Abstract
A monosomic addition plant of Beta vulgaris, containing an extra chromosome of B. procumbens with a gene for resistance to the beet cyst nematode, gave rise to a diploid descendant with an additional chromosome fragment. This fragment was found to bear the gene for resistance. A study of its morphology and behaviour made it likely that it represents the short arm of the alien chromosome and may have originated by centric fission of the single addition chromosome at anaphase I or II. The fragment showed incomplete generative transmission. In part of the somatic cells the fragment was missing and occasionally more than one fragment per cell was observed. The transmission of resistance in the telosome addition material was about 10%, which is 2.5 times lower than that of the monosomic addition. The pattern of transmission can be explained by an incomplete centromere, otherwise disturbed centric activity, or chromatid cohesiveness. Comparison with other monosomic addition plants showed that the resistance gene is located on the short arm of the satellite chromosome of B. procumbens and thus is linked to the nucleoar organizer region. The long arm of this chromosome possesses genes influencing annuality and leaf shape.Key words: Beta vulgaris, Beta procumbens, beet cyst nematode, monosomic addition, centric chromosome fragment.
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