Fluorescence in situ hybridization (FISH) is used to visualize the distribution of DNA elements within a genome. Conventional methods for FISH take 1–2 days. Here, we developed a simplified, rapid FISH technique using pre-labeled oligonucleotide probes (PLOPs) and tested the procedure using 18 PLOPs from 45S and 5S rDNA, Arabidopsis-type telomere, and newly-identified Panax ginseng-specific tandem repeats. The 16 developed rDNA PLOPs can be universally applied to plants and animals. The telomere PLOPs can be utilized in most plants with Arabidopsis-type telomeres. The ginseng-specific PLOP can be used to distinguish P. ginseng from related Panax species. Differential labeling of PLOPs allowed us to simultaneously visualize different target loci while reducing the FISH hybridization time from ~16 h to 5 min. PLOP-FISH is efficient, reliable, and rapid, making it ideal for routine analysis, especially of newly sequenced genomes using either universal or specific targets, such as novel tandem repeats identified from whole-genome sequencing data.

Tandem repeats can occupy a large portion of plant genomes and can either cause or result from chromosomal rearrangements, which are important drivers of dysploidy-mediated karyotype evolution and speciation. To understand the contribution of tandem repeats in shaping the extant Senna tora dysploid karyotype, we analyzed the composition and abundance of tandem repeats in the S. tora genome and compared the chromosomal distribution of these repeats between S. tora and a closely related euploid, Senna occidentalis. Using a read clustering algorithm, we identified the major S. tora tandem repeats and visualized their chromosomal distribution by fluorescence in situ hybridization. We identified eight independent repeats covering ~85 Mb or ~12% of the S. tora genome. The unit lengths and copy numbers had ranges of 7–5,833 bp and 325–2.89 × 10⁶, respectively. Three short duplicated sequences were found in the 45S rDNA intergenic spacer, one of which was also detected at an extra-NOR locus. The canonical plant telomeric repeat (TTTAGGG)_n was also detected as very intense signals in numerous pericentromeric and interstitial loci. StoTR05_180, which showed subtelomeric distribution in Senna occidentalis, was predominantly pericentromeric in S. tora. The unusual chromosomal distribution of tandem repeats in S. tora not only enabled easy identification of individual chromosomes but also revealed the massive chromosomal rearrangements that have likely played important roles in shaping its dysploid karyotype.

The Araliaceae contain many valuable species in medicinal and industrial aspects. We performed intensive phylogenomics using the plastid genome (plastome) and 45S nuclear ribosomal DNA sequences. A total of 66 plastome sequences were used, 13 of which were newly assembled in this study, 12 from new sequences, and one from existing data. While Araliaceae plastomes showed conserved genome structure, phylogenetic reconstructions based on four different plastome datasets revealed phylogenetic discordance within the Asian Palmate group. The divergence time estimation revealed that splits in two Araliaceae subfamilies and the clades exhibiting phylogenetic discordances in the Asian Palmate group occurred at two climatic optima, suggesting that global warming events triggered species divergence, particularly the rapid diversification of the Asian Palmate group during the Middle Miocene. Nucleotide substitution analyses indicated that the Hydrocotyloideae plastomes have undergone accelerated AT-biased mutations (C-to-T transitions) compared with the Aralioideae plastomes, and the acceleration may occur in their mitochondrial and nuclear genomes as well. This implies that members of the genus Hydrocotyle, the only aquatic plants in the Araliaceae, have experienced a distinct evolutionary history from the other species. We also discussed the intercontinental disjunction in the genus Panax and proposed a hypothesis to complement the previously proposed hypothesis. Our results provide the evolutionary trajectory of Araliaceae and advance our current understanding of the evolution of Araliaceae species.