Classification of Isatis indigotica Fortune and Isatis tinctoria Linnaeus via comparative analysis of chloroplast genomes

BACKGROUND: Isatis tinctoria Linnaeus and Isatis indigotica Fortune are very inconsistent in their morphological characteristics, but the Flora of China treats them as the same species. In this work, a new technology that differs from conventional barcodes is developed to prove that they are different species and to clarify their classification. RESULTS AND METHODS: I. indigotica was indistinguishable from I. tinctoria when using ITS2. CPGAVAS2 was used to construct the chloroplast genomes. MAFFT and DnaSP were used to calculate nucleotide polymorphism, the chloroplast genomes of the two have high diversity in the rpl32 ~ trnL-UAG short region. When using this region as a mini barcode, it was found that there are obvious differences in the base numbers of I. tinctoria and different ploidy I. indigotica were found, but diploid and tetraploid I. indigotica had the same number of bases. Moreover, the reconstruction of the maximum likelihood (ML) tree, utilizing the mini-barcode, demonstrated that I. tinctoria and both diploid and tetraploid I. indigotica are located on distinct branches. The genome size of tetraploid I. indigotica was approximately 643.773 MB, the heterozygosity rate was approximately 0.98%, and the repeat sequence content was approximately 90.43%. This species has a highly heterozygous, extremely repetitive genome. CONCLUSION: A new method was established to differentiate between I. indigotica and I. tinctoria. Furthermore, this approach provides a reference and basis for the directional breeding of Isatis. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1186/s12864-023-09534-8.