As an explanation for rDNA site loss, the hypothesis
based on nucleolar dominance could be integrated with
that of high initial copy number if association of rDNA
sites is demonstrated to occur only between those
derived from a subset of the diploid ancestors. Although genome composition of octoploid strawberries has not
been fully elucidated [44], putative subgenome donors
including ancestors of F. vesca and F. iinumae have
been strongly supported by phylogenetic analysis on
DNA sequence data of multiple nuclear genes [1,13],
whereas in the phylogenetic tree constructed on nuclear
ITS sequences [45], F. iinumae is not clustered with any
octoploids but is a sister species to all the others. As
nuclear ITS is included in the 45S rDNA unit, it is reasonable
to speculate that rDNA site loss could be subgenome-
specific and that rDNA repeats from the ancestral
F. iinumae subgenome(s) were lost during or after the
establishment of the ancestral octoploid(s).