| The complexity of the metaphases of nullisomic-5B haploids could be caused in two
ways, first an increase in the chiasma frequency of cells that already have a high degree
of synapsis or secondly, an increase in synapsis and either as a result of this or by a
separate and simultaneous action an increase in the chiasma frequency. If the absence of chromosome 5B simply allowed an increase in chiasma frequency then in nullisomics of 5B, where there is already total homologous synapsis, a change in the chiasma frequency would be directly detectable. Table 1 gives the chromosome pairing and the chiasma frequency per paired chromosome in examples of 42-chromosome euploid, 21-chromosome euhaploid, 40-chromosome nullisomic 5B and 20-chromosome nullisomic-5B haploid plants. The number of chiasmata per paired chromosome in the nullisomic is not significantly different from that of euploid and therefore the increased pairing in the nullisomic-5B haploid is not a result of an increase in chiasma frequency in cells that already have a high degree of synapsis. Also there cannot be a separate and simultaneous increase in chiasma frequency for this too should lead to a detectable increase in the chiasma frequency in the nullisomic. Therefore, the most plausable hypothesis describing the action of chromosome 5B at prophase is that it prohibits synapsis between homoeologous chromosomes in both euploid and euhaploid so that only bivalents are formed in the 42 chromosome plants and mainly univalents in the euhaploids where no homologous chromosomes are present. The removal of 5B in nullisomic-5B haploids allows synapsis to occur between presumably homoeologous chromosomes and there is consequently a rise in the chiasma frequency. Whilst the absence of chromosome 5B in nullisomic 5B plants cannot increase the already total synapsis it can, and does, allow the attraction and synapsis of homoeologous chromosomes so that the highly characteristic metaphase picture is produced. |
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