Distinctive origin and evolution of endemic thistle of Korean volcanic island: Structural organization and phylogenetic relationships with complete chloroplast genome

Abstract

Unlike otherCirsiumin Korea,Cirsium nipponicum(Island thistle) is distributed only on Ulleung Island, a volcanic island off the east coast of the Korean Peninsula, and a unique thistle with none or very small thorns. Although many researchers have questioned the origin and evolution ofC.nipponicum, there is not much genomic information to estimate it. We thus assembled the complete chloroplast ofC.nipponicumand reconstructed the phylogenetic relationships within the genusCirsium. The chloroplast genome was 152,586 bp, encoding 133 genes consisting of 8 rRNA genes, 37 tRNA genes, and 88 protein-coding genes. We found 833 polymorphic sites and eight highly variable regions in chloroplast genomes of sixCirsiumspecies by calculating nucleotide diversity, as well as 18 specific variable regions distinguishedC.nipponicumfrom otherCirsium. As a result of phylogenetic analysis,C.nipponicumwas closer toC.arvenseandC.vulgarethan nativeCirsiumin Korea:C.rhinocerosandC.japonicum. These results indicate thatC.nipponicumis likely introduced through the north Eurasian root, not the mainland, and evolved independently in Ulleung Island. This study contributes to further understanding the evolutionary process and the biodiversity conservation ofC.nipponicumon Ulleung Island.