Chloroplast Phylogenomics and Evolutionary History of the Alpine Endemic Eutrema scapiflorum

In this study, we sequenced, assembled, and characterized the first complete chloroplast (cp) genome of Eutrema scapiflorum, an alpine species endemic to the Qinghai-Tibet Plateau (QTP). The assembled plastome is 153,041 bp in length and exhibits a typical quadripartite structure, comprising a large single-copy (LSC) region of 83,547 bp and a small single-copy (SSC) region of 17,506 bp, which are separated by two inverted repeats (IRs) of 25,994 bp each. Structurally, the genome encodes 132 unique genes, including 87 protein-coding genes, 37 tRNA genes, and 8 rRNA genes. Comparative analysis across eight species revealed that genome size variation is primarily driven by the SSC region. Notably, the IR/SC boundaries in E. scapiflorum are highly conserved, which contrasts with the significant IR expansion observed in Capsella tenella. Furthermore, simple sequence repeat (SSR) analysis identified 78 loci, predominantly mononucleotide A/T repeats located in intergenic spacers. Nucleotide diversity analysis pinpointed accD and ycf1 as the most variable genes. Selection pressure analysis indicated that most genes are under purifying selection, while seven protein-coding genes (ycf2, nadhE, cemA, clpP, psbH, ycf4, nadhB) exhibited signatures of positive selection (Ka/Ks > 1). Subsequently, phylogenomic analyses robustly resolved E. scapiflorum within the tribe Arabideae, showing its closest relationship to Alliaria petiolata. Divergence time estimation dated the split between E. scapiflorum and its closest relative to the middle Miocene (~17.57 Ma). Collectively, these findings provide crucial genomic resources and new insights into the structural evolution, phylogenetic placement, and potential adaptive mechanisms of this alpine species within the Brassicaceae family.