Abstract
BackgroundPaphiopedilum is a horticulturally and ecologically important genus of ca. 80 species of lady's slipper orchids native to Southeast Asia. These plants have long been of interest regarding their chromosomal evolution, which involves a progressive aneuploid series based on either fission or fusion of centromeres. Chromosome number is positively correlated with genome size, so rearrangement processes must include either insertion or deletion of DNA segments. We have conducted Fluorescence In Situ Hybridization (FISH) studies using 5S and 25S ribosomal DNA (rDNA) probes to survey for rearrangements, duplications, and phylogenetically-correlated variation within Paphiopedilum. We further studied sequence variation of the non-transcribed spacers of 5S rDNA (5S-NTS) to examine their complex duplication history, including the possibility that concerted evolutionary forces may homogenize diversity.Results5S and 25S rDNA loci among Paphiopedilum species, representing all key phylogenetic lineages, exhibit a considerable diversity that correlates well with recognized evolutionary groups. 25S rDNA signals range from 2 (representing 1 locus) to 9, the latter representing hemizygosity. 5S loci display extensive structural variation, and show from 2 specific signals to many, both major and minor and highly dispersed. The dispersed signals mainly occur at centromeric and subtelomeric positions, which are hotspots for chromosomal breakpoints. Phylogenetic analysis of cloned 5S rDNA non-transcribed spacer (5S-NTS) sequences showed evidence for both ancient and recent post-speciation duplication events, as well as interlocus and intralocus diversity.ConclusionsPaphiopedilum species display many chromosomal rearrangements - for example, duplications, translocations, and inversions - but only weak concerted evolutionary forces among highly duplicated 5S arrays, which suggests that double-strand break repair processes are dynamic and ongoing. These results make the genus a model system for the study of complex chromosomal evolution in plants.
Highlights
Paphiopedilum is a horticulturally and ecologically important genus of ca. 80 species of lady’s slipper orchids native to Southeast Asia
Two to four 25S ribosomal DNA (rDNA) signals are apparent (Figure 2) among 2n = 26 chromosomes, with two signals most parsimoniously interpretable as the basal condition since this state is shared by the outgroup genera Mexipedium and Phragmipedium
The inferred primitive condition, rearrangement by duplication is observed in Paphiopedilum armeniacum, P. emersonii and P. hangianum, which have more loci. 5S rDNA patterns are stable, showing 2 subtelomeric signals that are usually closely linked with one pair of 25S signals (Table 1)
Summary
Paphiopedilum is a horticulturally and ecologically important genus of ca. 80 species of lady’s slipper orchids native to Southeast Asia. 80 species of lady’s slipper orchids native to Southeast Asia. These plants have long been of interest regarding their chromosomal evolution, which involves a progressive aneuploid series based on either fission or fusion of centromeres. Chromosome number is positively correlated with genome size, so rearrangement processes must include either insertion or deletion of DNA segments. We have conducted Fluorescence In Situ Hybridization (FISH) studies using 5S and 25S ribosomal DNA (rDNA) probes to survey for rearrangements, duplications, and phylogenetically-correlated variation within Paphiopedilum. Chromosome number has been shown to be positively correlated with genome size [4], so rearrangement processes must include either insertion or deletion of DNA segments
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