An analysis of temperature-dependent asynapsis inTriticum aestivum

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Abstract
Measurements of chiasma frequency over a range of temperatures from 9 to 35 °C in euploid wheat and lines nullisomic for chromosome 5D showed the absence of chromosome 5D greatly to increase the sensitivity of chiasma frequency to temperature, resulting in complete univalence at the lowest temperatures. Careful examination of meiotic prophase showed the lowering of chiasma frequency to be correlated with failure of zygotene chromosome pairing, justifying the use of the term asynaptic. Observation of the pairing behaviour of an isochromosome in the nullisomic 5D background suggested that the asynapsis was due to a failure in the mechanism of chromosome pairing rather than of the pre-alignment of homologues. The observed effects in the absence of chromosome 5D could be due to the presence of a recessive allele at the locus of chromosome 5A, also known to control stability of chiasma frequency to temperature. In conclusion it is pointed out that in euploid wheat the sensitivity of chiasma frequency to temperatures within the normal meteorological range could influence the cytological stability of the wheat crop.