Intragenomic Polymorphism of the ITS1-5.8S-ITS2 Region in Invasive Species of the Genus Reynoutria

The ITS1-5.8S-ITS2 (ITS1-2) region of the 35S rDNA is widely used for molecular barcoding and in the phylogenetics of plants. It is believed that, due to concerted evolution, all copies of 35S rDNA in eukaryotic genomes should be effectively homogenized. However, the existence of intragenomic polymorphism of the ITS1-2 region in plant genomes has recently been demonstrated, which may be a consequence of hybridization within or between species. In this study, the intragenomic polymorphism of the ITS1-2 region was evaluated using Illumina amplicon sequencing in accessions of two invasive species of the genus Reynoutria, R. japonica and R. sachalinensis, from Ukraine and Romania. Hybridization between these species can lead to the emergence of more aggressive invasive forms. The ITS1-2 sequences of the species studied were found to be represented by some major and minor subclasses/variants, indicating their incomplete homogenization. The number of major variants range from two in R. japonica to six in R. sachalinensis. The ITS1-2 variants that are widespread in the genome of one species may be present at low levels in another species, indicating possible interspecies hybridization. The obtained results show that the ITS1-2 intragenomic polymorphism must be taken into account when performing barcoding, reconstructing the phylogeny of low-level taxa, and for the identification of hybrid forms.