In three tetraploids, multiples of 5S and 25S rDNA
site numbers (Figure 3B-3D) are increased in proportion
to the increase in whole genome copy number (i.e., in comparison to the typical diploids, the tetraploids had
twice as many chromosomes and twice as many
detected 5S and 25S rDNA sites). To date, the alternate
possibilities of auto- or allo- polyploidy origin have not
been resolved in these tetraploids [13]. Various diploid
species occurring in respectively overlapping geographical
areas have been proposed as putative ancestors. Due
to the highly conserved genomic distribution pattern of
rDNA sites among diploids, no specific species were
identifiable as putative ancestors of the tetraploids based
on rDNA-FISH data. However, variable allocation of
25S rDNA signal intensities among loci between these
tetraploids at least implies that the diploid ancestry of F.
corymbosa may be distinct from those of either F. gracilis
or F. tibetica. Lundberg et al. [39] suggested that F.
corymbosa was an allotetraploid. The size polymorphism
of 5S rDNA signals in F. corymbosa observed in our
work is possibly supportive to this hypothesis.