| The effect of chromosome 5B at prophase G. KIMBER Plant Breeding, Institute, Cambridge, England The genetical control of the diploid-like meiosis of the polyploid wheats has been shown to be located on the long arm of chromosome 5B (V). (See publications of Riley et.al.). The effect of 5B is to limit the pairing in the polyploid wheats to strictly homologous chromosomes and thus regimenting as bivalents chromosomes that would form multivalents in its absence. This control of pairing is recognised by its effects at first meiotic metaphase, that is after both pairing and chiasma formation have taken place. Consequently it is impossible to decide whether the mechanism acts through the control of either or both of these agencies. To make this distinction it is necessary to examine the prophase of certain derivatives of T. aestivum. Prophase studies in T. aestivum are complicated by the difficulty of making good squash preparations because of the large number of chromosomes present. The analysis is further complicated since all of the centromeres are essentially submedian in position, and consequently it is not possible to recognise individual chromosomes by this feature. Also as two chromosomes (1B and 6B) are satellited the usefulness of this diagnostic character is limited. However, certain general conclusions concerning the group action of the chromosomes can be gleaned from an examination of prophase preparations. The prophase of euploid T. aestivum exhibits clearly all of the stages that are recognized as typical of a normal meiotic prophase. No anomalous chromosome behaviour was observed. The prophase of 21-chromosome euhaploids of T.aestivum was quite distinct from that of 42-chromosome euploids. There was some, but very little, chromosome pairing in the euhaploids and a certain degree of desynapsis took place so that the metaphase pairing gave a minimal estimate of the potential pairing ability. The prophase of haploids nullisomic for chromosome 5B presents an entirely different picture. In the majority of the cells examined much more pairing was observed and configurations of great complexity were seen quite frequently. Often three and occasionally more chromosomes were seen to be involved in one figure giving a preview of the multivalent configurations seen at meta-anaphase. In addtion to this increased complexity the chromosomes lay closer together than in euhaploids and the prophases of nullisomic-5B haploids were comparable to those observed in euploids in this respect. With the restriction that each chromosome has a homologous partner in nullisomics, the prophase pairing of nullisomic 5B was remarkably similar, in its general appearance,to that of nullisomic-5B haploids. There was aparently no change in the forces initiating homologous pairing in nullisomic 5B but superimposed on the normal and regular pattern was a freedom of presumably homoeologous chromosomes to pair with each other. The complex multivalents that were visible at metaphase in nullisomic 5B are a reflection of this intergenomic synapsis at prophase. |
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