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Conservation and Loss of Ribosomal RNA Gene Sites in Diploid and Polyploid Fragaria (Rosaceae)


Conservation and Loss of Ribosomal RNA Gene Sites in Diploid and Polyploid Fragaria (Rosaceae)

Bo Liu et al. BMC Plant Biol.


Background: The genus Fragaria comprises species at ploidy levels ranging from diploid (2n = 2x = 14) to decaploid (2n = 10x = 70). Fluorescence in situ 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 Fragaria.

Results: 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, F. nilgerrensis (CFRA 1358 and CFRA 1825) and F. vesca '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 F. ×bifera 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 F. moschata, 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 F. virginiana subsp. platypetala, and six and twelve in F. iturupensis.

Conclusions: Among diploid Fragaria 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 6x versus higher ploidy strawberries. Site number comparisons suggest common genomic composition among natural octoploids, and independent origins of the two divergent decaploid accessions.


Figure 1
Figure 1
FISH on diploid Fragaria genotypes with 5S (green signals) and 25S (red signals) rDNA probes. A: F. bucharica (CFRA 520); B: F. mandshurica (CFRA 1947); C: F. vesca subsp. vesca 'Hawaii 4'; D: F. daltoniana (CFRA 1685); E: F. pentaphylla 'GS34'; F: F. viridis (CFRA 333); G: F. iinumae (CFRA 1850); H: F. nilgerrensis (CFRA 1358); I: F. nipponica (CFRA 1862); J: F. vesca subsp. americana 'Pawtuckaway' (CFRA 1948); K: F. vesca subsp. vesca 'Yellow Wonder'. Arrows show locations of satellites that are visible under DAPI counterstain. Bar = 5 μm.
Figure 2
Figure 2
Variable distribution patterns of 5S (green) and 25S (red) rDNA sites among diploid Fragaria genotypes. Five cells (numbered 1-5) are selected from each of six diploid accessions, which show divergent distribution patterns of rDNA sites. Only chromosomes displaying (or "expected" to display) rDNA FISH signals are shown here (M: the medium-sized pair "single-marked" by 25S rDNA signals; S1: the small-sized pair "single-marked" by 25S rDNA signals; S2: the small-sized pair "double-marked" by both 25S and 5S rDNA signals in the typical pattern). The pattern represented by F. vesca 'Hawaii 4' here (A) is the typical one (6 = 2M+2S1+2S2) shared by most diploid Fragaria taxa, while distinctly divergent patterns are observed in very a few accessions shown in B. 5 = 2M+2S1+1S2 with an exception as 6 = 2M+2S1+2S2; C. 4 = 2M+2S1; D. 6 = 2M+2S1+2S2 with an exception as 5 = 2M+1S1+2S2; E. 5 = 2M+1S1+2S2; F. 4 = 2M+2S2. Within each of most accessions, the distribution pattern among cells is consistent, except for F. vesca 'Pawtuckaway' and F. iinumae. In F. vesca 'Pawtuckaway' (B), the S2 pair displays only one 25S rDNA signal in cells 1 to 4 but two signals in cell 5. In F. iinumae (D), the S1 pair exhibits two distinct 25S rDNA signals in cells 1 to 3, one distinct and one weak signal in cell 4, and only one signal in cell 5.
Figure 3
Figure 3
FISH on polyploid Fragaria genotypes with 5S (green signals) and 25S (red signals) rDNA probes. A: F. ×bifera 'GS104' (2n = 3x = 21); B: F. corymbosa (CFRA 1911, 2n = 4x = 28); C: F. gracilis 'GS31' (2n = 4x = 28); D: F. tibetica 'GS28' (2n = 4x = 28); E: F. moschata (CFRA 157, 2n = 6x = 42); F: F. moschata (CFRA 376, 2n = 6x = 42); G: F. chiloensis subsp. lucida (CFRA 1691, 2n = 8x = 56); H: F. virginiana subsp. glauca (CFRA 1992, 2n = 8x = 56); I: F. virginiana subsp. platypetala (CFRA 110, 2n = 10x = 70); J: F. iturupensis (CFRA 1841, 2n = 10x = 70). Bar = 5 μm.

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