Conservation and loss of ribosomal RNA gene sites in diploid and polyploid <it>Fragaria </it>(Rosaceae)

• Main Authors: Liu Bo, Davis Thomas M
• Format: Article
• Language: English
• Published: BMC 2011-11-01
• Series: BMC Plant Biology
• Online Access: http://www.biomedcentral.com/1471-2229/11/157

<p>Abstract</p> <p>Background</p> <p>The genus <it>Fragaria </it>comprises species at ploidy levels ranging from diploid (2<it>n </it>= 2<it>x </it>= 14) to decaploid (2<it>n </it>= 10<it>x </it>= 70). Fluorescence <it>in situ </it>hybridization with 5S and 25S rDNA probes was performed to gather cytogenetic information that illuminates genomic divergence among different taxa at multiple ploidy levels, as well as to explore the evolution of ribosomal RNA genes during polyploidization in <it>Fragaria</it>.</p> <p>Results</p> <p>Root tip cells of diploid taxa were typified by two 5S and six 25S rDNA hybridization signals of varying intensities, providing a baseline for comparisons within the genus. In three exceptional diploid genotypes, <it>F. nilgerrensis </it>(CFRA 1358 and CFRA 1825) and <it>F. vesca </it>'Yellow Wonder', two 5S but only four 25S rDNA sites were found but with differing site losses. The numbers of 5S and 25S rDNA signals, respectively were three and nine in a triploid <it>F</it>. ×<it>bifera </it>accession, and were four and twelve in three tetraploids, thus occurring in proportional 1.5× and 2× multiples of the typical diploid pattern. In hexaploid <it>F</it>. <it>moschata</it>, a proportional multiple of six 5S rDNA sites was observed, but the number of 25S rDNA sites was one or two less than the proportionate prediction of eighteen. This apparent tendency toward rDNA site loss at higher ploidy was markedly expanded in octoploids, which displayed only two 5S and ten 25S rDNA sites. In the two decaploids examined, the numbers of 5S and 25S rDNA signals, respectively, were four and fifteen in <it>F. virginiana </it>subsp. <it>platypetala</it>, and six and twelve in <it>F. iturupensis</it>.</p> <p>Conclusions</p> <p>Among diploid <it>Fragaria </it>species, a general consistency of rDNA site numbers implies conserved genomic organization, but highly variable 25S signal sizes and intensities and two instances of site loss suggest concurrent high dynamics of rDNA copy numbers among both homologs and non-homologs. General conservation of rDNA site numbers in lower ploidy, but marked site number reductions at higher ploidy levels, suggest complex evolution of rDNA sites during polyploidization and/or independent evolutionary pathways for 6<it>x </it>versus higher ploidy strawberries. Site number comparisons suggest common genomic composition among natural octoploids, and independent origins of the two divergent decaploid accessions.</p>