The ribosomal DNA of Luzula pilosa (L.) Willd, a plant with holocentric chromosomes

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Abstract
The ribosomal DNA (rDNA) of Luzula pilosa (L.) Willd, a plant with holocentric chromosomes, has been cloned and characterized by restriction mapping. The major variant is present in about 730 tandemly arranged copies per haploid genome which occupy nearly an entire chromosome. We propose that much of the rDNA is flanked or interspersed by kinetochores, so that reciprocal interchromatid exchanges in this region would lead to chromosome breakage. Homogenization and amplification of rDNA spacer length variants may occur largely by intrachromatid exchanges and gene conversion. By trying to isolate rDNA junction fragments which might be closely linked to centromeric DNA, we have isolated three minor rDNA variants whose spacer regions do not cross-hybridize with one another or with the major variant. The possibility that these minor variants may be rRNA pseudogenes is discussed.Key words: centromere, holocentric, Luzula, rDNA, recombination.