Nuclear genomes of higher plants vary enormously in size. A substantial proportion of this variation is due to polyploidy, and it is assumed that 50% or more of angiosperms are polyploids. However, although all plants, at the diploid level, require the same number of genes and regulatory DNA sequences for physiological processes like germination, growth, flowering and reproduction, the total amount of the chromosomal DNA still ranges from some 130 Mbp in the model species Arabidopsis thaliana L. up to more than 25,000 Mbp in the timber species Pinus elliottii Engelm. var. elliottii. The differences in complexity of diploid nuclear genomes over several orders of magnitude (Bennett and Leitch, 1995) are caused by repetitive DNA which consists of repeated sequences varying in size from dinucleotides to motifs longer than 10,000 bp and which is present in copy numbers from many hundreds to hundred thousands.