Any rDNA site is a stretch of DNA with sequence
homology to other rDNA sites on other chromosomes
[35]. Thus, physical association of rDNA clusters
between both homologous and nonhomologous sites is
possible, supported by the fact that rDNA-bearing chromosomes
appear to be non-randomly associated with
each other at mitotic metaphase [36]. It is widely
accepted that association of genes with highly repetitive
sequences would increase the opportunity for unequal
exchanges. The distal chromosomal position of rDNA
sites in strawberry may facilitate this association, which
could lead to unequal exchanges and rDNA repeat
duplications/deletions, and therefore changes (both
increases and decreases) in copy number between both
homologous and nonhomologous sites. Size polymorphism
of the hybridization signals, among and within homologous sites could be explained by such events. On
the other hand, somatic exchanges taking place between
different sites could make homogenization and changes
in rDNA copy number occur especially quickly [36].
Therefore, it is reasonable to suggest that the rDNA
repeats in strawberry are in a highly dynamic state
because of their terminal positions and potentially high
degree of association between sites. However, the high
conservation in rDNA site number and chromosomal
location, despite their apparent high dynamics of copy
number among sites, may indicate conserved genome
organization among strawberry species, at least for chromosomal
segments involving rDNA sites.