Introgression may act as an important source of new genetic variation to facilitate the adaptation of organisms to new environments, yet how introgression might enable tree species to adapt to higher latitudes and elevations remains unclear. Applying whole-transcriptome sequencing and population genetic analyses, we present an example of ancient introgression from a cypress species (Cupressus gigantea) that occurs at higher latitude and elevation on the Qinghai-Tibet Plateau into a related species (C. duclouxiana), which has likely aided the latter species to extend its range by colonizing cooler and drier mountain habitats during postglacial periods. We show that 16 introgressed candidate adaptive loci could have played pivotal roles in response to diverse stresses experienced in a high-elevation environment. Our findings provide new insights into the evolutionary history of Qinghai-Tibet Plateau plants and the importance of introgression in the adaptation of species to climate change.

The complete chloroplast genome sequence of Swertia souliei, an alpine annual herbaceous plant endemic to the Qinghai-Tibetan Plateau (QTP), is reported in this study. The plastome is 152,804 bp in length, with one large single-copy region of 83,195 bp, one small single-copy region of 18,096 bp, and two inverted repeat (IR) regions of 25,756 bp. It contains 130 genes, including 85 protein-coding, 8 ribosomal RNA, and 37 transfer RNA genes. Phylogenetic tree shows that this species is a sister to the clade containing S. leducii, S. verticillifolia, and S. mussotii. The complete chloroplast genome could provide significant insight for understanding the phylogenetic relationship of taxa within Gentianaceae.