In the hexaploid species F. moschata, six 5S and eighteen
25S rDNA sites would be expected as three times
those in most diploids. In fact, six 5S rDNA sites
appeared as distinct FISH signals, but one or two fewer
signals for 25S rDNA sites were shown in, respectively,
F. moschata genotypes CFRA 376 and CFRA 157 (Figure
3E and 3F). F. moschata was shown to be an allopolyploid
and its subgenome donors include F. vesca and F.
viridis, which were suggested by DNA molecular studies
[13,39,40]. In F. vesca, some genotypes (e.g., ‘Yellow
Wonder’ and ‘Pawtuckaway’) have fewer 25S rDNA sites
than six. Thus, the 25S rDNA site number less than eighteen in a hexaploid would not be surprising, if any
F. vesca genotype(s) having less than six 25S rDNA sites
were involved in its origin. Alternatively, one or two
rDNA sites could have been diminished or lost by loss
of most or all of its repeats during or after the arising of
the hexaploid. When Rousseau-Gueutin et al. [13] studied
two low-copy gene sequences for the construction
of phylogenetic trees of Fragaria species, they linked F.
moschata to both F. vesca and F. viridis in the tree
based on the sequence analysis of the gene GBSSI-2, but
detected no affinity between F. moschata and F. viridis
in the tree from the DHAR gene, for which physical
elimination of one homoeologous copy of this gene was
proposed to be a possible reason.