Conclusions
This report describes the first molecular cytological
study of comparative genome organization in Fragaria,
and reveals the extent to which the genomic distribution
of rDNA sites is conserved among and within species
and ploidy levels, as well as dynamics of 25S rDNA
repeats, in this economically important genus. Per basic
genome copy, one proximal 5S and three terminal 25S
rDNA loci were largely but not uniformly conserved in
diploids (2x) and lower polyploids (3x, 4x and 6x), but a
marked reduction in site number was seen in higher
polyploids (8x and 10x). Based upon shared genomic
distribution patterns of rDNA sites, a common origin of
the Fragaria octoploids is suggested; however, the distinctly
different patterns seen in the two recently identified
decaploids suggest that these originated
independently. In the Rosaceae family, Fragaria was the
first genus in which a systematic molecular cytogenetic
study has been done, thereby providing a comparator
for genomic studies in other rosaceous species that could, for instance, reveal common or differing trends in
rDNA locus evolution following polyploidy.