In most diploid accessions, remarkable variability in
signal size and intensity was observed among the three 25S rDNA loci as well as between homologs at each 25S
locus. (Figures 1 and 2). Yet of the three typically
marked chromosome pairs, no one pair had consistently
the brightest or least bright 25S signals across the
diploid accessions. For instance, in F. vesca ’Hawaii 4’
the 25S rDNA FISH signals on the M pair were remarkably
smaller and weaker than the ones on the S1 and S2
pairs, thus showing a “minor M - major S1 - major S2”
pattern (Figure 2A); while in F. nipponica the S1 pair
showed the smallest and weakest 25S rDNA signals,
thus presenting a “major M - minor S1 - major S2” pattern
(Figure 2E). This variability in allocation patterns of
“major” and “minor” 25S rDNA signals were not only observed among different diploid species but also
between subspecies (e.g., “minor M - major S1 - major
S2” in F. vesca subsp. vesca ’Hawaii 4’ versus “major M
- major S1 - major S2” in F. vesca subsp. americana
’Pawtuckaway’, Figures 2A and 2B), or even between different
accessions within a subspecies (e.g., F. vesca
subsp. vesca ’Hawaii 4’ versus “major M - major S1 -
none S2” in F. vesca subsp. vesca ’Yellow Wonder’, Figures
2A and 2C). Within a single accession, on the
other hand, the allocation patterns were generally consistent
among examined cells (Figure 2). Contrastingly,
one or two satellites and secondary constrictions, which
are cytological markers for transcriptional active rDNA
sites, are only sometimes visible in metaphase chromosome
preparations in some diploids (Figure 1) and were
variously associated with either “major” or “minor” FISH
signals.