Another type of chromosomal variation, reported in about 1300 plant species, concerns the presence of B (= accessory, supernumerary) chromosomes (Trivers et al. 2004). They are dispensable, often heterochromatic and sometimes demonstrably selfish genetic elements, which do not pair with A complement members. They are found in some (but not all) individuals of a species and may even vary within the individual. When present in higher numbers, their effect on the phenotype is negative. In FCM research, their presence may result in apparent intraspecific or even intra-plant genome size variation that is usually difficult to interpret. The most thoroughly studied plant group with respect to B chromosomes is the Boechera holboellii complex (Brassicaceae). FCM analyses of leaf tissues revealed a bimodal distribution of nuclear fluorescence, corresponding to the absence/presence of accessory chromosomes (Sharbel et al. 2004). Interestingly, supernumerary chromosomes at the diploid level seemed to trigger apomic-tic reproduction (Sharbel et al. 2005).
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