A plant carrying a small extra chromosome was found in Landsberg erecta ecotype of Arabidopsis thaliana. Fluorescence in situ hybridization revealed that this minichromosome was derived from the short arm of chromosome 4. The size of this "mini4S" chromosome was estimated to be ∼7.5 Mb on the basis of previously reported data and the amount of the centromeric major satellite (180-bp family) present, which was determined to be about 1 Mb, or about one third of that in the normal chromosome 4. No pairing between mini4S and its original chromosome 4 was observed at pachytene and metaphase 1 stages. The transmission of mini4S through pollen was limited, but about 30% of selfed progeny carried the mini4S chromosomes. The transmission rates considerably increased when the mini4S chromosomes were transferred to plants with a Columbia background by successive backcrosses. This suggests that the stability of the minichromosomes is controlled genetically by factors that can vary between ecotypes.
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