New Cave-Dwelling Coffee Relative Discovered in Guangxi’s Karst Caves

Deep inside a limestone cave in Xincheng County, in China’s Guangxi Zhuang Autonomous Region, botanists have found a plant so unusual that it has now been formally named as a species new to science. The discovery, published in the journal PhytoKeys, introduces Ophiorrhiza xinchengensis, a delicate perennial herb belonging to the coffee family, Rubiaceae. With only a single known population of roughly 60 to 100 mature individuals clinging to existence at an elevation of about 400 metres, the plant has already been provisionally assessed as Critically Endangered under the criteria of the International Union for Conservation of Nature.

The story began in May 2025, when researchers conducting field surveys in the karst landscapes of central Guangxi encountered a fruiting Ophiorrhiza growing within a cave. Karst terrain, formed by the dissolution of soluble limestone bedrock, produces a mosaic of caves, sinkholes and shaded gorges that harbour many narrow endemics. These habitats act as evolutionary islands, and the team immediately recognised that the plant before them did not match any known species in the region. A follow-up collection in February 2026, when the plants were in flower, allowed a full morphological and genetic characterisation.

Ophiorrhiza is a large and notoriously difficult genus within Rubiaceae, comprising predominantly annual or perennial herbs, with subshrubs appearing only rarely. Species in the genus are most readily recognised by their distinctive fruits: obcordate, flattened capsules that split open along a transverse slit at the apex into two valves. The genus has long been entangled with Spiradiclis, a related group of roughly 60 species concentrated in the subtropical and tropical regions of Southeast Asia, with particular richness in the karst landscapes of southern China and northern Vietnam. The two groups have traditionally been separated by fruit architecture, since Spiradiclis produces linear-oblong or subglobose capsules that dehisce into four valves, in contrast to the two-valved, compressed capsules of Ophiorrhiza.

In 2019, however, Razafimandimbison and Rydin proposed, on the basis of molecular phylogenetic analyses, that Spiradiclis should be treated as a synonym of Ophiorrhiza. Subsequent taxonomic transfers have followed, yet some botanists continue to describe new species under the traditional name Spiradiclis, which generates a steady stream of unnecessary recombination. The authors of the new study adopt the molecularly supported, broader circumscription and therefore describe their discovery under Ophiorrhiza. The tension between molecular evidence and morphologically coherent field concepts remains one of the classic conflicts of modern taxonomy, and the team notes that the boundaries between the two genera still require further investigation.

Morphologically, Ophiorrhiza xinchengensis is a decumbent perennial herb with terete, densely villous stems that do not root at the nodes. Its leaves are elliptic to oblanceolate, 2 to 5 centimetres long and 1 to 1.8 centimetres wide, densely pubescent on both surfaces, with an attenuate base and five to seven pairs of secondary veins. The stipules are linear and only 2 to 3 millimetres long. The white, funnel-form flowers are distylous, a breeding system in which two floral morphs coexist: long-styled flowers bear exserted styles of 12 to 15 millimetres with stamens attached at the middle of the corolla tube, while short-styled flowers carry exserted stamens with filaments adnate at the throat and shorter styles of 5 to 6 millimetres. The corolla tubes measure 10 to 12 millimetres, and the ellipsoid capsules, 2 to 3 millimetres long, are densely pubescent and split into four valves.

These characters separate the new species clearly from its closest look-alikes. It resembles Ophiorrhiza longanensis in its densely pubescent stems, linear stipules and terminal or upper axillary inflorescences, but differs in leaf shape and base, shorter stipules, smaller bracteoles, dramatically longer corolla tubes and the position of filament attachment. Compared with Ophiorrhiza scabrida, the new species has attenuate rather than obtuse leaf bases, much shorter corolla tubes, and a different stamen-style arrangement. Against Spiradiclis hechiensis, which it resembles in its villous stems, terminal inflorescences and corolla colour, the new plant shows linear rather than broad triangular stipules, a straight rather than constricted corolla, a ring of long white hairs inside the tube rather than dense yellow throat villi, glabrous rather than glandular-pubescent corolla lobes, and much longer styles.

To test whether the morphology alone justified species status, the team turned to DNA barcoding of the nuclear internal transcribed spacer, or ITS, region. Total genomic DNA was extracted from silica-dried leaves using a modified CTAB protocol and sequenced by the Sanger method. The researchers generated two new sequences and combined them with 43 additional sequences downloaded from the NCBI database, producing a matrix of 45 taxa. Sequences were aligned with MAFFT, and phylogenetic trees were constructed under both Maximum Likelihood, using IQ-TREE, and Bayesian inference, with substitution models selected automatically under the Bayesian and Akaike information criteria. Nodal support was evaluated with 1000 bootstrap pseudo-replicates.

The phylogenetic results were unambiguous. Ophiorrhiza xinchengensis emerged as an independent terminal branch positioned close to Ophiorrhiza havilandii and Ophiorrhiza hayatana yet clearly separated from them, with strong statistical support of 95 percent bootstrap and a Bayesian posterior probability of 0.97. This exclusive branching pattern indicates an evolutionary trajectory independent of its morphologically similar relatives and firmly supports its recognition as a distinct taxon. Intriguingly, the broader analysis also showed that species of Ophiorrhiza do not form a monophyletic group; they are intermingled with representatives of Keenania, Spiradiclis and Coptophyllum. This finding suggests that, at least based on the ITS marker alone, generic boundaries within the tribe Ophiorrhizeae remain unresolved, and that integrative evidence combining morphology with broader population sampling will be needed to redraw them.

The conservation outlook for the new species is precarious. Field surveys conducted in 2025 and 2026 estimated the single known population at approximately 60 to 100 mature individuals, with an extent of occurrence of roughly 200 square metres and an area of occupancy of about 30 square metres. Under IUCN criterion B, the species is provisionally assessed as Critically Endangered, reflecting its extremely restricted distribution, single location and the potential for habitat degradation. Cave-dwelling plants are particularly vulnerable because their specialised microhabitats cannot be recreated elsewhere, and any disturbance to the cave environment, whether from tourism, quarrying or changing hydrology, could eliminate the entire global population in a single event.

The authors suggest that future studies employing multi-gene phylogenetic analyses and scanning electron microscopy of pollen morphology may help clarify the generic boundaries within Ophiorrhizeae. Until such integrative work is complete, two parallel classification systems are likely to coexist: a phylogenetic one following the expanded circumscription of Ophiorrhiza, and a traditional one maintained by researchers working on regional floras of China and Southeast Asia. As molecular data accumulate for more species, the balance of evidence is expected to favour the monophyletic concept. For now, Ophiorrhiza xinchengensis stands as both a reminder of how much biodiversity remains hidden in the world’s karst caves and a warning that many such discoveries may be documented only just in time to watch them disappear.

Subject of Research: Taxonomic description and phylogenetic placement of a new critically endangered plant species from a karst cave in Guangxi, China

Article Title: Ophiorrhiza xinchengensis ( Rubiaceae ), a new species from Guangxi, China

Article References: Wei, G.-Y., Li, J.-Y., Chen, R., Nong, Y., Hu, Q.-M., Li, Y.-J., Ji, X.-Y., Xu, C.-G., & Nong, Y.-K. (2026). Ophiorrhiza xinchengensis (Rubiaceae), a new species from Guangxi, China. PhytoKeys, 278, 1-14. https://doi.org/10.3897/phytokeys.278.194094

Image Credits: AI Generated

DOI: 10.3897/phytokeys.278.194094

Keywords: Ophiorrhiza xinchengensis, Rubiaceae, new species, Guangxi, karst cave, taxonomy, phylogenetics, ITS barcoding, Critically Endangered, Spiradiclis, China biodiversity, plant conservation