Remarkable rDNA site number reduction in octoploid
strawberries
Each of the two wild octoploid species includes multiple
subspecies. In this work, we examined three subspecies
of F. chiloensis and of F. virginiana, respectively. Numbers
of 5S and 25S rDNA sites are consistent among all
these subspecies, and a strong reduction in rDNA site
number for both kinds of rDNA was observed as compared
with proportionate multiples of the typical diploid
numbers. The strong reduction in rDNA site number
might be attributable to two factors. First, more than a
half of the rDNA sites inherited from the lower-ploidy
ancestors might have failed to participate in associations
occurring among other rDNA-bearing chromosomes,
where homogenization through unequal crossing-over
and gene conversion could have maintained homology
of their DNA sequences and therefore their transcriptional
function. Thus, susceptibility to loss of some
rDNA arrays may be due simply to the high initial number
of rDNA sites. Though rDNA site number is
expected to be correlated with genome size, perhaps
only a restricted number of rDNA sites under a certain
threshold could be associated together and be maintained
in homology, and this low number might be sufficient
to support normal cellular activity. Thus, extra
sites beyond that restricted or necessary number would
suffer accumulation of mutations from lack of homogenization
forces, and be subject to elimination.