Assembly of the Delphinium densiflorum Chloroplast Genome and Comparative Genomics Within Delphinium

<b>Background/Objectives</b>: Chloroplast genomes are essential for understanding the systematics and adaptive evolution of alpine plants, yet genomic data for high-altitude <i>Delphinium</i> species remain scarce. <i>Delphinium densiflorum</i>, a medicinal plant endemic to the Qinghai-Tibet Plateau, exhibits notable high-altitude adaptations, but its plastome features and evolutionary position are still unclear. This study aims to assemble and characterize its complete chloroplast genome and clarify its phylogenetic placement within <i>Delphinium</i>. <b>Methods</b>: Using Illumina NovaSeq data, we de novo assembled the <i>D. densiflorum</i> plastome, annotated it with CPGAVAS2, and compared it with 12 published Ranunculaceae plastomes. We analyzed IR-boundary dynamics, genome-wide sequence variation, and codon-usage bias and constructed a maximum-likelihood phylogeny based on 69 shared protein-coding genes. <b>Results</b>: The plastome is 154,161 bp (GC 38.24%) with a canonical quadripartite structure, encoding 131 genes (87 CDS, 8 rRNA, 37 tRNA). An IR expansion into the SSC region yields the shortest SSC reported among the compared <i>Delphinium</i> species and produces unique structural variants. Photosynthetic genes are extremely conserved (nucleotide diversity Pi ≤ 0.01), whereas several loci (e.g., <i>ycf1</i> and <i>psaC</i>) are highly divergent (Pi ≥ 0.05). Codon usage shows a strong bias toward AU-ending triplets. Phylogenetically, <i>D. densiflorum</i> forms a 100%-bootstrap clade with other high-altitude congeners, supporting the non-monophyly of <i>Delphinium</i>. <b>Conclusions</b>: This study delineates the plastome architecture and putative adaptive signatures of <i>D. densiflorum</i>, identifies robust candidate loci for DNA barcoding, and provides molecular evidence for taxonomic revision and conservation strategies in <i>Delphinium</i>.