Conclusions The variation occurs across the whole spectrum of sequence repetition and has been shown to occur in the highly repeated, middle repetitive and low copy number sequences. Although the variation has been shown to be spread throughout the genome it does not occur at random, as similar molecular events haw been shown to occur repeatedly. The changes in two labile regions in the nucleus. the ribosomal RNA genes and a site-Specific insertion event, have been shown to occur within the period of vegetative growth and over a relatively short period of that growth. The gradual changes in total nuclear DNA that has been described would then need to have arisen through an accumulation of chances accumulation of changes occuring over the whole, or most of the, period of growth prior to flowering. The polymorphism that result from these rapidly occurring genomic events have also been observed in many other flax and linseed varieties as well as in the wild progenitors of flax. (C) 2005 Annals of Botany Company.