued with plants of T. aestivum. Holdfast with various telocentric conditions of chromosome 5B. In the course of this investigation 42 - chromosome plants with one complete and one short arm telocentric 5B were pollinated with the pollen of individuals ditelocentric for the long arm telocentric. Amongst the F1 generation of this cross there were plants with 42 chromosomes in which 5B was represented as one long arm and one short arm telocentric. At meiosis both telocentrics were univalents and they were transmitted irregularly to the progeny of the doubly telocentric F1 plants. Table 1 shows the numbers of plants with various chromosome constitutions, as determined at meiosis, that were obtained in these progenies. |
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