Intact rDNA arrays of Potentilla-origin detected in Erythronium nucleus suggest recent eudicot-to-monocot horizontal transfer
The occurrence of horizontal gene transfer (HGT) in Eukarya is increasingly gaining recognition. Nuclear-to-nuclear jump of DNA between plant species at high phylogenetic distance and devoid of intimate association (e.g., parasitism) is still scarcely reported. Within eukaryotes, components of ribosomal DNA (rDNA) multigene family have been found to be horizontally transferred in protists, fungi and grasses. However, in neither case HGT occurred between phylogenetic families, nor the transferred rDNA remained tandemly arrayed and transcriptionally active in the recipient organism. This study aimed to characterize an alien eudicot-type of 45S nuclear rDNA, assumingly transferred horizontally to the genome of monocot European Erythronium (Liliaceae). Genome skimming coupled by PacBio HiFi sequencing of a BAC clone were applied to determine DNA sequence of the alien rDNA. A clear phylogenetic signal traced the origin of the alien rDNA of Erythronium back to the Argentea clade of Potentilla (Rosaceae) and deemed the transfer to have occurred in the common ancestor of E. dens-canis and E. caucasicum. Though being discontinuous, transferred rDNA preserved its general tandemly arrayed feature in the host organism. Southern blotting, molecular cytogenetics, and sequencing of a BAC clone derived from flow-sorted nuclei indicated integration of the alien rDNA into the recipients nuclear genome. Unprecedently, dicot-type alien rDNA was found to be transcribed in the monocot Erythronium albeit much less efficiently than the native counterpart. This study adds a new example to the growing list of naturally transgenic plants while holding the scientific community continually in suspense about the mode of DNA transfer. Significance StatementRibosomal DNA is an essential component of all cellular genomes. In plants, accidental movement of rDNA via horizontal gene transfer has only been reported in sexually incompatible grasses (monocots) where it involved non-functional rDNA units. In this study, we propose that evolutionary trajectories of eudicots and monocots were bypassed by the jump of rDNA from a Potentilla species (Rosaceae) to a common ancestor of Erythronium dens-canis and E. caucasicum (Liliaceae). The alien eudicot-type rDNA appeared relatively well conserved in the examined host Erythronium genome, being able to be expressed while preserving its general tandemly repeated feature, evidences that have no match in earlier literature.