Polymorphisms were also detected in size and intensity
of 25S rDNA FISH signals, which we described as
“major” (large and strong) and “minor” (small and weak)
signals, between different loci in the various diploid
taxa. Yet of the three typically marked chromosome
pairs, no one pair had consistently the brightest or least
bright 25S signals across the diploid genotypes. Thus, in
contrast to the conservation of a typical pattern of
rDNA site distribution, the allocation pattern of 25S
rDNA signal intensities among chromosome pairs varied
among diploid genotypes to an extent that precluded typification. Furthermore, polymorphism in signal intensity
and/or presence versus absence was observed
between homologous 25S rDNA sites in many diploid
genotypes. Signal absences from one but not both members
of a typically marked pair occurred in three cases:
F. nipponica (Figure 2E), F. iinumae (Figure 2D), and a
genotype (’Pawtuckaway’) of F. vesca subsp. americana
(Figure 2B). The latter pattern of variability between
homologs was consistently seen on separately prepared
slides suggesting that this variability was not due to
experimental artifact. Based upon available data, it
would be premature to speculate about the possibility
that unbalanced signal intensity between homologs is a
precursor to eventual locus loss, culminating in a
diminution of 25S site number from the typical six to
four. However, all the variations summarized above indicate
that the 25S rDNA arrays in diploid Fragaria exist
in a dynamic state.
Polymorphisms were also detected in size and intensity
of 25S rDNA FISH signals, which we described as
“major” (large and strong) and “minor” (small and weak)
signals, between different loci in the various diploid
taxa. Yet of the three typically marked chromosome
pairs, no one pair had consistently the brightest or least
bright 25S signals across the diploid genotypes. Thus, in
contrast to the conservation of a typical pattern of
rDNA site distribution, the allocation pattern of 25S
rDNA signal intensities among chromosome pairs varied
among diploid genotypes to an extent that precluded typification. Furthermore, polymorphism in signal intensity
and/or presence versus absence was observed
between homologous 25S rDNA sites in many diploid
genotypes. Signal absences from one but not both members
of a typically marked pair occurred in three cases:
F. nipponica (Figure 2E), F. iinumae (Figure 2D), and a
genotype (’Pawtuckaway’) of F. vesca subsp. americana
(Figure 2B). The latter pattern of variability between
homologs was consistently seen on separately prepared
slides suggesting that this variability was not due to
experimental artifact. Based upon available data, it
would be premature to speculate about the possibility
that unbalanced signal intensity between homologs is a
precursor to eventual locus loss, culminating in a
diminution of 25S site number from the typical six to
four. However, all the variations summarized above indicate
that the 25S rDNA arrays in diploid Fragaria exist
in a dynamic state.
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