Glabrous New Milk-Vetch Species Discovered on Baima Mountain in Yunnan

High on the wind-scoured slopes of Baima Mountain in northwestern Yunnan, China, botanists conducting field surveys in July 2024 encountered a perennial herb that looked unlike any milk-vetch they had documented before. The plant, now formally described as Astragalus baimaensis in the journal PhytoKeys, belongs to the most species-rich genus of seed plants on Earth, a group of roughly 3,000 species that has long frustrated taxonomists with its blurred boundaries and bewildering morphological variation. The discovery, based on an integrative analysis of morphology and molecular phylogenetic evidence, adds a new branch to the evolutionary tree of a genus whose members fix nitrogen, enrich soils, and supply bioactive compounds prized in medicine.

The new species was found in the alpine zone of Baima Mountain in Deqin County, Shangri-La, at coordinates 28°20’10.896″N, 99°4’58.332″E, growing among alpine shrubs and in alpine meadows at elevations of 4,200 to 4,400 meters. What makes A. baimaensis immediately recognizable is a trait that is rare in the genus: it is completely glabrous throughout. No trichomes, or plant hairs, are present on the leaves, bracts, calyces, petals, or ovary, and every surface is uniformly smooth. The species bears narrowly oblong to obovate leaflets, green-yellow to yellow flowers, and inflated, obliquely ellipsoid legumes that hang pendulously from a stipe 10 to 14 millimeters long. Plants stand 20 to 50 centimeters tall with several erect, angulate stems, and they flower and fruit from June to September.

China harbors a documented total of 401 Astragalus species, 221 of which are nationally endemic, and the genus’s distribution across the country is far from uniform. The richest hotspot extends across the Hengduan Mountains and the Qinghai–Tibet Plateau, centered on the interconnected high-mountain and deep-gorge landscapes of northwestern Yunnan, western Sichuan, eastern Xizang, and Qinghai. There, extreme elevational gradients spanning dry, hot valley bottoms to exposed alpine scree habitats have acted as a powerful driver of rapid ecological speciation. A second major center lies in the arid biomes of northwestern China, where species are adapted to steppe, desert, and Gobi habitats. The Hengduan Mountains, with their intricately dissected topography, steep gradients, and heterogeneous microclimates, generate the geographic isolation and ecological filtering required for allopatric speciation, making them a primary center of recent species radiation within the genus.

To confirm that the Baima Mountain plant was genuinely new rather than an extreme variant of a known species, the team pursued a two-track phylogenetic strategy. Genomic DNA was extracted from silica-gel-dried leaf tissue using a modified CTAB method, and sequencing was performed on the DNBSEQ-T7 platform. The researchers assembled the complete chloroplast genome de novo using GetOrganelle and annotated it with CPGAVAS2, while the nuclear ITS region was amplified by PCR under a standard cycling protocol and sequenced separately. Phylogenetic reconstruction then proceeded along two complementary pipelines: a multilocus concatenation analysis using the standard plant barcode set of ITS, rbcL, and matK across 40 ingroup samples, and a phylogenomic analysis based on whole chloroplast genome sequences across 45 accessions, with Caragana species serving as outgroups. Maximum likelihood inference in IQ-TREE with 1,000 bootstrap replicates and Bayesian analysis in MrBayes with two independent Markov chain Monte Carlo runs of 1,000,000 generations produced highly congruent topologies.

The chloroplast genome of A. baimaensis is a single circular DNA molecule 124,633 base pairs in length with a GC content of 34.01%. In a striking departure from most angiosperms, the genome has undergone complete loss of the inverted repeat regions, obliterating the usual demarcation between the large single-copy and small single-copy regions. Annotation revealed 110 genes, including 76 unique protein-coding genes, 30 tRNA genes, and four rRNA genes, distributed across functional categories covering photosynthesis, self-replication, other roles, and unknown functions. This IR loss is a hallmark of the Inverted Repeat-Lacking Clade within the Papilionoideae, a species-rich group encompassing approximately 48 genera and more than 4,000 species, including Astragalus. The elimination of the IR releases tight structural constraints on plastid genomes and promotes widespread rearrangements, making IRLC plastomes the most structurally labile group documented in Fabaceae, while substitution rates across all functional compartments are consistently elevated compared with legume lineages that retain canonical IR copies.

The phylogenomic results delivered a fascinating twist. Analyses based on complete chloroplast genomes robustly placed A. baimaensis as sister to A. tongolensis, with fully supported statistics of 100 bootstrap support and a posterior probability of 1.00, within one of three segregated clades of the conventionally defined section Cenantrum. Yet the combined nuclear ITS and plastid rbcL and matK multilocus dataset supported a closer evolutionary affinity between A. baimaensis and A. chilienshanensis, a relationship recovered with maximum likelihood support of 100 but a relatively low Bayesian posterior probability of 0.63. Two non-mutually exclusive mechanisms may explain this cytonuclear discordance. First, recurrent chloroplast capture events have been well documented in Hengduan Mountain endemics, so the plastome of A. baimaensis may have been acquired through historical introgressive hybridization with A. tongolensis or its direct ancestor, while the nuclear ribosomal ITS region retains a closer affinity to A. chilienshanensis. Second, the full plastome dataset contains roughly 40 times more phylogenetically informative characters than the three-locus alignment, and because the plastome is strictly maternally inherited in most legumes, it captures only a single evolutionary lineage, whereas the concatenated dataset integrates markers with distinct inheritance modes and evolutionary signals.

Despite these close phylogenetic relationships, the new species exhibits a distinct set of consistent morphological differences that clearly distinguish it from both putative relatives. A. tongolensis, which occurs sympatrically across the Baima Mountain range, is sparsely hairy, with narrowly elliptic to obovate leaflets up to 60 millimeters long, a turbinate-tubular calyx that is densely black-hairy inside, a densely pubescent ovary, and papery legume valves that are very densely black-hairy. A. chilienshanensis, from Qinghai and Xizang, is sparsely hairy with ovate to elliptic leaflets and sparsely black-hairy legume valves. Another completely glabrous congener, A. luculentus from Xinjiang, can be distinguished by its much larger leaves of 25 to 30 centimeters with 8 to 11 centimeter petioles, a standard petal around 25 millimeters long, and subsessile, straight, incompletely bilocular legumes with thick, leathery valves. All diagnostic sectional characters of section Cenantrum, including subsessile leaves, long-ascending pedunculate racemes, multiflorous inflorescences, campanulate calyces, yellow petals that darken upon desiccation, and papery legume valves, are fully exhibited by the new species.

The study also highlights the considerable taxonomic value of variation in ovary pubescence within Astragalus. The ovaries of A. baimaensis were completely glabrous, a trait that showed no intraspecific variation among all 50 mature individuals sampled throughout its known distribution, while both A. tongolensis and A. chilienshanensis bear dense black or white trichomes on their ovaries. Crucially, the ovary trichome phenotype exhibits perfect developmental correlation with legume indumentum: species with pubescent ovaries invariably produce pubescent legumes, and this fruit pubescence trait has long served as a section-level diagnostic character in the genus’s infrageneric classification. A parallel precedent can be found in the Flora of China treatment, which formerly merged A. membranaceus into A. mongholicus, yet the two entities differ consistently in ovary pubescence, and the phylogenetic analyses in this study support their status as well-separated species. The combined evidence indicates that the presence or absence of ovary pubescence represents an exceptionally stable, low-homoplasy diagnostic character for species delimitation, reducing the ambiguity that commonly arises from overreliance on phenotypically plastic features such as leaf outline and flower color.

The phylogenomic analyses also exposed deep problems in the genus’s traditional classification. Of the 20 sections circumscribed in Flora of China that were sampled, the vast majority were not recovered as reciprocally monophyletic assemblages. Section Cenantrum was partitioned into three deeply divergent, non-sister clades, while section Poliothrix was similarly separated into two independent lineages. Only a small subset of sections, including Chlorostachys and Skythropos, demonstrated full monophyletic congruence with the molecular inferences. This nonmonophyletic pattern confirms that the current morphology-based infrageneric classification does not accurately reflect the full evolutionary history of Astragalus, a problem attributed to intricate patterns of continuous morphological variation, rampant interspecific hybridization, persistent undersampling of remote regions, and exceptionally high homoplasy in functional traits such as legume and foliar characters under strong environmental selection, which leads to widespread phenotypic convergence among distantly related species.

For now, A. baimaensis is known exclusively from Baima Mountain, with an estimated area of occupancy of less than 500 square kilometers, meeting the requirements for the Endangered category under IUCN Red List criterion B2. The species occurs at fewer than five threat-defined locations and faces a continuing decline in habitat quality driven by anthropogenic disturbances and intensified climate warming, two stressors with disproportionately severe effects on high-altitude alpine species. Its long-stiped, pendulous pods likely represent an adaptive seed-dispersal strategy suited to the strong winds prevalent at high elevations, while its dwarf growth habit and putatively unique physiological mechanisms enable it to tolerate intense solar radiation, low temperatures, and an extremely short growing season. The type specimens are deposited in the Herbarium of the Kunming Institute of Botany, and the chloroplast genome and ITS sequences have been submitted to GenBank. As the alpine flora of the Hengduan Mountains continues to yield hidden diversity, the new species stands as both a reminder of how much remains undocumented and an ideal model for investigating the adaptive mechanisms of Astragalus in extreme environments.

Subject of Research: Botany

Article Title: Astragalus baimaensis ( Fabaceae ), a new species from Yunnan, China

Article References: Chen, Q.-Y., Ma, H.-T., Rao, J.-H., Li, H.-F., Jiang, B., Zhou, Z., & Zhang, Y.-Z. (2026). Astragalus baimaensis (Fabaceae), a new species from Yunnan, China. PhytoKeys, 278, 55-74. https://doi.org/10.3897/phytokeys.278.191151

Image Credits: AI Generated

DOI: 10.3897/phytokeys.278.191151

Keywords: Astragalus, new species, Yunnan, phylogenomics, chloroplast genome, Hengduan Mountains, taxonomy, Fabaceae, baimaensis, species, China, scientific research