Abstract

Background and aims – The newly described species Arcytophyllum leymebambense sp. nov. (Rubiaceae) from the Amazonas department of northern Peru represents an isolated lineage within the monophyletic genus Arcytophyllum. This study aims to describe its distinctive morphological features and ecological context, assess its phylogenetic position, and discuss its implications for the diversification of the genus in the Andes.

Material and methods – Morphological observations were conducted on herbarium specimens and field collections from the type locality. Phylogenetic analyses were performed using nuclear and plastid DNA sequences from the tribe Spermacoceae, with particular emphasis on Arcytophyllum.

Key results – Arcytophyllum leymebambense sp. nov. is morphologically distinct from its congeners by its acuminate leaf and sepal apices, hirsute-lanuginose seeds, and the presence of protective hooks surrounding the ovary. It inhabits humid, high-elevation grasslands known as “Jalca,” where it coexists with a diverse assemblage of montane species. The species appears to be endemic to the region and is currently known from only three populations. Its restricted distribution, combined with potential threats such as overgrazing and fire, supports a preliminary conservation assessment as Critically Endangered (CR).

Conclusion – The discovery of Arcytophyllum leymebambense sp. nov. highlights the underestimated diversity of Andean Rubiaceae and provides evidence that Andean species form a grade within the diversification of Arcytophyllum. This pattern suggests a historical northward expansion of the lineage into the high mountains of Panama and Costa Rica.

Keywords

Amazonian, endemism, Spermacoceae, taxonomy, tropical Andes

Introduction

The tropical Andes is recognized as one of the world’s richest regions in terms of biodiversity, and studies predict that many species have yet to be described there (Swenson et al. 2012; Ondo et al. 2024). This hotspot is believed to harbour approximately 45,000 species of vascular plants (Olson and Dinerstein 2002; Mittermeier et al. 2005; Orme et al. 2005), making it a critical area for ecological research and conservation. Despite its exceptional species richness, many aspects of its flora and ecosystems are still poorly understood, highlighting the need for further exploration and documentation. Such efforts are essential not only to enhance scientific knowledge but also to support strategies aimed at preserving this globally significant centre of biodiversity in the face of ongoing environmental changes and anthropogenic pressures. The Rubiaceae family, commonly known as the madder or coffee family, is one of the largest families of flowering plants with a cosmopolitan distribution but a significant concentration in tropical regions (Robbrecht 1988; Robbrecht and Manen 2006; Bremer and Eriksson 2009) with 14,181–14,266 species in approximately 586–615 genera (Razafimandimbison and Rydin 2024a, 2024b; Verstraete et al. 2025). In South America, the family is notably diverse and represents a significant component of South America’s plant biodiversity, especially in humid tropical regions like the Amazon. The tribe Spermacoceae Bercht. & J.Presl is a pantropical lineage of 1411 species (Verstraete et al. 2025) and 86 genera (Razafimandimbison and Rydin 2024b), dominated by herbaceous and suffruticose taxa that frequently colonize disturbed environments (Delprete and Jardim 2012). In America, Spermacoceae are currently represented by 23 genera, including recently described genera such as Paganuccia R.M.Salas (Nuñez-Florentin et al. 2022) and Januaria R.M.Salas & Nuñez Florentin (Nuñez-Florentin et al. 2023).

Spermacoceae is one of the largest lineages of Rubiaceae with major Neotropical representatives from the Amazon lowlands (e.g. Borreria G.Mey, Hexasepalum Bartl. ex DC., Diodia Gronov., Mitracarpus Zucc., Galianthe Griseb., Psyllocarpus Mart. & Zucc., Spermacoce L.) and a conspicuous Andean element such as Arcytophyllum Schult. & Schult.f. which is a small but ecologically important group inhabiting high-altitude regions of the Andes including the norward extension of the Cordillera de Mérida in Venezuela and the Talamanca mountain range in western Panama and Costa Rica (Mena 1990). Members of Arcytophyllum are cushion or mat-forming shrubs and constitute important components in montane and paramo ecosystems, adapted to cold and humid conditions (Sturm and Rangel 1985; Mena and Balslev 1986; Mena 1990; Andersson et al. 2002).

Mena (1990) recognized 15 species in his revision, and Taylor et al. (1993) listed eight species for Peru including Arcytophyllum weberbaueri K.Krause, that was not considered by Mena (1990). Terrell (1999) merged the Mexican Hedyotis serpyllacea Schltdl. into Arcytophyllum, what was rejected by Andersson et al. (2002) and Groeninckx et al. (2009) who found this species as sister to Bouvardia ternifolia (Cav.) Schltdl., in a clade close to the traditional narrowly circumscribed Spermacoceae, but not related to the assemblage of Arcytophyllum, Hedyotis L., and Houstonia Gronov. as a more distant group in Spermacoceae sensu lato. Another species, Arcytophyllum fasciculatum (A.Gray) Terrell & H.Rob. with a distribution in the northern deserts of Mexico and reaching into Texas was transferred from Houstonia to Arcytophyllum by Terrell and Robinson (2011).

This group has drawn attention for its taxonomic diversity and endemism, particularly in Peru, Colombia, and Ecuador (Andersson et al. 2002). Morphologically, Arcytophyllum species are characterized by small, opposite leaves, interpetiolar stipules, and diminutive, often inconspicuous flowers (Wolff and Liede-Schumann 2007). Despite these similarities, the genus displays notable variations in floral disposition (pedicellate or sessile), leaf shape, margin pubescence, and seed hilum position (Mena 1990; Terrell and Robinson 2011), which have been key in distinguishing species within the genus.

As part of the ongoing comprehensive investigation of the flora of the mountainous regions of northern Peru several collections of Arcytophyllum were made that at first glance showed a similar habit as other cushion or mat forming species of Arenaria L. or Stellaria L. in Caryophyllaceae, which are frequent in the same habitats (e.g. Iamonico and Montesinos-Tubée 2023; Montesinos-Tubée and Iamonico 2023; Montesinos-Tubée and Borsch 2023). When analysing these specimens closely, they became evident as members of the coffee family, and subsequent phylogenetic analysis indicated that they correspond to a hitherto undescribed species of Arcytophyllum. This paper aims to describe this new species as new for science, including a detailed morphological description, illustrations, and an evaluation of its phylogenetic position.

Material and methods

Plant material

The first author collected specimens in the highlands of the Central Andes from 2015 to 2025. Specimens were studied using appropriate literature (Terrell and Robinson 2011) and compared to material deposited at AMAZ, B, CPUN, HUT, MEXU, and USM (herbarium acronyms according to Thiers 2025). Additionally, images of type specimens from diverse species were checked via JSTOR Global Plants (JSTOR 2025). Since these collections did not match any of the known species in Arcytophyllum, they were hypothesized to represent an undescribed species.

Morphological studies

The description presented here is based on our field observations, herbarium specimens, and spirit material, while the terminology follows Harris and Harris (2001).The primary habit characteristics of collected specimens were recorded directly during fieldwork. Additional morphological features were examined using herbarium samples using Olympus SZX10 and NSZ-405 stereo microscopes (1X-4.5X magnification). To further document the specimens, detailed field photographs were also captured during the collection process.

Distribution and conservation status

The distribution maps were drawn using locality data from our collections using QGIS Desktop v.3.4.11 (QGIS Development Team 2021). The maps were prepared using cartography shapefiles acquired from MapBiomas Peru (2025). Conservation status was assessed by applying the IUCN Red List Category criteria (IUCN 2024). We used the GeoCAT program (Bachman et al. 2011) to estimate the extent of occurrence (EOO) and area of occupancy (AOO), based on 2 × 2 km cells.

Molecular laboratory works

Genomic DNA was extracted from silica-gel-dried or herbarium samples using the CTAB method (Borsch et al. 2003). The ITS region was amplified using the primers 18S 1830 and 26S 25 (Nickrent et al. 1994). PCR conditions were: 40 cycles of denaturation (60 s at 97°C), annealing (50 s at 53°C), extension (20 s at 72°C), and a final extension step (7 min at 72°C). The rps16 region was amplified using the primers rpsF and rpsR2 (Oxelman et al. 1997). PCR conditions were 90 s at 96°C, 1 min at 50°C, 90 s at 72°C, 35 cycles of 30 seconds at 96°C, 1 min at 50°C, 90 s at 72°C, and a final extension time of 10 min at 72°C. Sequencing was outsourced to Macrogen Europe using the BigDye® Terminator Kit on an ABI 3730xl Analyzer.

Sequence alignment and phylogenetic analysis

All sequences were manually aligned following the criteria proposed by Löhne and Borsch (2005). The newly generated pherograms were edited and assembled using PhyDE v.0.995 (Müller et al. 2005). The matrix comprises 82 terminals, corresponding to 19 genera and 68 species of the tribe Spermacoceae. Of all the accessions, two correspond to A. leymebambense sp. nov. generated for this study, the other 80 sequences were generated for previous studies and downloaded from GenBank (Andersson and Rova 1999; Andersson et al. 2002; Wolff and Liede-Schumann 2007; Neupane et al. 2015, 2017; Miguel et al. 2018; Gibbons 2020). Taxon names with authors and GenBank accession numbers are presented in Suppl. material 1.

For Bayesian inference (BI), best-fit models of sequence evolution were selected for the molecular marker following the results of the Akaike Information Criterion in Modeltest v.3.6 (Posada and Crandall 1998) with parameters of 203 substitution schemes, +F base frequencies, +I and +G rate variation, nCat = 4, and SPR tree search. BI analyses were conducted using MrBayes v.3.1.2 (Huelsenbeck and Ronquist 2001). Four independent Markov Chain Monte Carlo (MCMC) runs were carried out, each with four parallel chains. Each chain was performed for 1,000,000 generations, saving one random tree every 1000 generations, the first 25% of the trees were discarded as burn-in. Trees were selected based on a 50% majority-rule consensus to estimate the posterior probabilities. A posterior probability (PP) equal or greater than 0.90 was considered as well supported (Alfaro and Holder 2006). Visualization and node confidence were annotated in TreeGraph v.2.15.0 (Stöver and Müller 2010).

Results and discussion

Comparative morphological studies

Morphological features place Arcytophyllum leymebambense sp. nov. unambiguously within Arcytophyllum. The mat-forming growth habit with slender and short herbaceous stems, together with the overall leaf and stipule morphology, conforms to the diagnostic features of the genus. Among the published species, A. leymebambense sp. nov. is morphologically most similar to A. filiforme (Ruiz & Pav.) Standl., particularly in its mat-forming habit and general leaf form, which may cause initial confusion between both species. However, A. leymebambense sp. nov. can be readily distinguished by a suite of diagnostic characters: it has acuminate leaf and sepal apices (vs acute or aristate in A. filiforme), lacks the long digitiform projections characteristic of A. filiforme, and exhibits longer stipules (1.3–2.0 vs 0.3–1.0 mm). Seed morphology provides an additional clear distinction, as A. leymebambense sp. nov. has hirsute-lanuginose seeds, whereas A. filiforme presents finely scaly seeds. The new species also resembles A. muticum (Wedd.) Standl. in its growth habit and stipule morphology but differs by its purplish flowers and the hirsute-lanuginose texture of its seeds, compared to the glabrate seeds of A. muticum. A noteworthy character of A. leymebambense sp. nov. is the presence of two hooks protecting the ovary, a feature unique within the genus, which may represent an ecological adaptation to the harsh environmental conditions of the northern Jalca. These hooks could act as a physical barrier against water damage, reducing the risk of seed rotting under prolonged moisture, and provide mechanical reinforcement against raindrop impact and herbivory. The main morphological differences between A. leymebambense sp. nov. and its most similar congeners are summarized in Table 1.

Table 1.

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Morphological comparison of A. leymebambense sp. nov. and related species.

Character	A. leymebambense sp. nov.	*A. filiforme*	*A. muticum*
Leaf shape	Ovate-lanceolate	Linear	Lanceolate to linear-lanceolate
Leaf apex	Acuminate	Acute or aristate	Acute to rounded
Sepal apex	Acuminate	Acute or aristate	Oblong
Stipule length	1.3–2.0 mm	0.3–1.0 mm	1 mm
Corolla tube length	1.2–1.5 mm	1 mm	3.0–4.5 mm
Flower colour	White	White	White with purplish tones
Seed ornamentation	Hirsute-lanuginose	Finely scaly	Glabrate
Ovary protective structures	Two hooks protecting the ovary	Absent	Absent

Phylogenetic analysis

The aligned matrix with all regions consisted of 1493 characters. The selected substitution model was GTR+I+G. Detailed matrix statistics regarding each marker contribution are given in Table 2.

Table 2.

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Contribution of the different genomic regions to the total matrix.

	ITS	rps16	Total
Number of sequences	71	61	82
Aligned positions	610	883	1493
Potentially informative characters for parsimony	218 (35.7%)	206 (23.3%)	424 (28.3%)
Model selected for BI	GTR+I+G	GTR+I+G

Our phylogenetic analysis recovered the new species as a member of Arcytophyllum with strong support (Fig. 1). Within Arcytophyllum, the new species is resolved as an independent branch separated from its closest morphological relatives, reinforcing its recognition as a separate species. However, the limited terminal sampling in our dataset prevents us from drawing definitive conclusions about its closest phylogenetic allies or the biogeographical history of the group. Despite these limitations, the congruence between the molecular results and the diagnostic morphological characters highlighted above underscores the importance of combining phylogenetic evidence with detailed comparative morphology for the delimitation of Arcytophyllum species from the Andean highlands.

Figure 1.

Reconstructed Bayesian phylogenetic tree of the concatenated markers ITS and rps16 showing the PP values at the nodes. The samples of Arcytophyllum leymebambense sp. nov. are marked in red. The scale bar represents the mean number of nucleotide substitutions per site.

Taxonomic treatment

Arcytophyllum leymebambense Montesin., Borsch & Torr.-Montúfar, sp. nov.

urn:lsid:ipni.org:names:77378619-1

Figs 2, 3, 4

Type

PERU – Amazonas department • Chachapoyas, Leymebamba, Laguna de los Cóndores track, antes de Abra el Rayo, scrubland vegetation, very humid; 6°46’06”S, 77°46’37”W; 2990 m; 21 Sep. 2023; D.B. Montesinos, C.E.L. Dasher & E. Chávez 9695; holotype: HUT [HUT-64624]; isotype: B [B 10 1330351].

Figure 2.

Arcytophyllum leymebambense sp. nov. A. Habit. B. Sterile stem. C. Stipule. D. Flower. E. Fruit. Drawn by Aldo Domínguez de la Torre based on Montesinos 9695.

Diagnosis

Arcytophyllum leymebambense sp. nov. can be recognized within the genus by the presence of two hooks protecting the ovary, a unique characteristic for this taxon and by the combination of acuminate leaf apex and hirsute-lanuginose seeds; it differs from the also herbaceous and mat-forming Arcytophyllum filiforme in leaf shape (ovate-lanceolate vs linear in A. filiforme) and sepal apex shape (acuminate vs acute or aristate in A. filiforme), the absence of long digitiform projections (present in A. filiforme), stipule length (1.3–2.0 vs 0.3–1.0 mm in A. filiforme), and seed ornamentation (hirsute-lanuginose vs finely scaly in A. filiforme). It also differs from A. muticum, another mat forming species by its sepal apex shape (oblong in A. muticum), corolla tube length (1.2–1.5 vs 3.0–4.5 mm long in A. muticum), and seed ornamentation (hirsute-lanuginose vs glabrous seeds in A. muticum).

Figure 3.

Arcytophyllum leymebambense sp. nov. A. Habit. B. Flowers. C. Fruits (black arrows). Photographs by Daniel Montesinos.

Description

Perennial herb with a decumbent, prostrate, or erect growth habit, forming a dense and ramified mat measuring approximately 5–10 cm in diameter. Roots subwoody-fibrous. Stems ranging from 5 to 8 cm in length, display frequent branching and are often concealed by older leaves; bark dark red hue, young shoots exhibit shades of dark green to pale red. Internodes short (2.0–3.5 mm) and narrow (0.2–0.3 mm), scarcely covered by irregular brown protuberances less than 0.05 mm in length, striate, deeply sulcate with 9–11 ribs. Stipules oblong-lanceolate, translucid, 1.3–2 × 0.8–1.8 mm, acuminate apex, irregular dentate margins, base truncate. Leaves opposite, ovate-lanceolate, 1.9–2.7 × 0.5–0.7 mm, sessile, apically subrevolute, slightly bulging midnerve, lamina dark green, thick, densely covered by sericeous trichomes on the underside and margins, sparsely and glandulous on the upperside, margin trichomes are whitish, measuring 0.10–0.25 mm long, multicellular, 18 to 40 per margin side, leaf apex largely acuminate, hirsute, hooked, with a long linear to sublinear ciliate hair, 0.5–0.8 mm long that tends to turn yellowish red with age, persisting on older foliage, leaf base truncate, narrow, densely hirsute on both sides and margins. Flowers solitary, axillary or terminal, tubular to shortly campanulate, tube 1.2–1.5 mm long, sessile or pedicellate, pedicels 0.1–0.2 mm long, sepals 4, ovate-lanceolate, 2.0–2.3 × 0.5–0.8 mm, covered with a thin translucid ovate stipule, densely hirsute on margins and upper surface, bearing a whitish acumen of approximately the same size as in the leaves, 2.0–2.2 mm long; petals 4, entire, white translucid, with a unique lobule, ovate-lanceolate, 1.8–2.0 × 0.8–1.0 mm, glabrous, hidden by the concave sepal, and fused with the stamens at the base. Androecium with 10 stamens, spreading and slightly curved filaments, 1.2–1.4 mm including the roundish and reddish anthers, which exceed the size of the ovary. Gynoecium 1 mm long, ovary ovate, 0.9–0.8 × 0.4–0.6 mm, with a slightly hirsute surface, ovary protected by two hooks. Fruits capsular, roundish, persistent, sparsely hirsute, otherwise glandulose, 1.1–1.3 mm in diameter, five carpels. Seeds about 6 to 8 per capsule, ovate to triangular, 0.4–0.6 mm in diameter, densely pubescent, trichomes dark maroon coloured and glandulose.

Figure 4.

Distribution map of Arcytophyllum leymebambense sp. nov.

Distribution

This species is exclusively documented in the Amazonas department, Peru. This area is situated within the Amotape–Huancabamba Zone, a major biodiversity hotspot extending from the Pacific coast, across the Andean cordilleras, to the tropical lowlands of southern Ecuador and much of northern Peru (Weigend 2002, 2004) with many narrowly endemic species concentrated (e.g. Weigend 2002; Weigend et al. 2005; Henning et al. 2019). It primarily thrives in the Jalca ecosystems, which are characterized by persistent humidity in forest patches amidst wet grasslands. These ecosystems are typically found at elevations ranging from approximately 2,990 to 3,791 m. Despite extensive surveys conducted by the first author across all Peruvian herbaria, no herbarium collections or additional specimens have been found, underscoring its status as an endemic species in northern Peru. The Jalca region, which borders dense montane tropical forests and transitions to lowland tropical forests, receives continuous rainfall year-round. Notably, the highest point of the collection site, around 3,800 m in Pampa del Rayo on the route to the Laguna de los Cóndores, receives an estimated 2,500 mm of precipitation annually.

Phenology

The new entity exhibits a flexible phenological pattern finely tuned to the extreme rainfall conditions of the northern Jalca in Peru, an ecosystem characterized by persistent and intense precipitation. Flowering and fruiting periods are highly variable, primarily influenced by the amount and timing of rainfall. Typically, flowering begins as early as August, coinciding with the onset of consistent rains, and may extend into October, depending on the intensity and duration of the wet season. This variability suggests a strong ecological adaptation to the region’s erratic precipitation patterns, enabling the species to optimize reproductive success during favourable moisture conditions.

Vernacular name

“Atunina”, name used by the locals in the communities of Atuén and Dos de Mayo of the Leymebamba district.

Etymology

Named after the Leymebamba district, several populations were identified in the vicinity of Laguna de los Cóndores. Although the lagoon itself lies just within San Martín (province of Huallaga) the type specimen’s collection site is clearly located in Amazonas in the Amotape–Huancabamba Zone as defined by Casper et al. (2020). This area represents a significant habitat for the species, emphasizing its ecological importance and potential for further research. The diverse environments found in the Jalca ecosystems in the uppermost west topographical areas of the lagoon may contribute to the species unique adaptations and distribution patterns.

Preliminary IUCN conservation assessment

The species is given a Red List status of Critically Endangered [CR B1ab(iii)+2ab(iii)]. So far, Arcytophyllum leymebambense sp. nov. has been identified in only three locations, making it a relatively uncommon species. The extent of occurrence (EOO) is of 1.2 km² and the area of occupancy (AOO) is of 8 km². Both EOO and AOO fall within the limits of the Critically Endangered (CR) category under subcriteria B1 and B2. According to the IUCN criteria and categories (IUCN 2024), it meets the conditions for the CR category. The preferred habitats for Arcytophyllum leymebambense sp. nov. are the summit slopes of Jalca ecosystems, adjacent to lowland tropical ecosystems in northern Peru. These mountain slopes face vulnerabilities from factors such as overgrazing, changes in annual rainfall, and fire, all of which could reduce the distribution area of this species. During a field visit by the first author in 2023, extensive burning of these ecosystems was observed. Despite their natural resistance to fire, rapid environmental changes threaten biodiversity in the region. Furthermore, recent fires in September 2024 have devastated much of the habitat where this species grows, leading to concerns about its potential disappearance unless immediate conservation protocols are implemented by the state.

Additional material examined

PERU – Amazonas Department • Chachapoyas Province, Leymebamba District, Camino a Laguna Yanacocha, humid grasslands, forest patches, continuously humid; 3791 m; 23 Sep. 2023; D.B. Montesinos, C.E.L. Dasher & E. Chávez 9706; HUT [HUT-64623], B [B 10 1330350] • Jalca ecosystem, highly moist, dense scrubland with tussock grasses; 3760 m; 22 Jul. 2024; D.B. Montesinos, C.E.L. Dasher & E. Chávez 10400; AMAZ, B [B 10 1330352].

Phylogenetic position

Arcytophyllum was retrieved as monophyletic and highly supported either in combined analysis (PP = 1.00) and in each individual molecular marker analysis (results not shown). Both samples of A. leymebambense sp. nov. form a clade with high Bayesian posterior probabilities in the combined analysis (PP = 1.00) and in the ITS region (PP = 1.00) (results not shown), only one sample amplified for the rps16 marker. Furthermore, the phylogeny also shows that A. leymebambense sp. nov. is closest to a clade formed by several Andean species, however, the sampling of both characters and terminals prevents establishing a biogeographic hypothesis and this molecular analysis only confirms the existence of A. leymebambense sp. nov. as a unique lineage within Arcytophyllum.

Conclusions

The importance of this work lies in the identification and formal description of a new lineage from the Andean region, belonging to a genus of Rubiaceae characteristic of the high-elevation zones of South America. This contribution not only expands the known diversity of the group but also underscores the value of continued botanical exploration in understudied areas. Moreover, it reinforces the relevance of integrative systematics, in which phylogenetic analyses are combined with detailed morphological studies to achieve a deeper understanding of the evolutionary patterns and diversification processes shaping these montane ecosystems.

Acknowledgements

This work was conducted as part of the postdoctoral project of the first author, sponsored by the Alexander von Humboldt-Stiftung through a Georg Forster Research Fellowship. Thanks are due to Anna Guengerich for her fieldwork support in the Amazonas region, financed by US National Science Foundation, award #2019425. We also thank the residents of Leymebamba, Dos de Mayo, and Atuén for their assistance in the field in the Amazonas region. Additionally, funding was previously obtained through J.S. Ingham. We express our gratitude to E.F. Rodríguez from the Herbario HUT (Universidad Nacional de Trujillo), and P. Hein and S. Bernhard (Botanischer Garten und Botanisches Museum Berlin), for their support with herbarium consultations. The independent illustrator Aldo Domínguez de la Torre made the illustration of the type. The various botanical collections carried out are covered by collection permits granted by the Peruvian State: Letter N° D000375-2023-MIDAGRI-SERFOR-DGGSPFFS-DGSPF, Letter N° RD-000130-2023-DGGSPFFS-DGSPF, and Letter N° RD-000026-2024-DGGSPFFS-DGSPF.

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Supplementary material

Supplementary material 1

Taxon names with authors and GenBank accession numbers.

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