Comparative chloroplast genomes of Incarvillea species (Bignoniaceae) unveiled genomic diversity and shed light on phylogenetic relationships.

Abstract

Background incarvillea: Juss. is a small herbaceous genus within the Bignoniaceae family. It comprises 16 species, which are subdivided into five subgenera. The species are distributed mainly in the Himalaya-Hengduan Mountains, although there are exceptions, including I. sinensis, I. algae, I. semiretschenskia, and I. potaninii. Phylogenetic analyses based on trnL-F and nr ITS sequences provided support for the monophyly of the genus and its subgenera. However, the interspecific relationships among subgenera remain unresolved, and further investigation is necessary to elucidate these relationships. In this study, we sequenced and assembled 34 chloroplast genomes from 12 Incarvillea species, representing all five subgenera, and explored the phylogeny of the genus based on the cp. genome data.

Results: The results demonstrated that 34 newly assembled chloroplast genomes exhibited lengths between 159,132 and 169,244 bp, and encoded a total of 129-141 genes. These included 84-95 protein-coding genes, 37 or 38 tRNA genes, and eight rRNA genes. A comparative analysis of the chloroplast genomes revealed the structural rearrangements and the expansions/contractions of the IR regions among the Incarvillea species. A total of 12 mutation hotspot regions were identified in the cp. genomes of the genus Incarvillea, encompassing six genes (atpI, psaI, rps18, trnQ-UUG, infA and ycf1) and six intergenic spacer regions (psbT-psbf1, rps11-rpl36, infA-rps8, trnN-GUU-ycf1, ndhE-ndhG and ndhI-ndhA). The Pi values of all highly variable regions exceeded 0.06. The phylogenetic analysis corroborated the monophyly of the genus and elucidated the relationships between five subgenere, namely ((Niedzwedzkia, Incarvillea), ((Amphicome, Olgaea),Pteroscleris)).

Conclusion: A comprehensive comparison of cp. genomic sequences revealed the diversity of the genus Incarvillea in terms of size, gene content and gene order of cp. genomes. Based on the cp. genome data, a robust phylogenetic tree of the genus Incarvillea was generated through phylogenetic analysis, with interspecific relationships well resolved. The results of this study enhance the understanding of the evolutionary history of the genus, and will facilitate further studies on the diversity and resource protection of the genus.