We have tested the influence of recombinantly-elongated chromosome arms on nuclear divisions in barley and confirmed a rule according to which half the length of the average spindle axis defines the upper tolerance limit for chromosome arm length. A slightly longer chromosome arm caused incomplete separation of sister chromatids in approximately 70% of mitotic telophase cells and >2.5% of daughter cells showing a micronucleus, due to disruption of non-separated sister chromatids by the newly forming cell wall. In homozygous condition, this elongated chromosome mediated a slower growth and reduced fertility of the carrier plants. Its meiotic transmission was not impaired because of the larger spindle dimensions in meiocytes as compared to those in mitotic cells.
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