Hiptage stenopterum K.Tan & M.X.Ren, a new species of Hiptage collected from a deep valley close to the Nujiang Gorge, northwest of Yunnan Province, China, is described and illustrated based on molecular and morphological data. The new species was found isolated in an entrenched valley of the Laowo River, a tributary of the Nujiang River, at the northern edge of the distribution range of the genus. H. stenopterum shares some morphological similarities with the narrowly endemic H. incurvatum and H. lushuiensis. However, H. stenopterum is easily distinguished by its oblanceolate lateral wing of winged mericarp, 10 to 12 calyx glands, and branchlets densely rusty tomentose. The new species status is also supported by molecular phylogenetic analyses based on nuclear ribosome internal transcribed spacer (nrITS), which showed distinct systematic boundaries from the most morphologically similar species, H. incurvatum and their morphological relatives, H. lushuiensis.

flora of Yunnan, Hengduan Mountains, Hiptage, Malpighiaceae, taxonomy

Hiptage Gaertn. (Gaertner 1791) is a large genus of Malpighiaceae, currently comprising ca. 40 species of woody shrubs or lianas growing at shrub forests or valleys on limestone hills or riverbanks of tropical Asia, such as Indo-China Peninsular, Malay Archipelago and Southern China (Sirirugsa 1991; Hô 1992; Srivastava 1992; Chen and Funston 2008; Ren 2015; Yang et al. 2018; Tan et al. 2019; Dong et al. 2020; Wei et al. 2022). Hiptage is derived from the Greek hiptamai, which means “to fly” and refers to its unique three-winged fruit. Such three-winged fruit means it is easily dispersed over long distances, explaining its widespread distribution in tropical Asia (Sirirugsa 1991; Srivastava 1992; Hô 1992; Chen and Funston 2008). Nonetheless, most species of this genus are narrow and endangered endemics, especially in the ravine and mountain peaks, such as H. multiflora F.N.Wei, H. lushuiensis S.P.Dong, K.Tan & M.X.Ren, and H. incurvatum K. Tan & M.X. Ren (Wei 2018; Tan et al. 2019; Dong et al. 2020). In recent years, five new species of Hiptage have been described. One species was found in Southern India (Sujana and Vadhyar 2020), and four were collected in Southwestern China (Yang et al. 2018; Tan et al. 2019; Dong et al. 2020).

Hiptage is currently classified within the Tetrapteroid clade, which is among the ten primary lineages identified for Malpighiaceae (Davis and Anderson 2010). Within that phylogenetic framework, Hiptage included six species and was resolved as closely related to Flabellariopsis R.Wilczek, a genus endemic to Africa. Hiptage and Flabellariopsis comprise a clade with weak support, sister to the Neotropical genus Carolus W.R.Anderson (Davis and Anderson 2010). Despite relying on a single marker (ITS) and yielding mainly weakly supported clades, recent molecular phylogenies of Hiptage indicate that H. stellulifera Arènes stands as the basal group within the genus (Tan et al. 2019; Almeida and van den Berg 2022).

During our fieldwork in the Hengduan Mountains, northwest of Yunnan Province, China, we collected specimens from a population of a distinct morphotype of Hiptage growing on a hillside in the valley of Laowo River, close to the Nujiang Gorge. After detailed analyses of morphological characters and molecular data of individuals from this population, we found out that this population is most morphologically similar to H. incurvatum and H. lushuiensis. But it differs in the number of calyx glands and the morphology of mericarp wings, both of which are critical morphological traits for species taxonomy in Hiptage (Chen and Funston 2008; Ren 2015). Therefore, we describe it as a new species, depicted and illustrated here.

The aligned matrix of ITS sequences consisted of 690 bp, of which 466 sites were identical, 132 (19.1%) were parsimony informative, and 92 parsimony-uninformative variable characters. The phylogenetic analysis showed that Hiptage is a monophyletic group (PP/BS=1/100), with H. stellulifera (PP/BS=1/100) being the first lineage to diverge, consistent with previous studies (Tan et al. 2019; Almeida and van den Berg 2022). The five samples of the proposed new species, H. stenopterum, formed a clade with strong support (PP/BS = 0.79/94) sister to a well-supported subclade alongside H. incurvatum (PP/BS=0.90/91) (Fig. 3). The results of molecular phylogenetics align with morphological observations, with the new species closely resembling H. incurvatum due to the presence of multiple glands. However, H. stenopterum is distinguished by its oblanceolate wings, a more significant number of calyx glands, and branchlets that are densely covered with rusty tomentose. Other features, like pedicels, differentiate the two species, as detailed in Table 2. The new species is also similar to H. lushuiensis due to the elliptic leaf blade and lanceolate bracteoles. However, there are significant discrepancies between the morphological and molecular phylogenetic findings (Fig. 2, Table 2). Especially in phylogenetics, H. lushuiensis was resolved as sister to H. minor (PP/BS = 1/99) (Fig. 2).

Figure 1. 

Hiptage stenopterum A habit B flowering branch C flower in side view D flowers in frontal view E–G calyx glands (E calyx glands with secretions attracting ants) H young branch (showing densely rusty tomentose) I petals J winged mericarp K young leaf. Photos A, B by M.X. Ren, E by T.T. Zhang, and C, D, F–K by K. Tan.

Figure 2. 

Molecular phylogenetic tree of Hiptage based on ITS sequences. Bayesian posterior probability (PP) and ML bootstrap values (BS) are shown above branches as PP/BS (only shown if BS > 50%).

Figure 3. 

Distribution map showcasing the new species, Hiptage stenopterum, alongside the other seven species of the genus found in adjacent regions

The zygomorphic, mirror-image, and floral structure of Hiptage represents a distinct evolutionary trait of biological significance that promotes adaptability (Ren et al. 2013; Qian et al. 2016). Currently, the presence, shape, and number of calyx glands, as well as their attachment to the pedicel, are critical diagnostic features for species identification in Hiptage (Sirirugsa 1991; Srivastava 1992; Chen and Funston 2008; Yang et al. 2018; Tan et al. 2019; Almeida and van den Berg 2022). However, Almeida and van den Berg (2022) contend that the presence and number of calyx glands in Hiptage are highly homoplasic and unsuitable for circumscribing infrageneric ranks. Calyx glands in Hiptage might be an evolutionary adaptation to attract myrmecophilous interactions for herbivore defense (Ren et al. 2013; Qian et al. 2016). Therefore, the absence and number of calyx glands may be related to local herbivorous insects, which are a significant factor in promoting speciation (McCall and Irwin 2006; Ramos and Schiestl 2019). Based on long-term field observations, we believe that although there is a slight probability of variation in the number of calyx glands, the overall characteristic remains relatively stable. In summary, we believe that the absence and number of calyx glands still hold significance in the infrageneric classification of Hiptage. However, it should be properly carried out in future phylogenetic studies to establish a new monophyletic infrageneric classification system.

Morphologically, H. stenopterum shares specific characters with H. incurvatum and H. lushuiensis in having elliptic leaf blades, lanceolate bracteoles, white to light pink flowers, and suborbicular petals. Based on the phylogeny tree, H. incurvatum is closely related to the new species, while H. lushuiensis clusters with H. minor, which has non-reflexed petals. Not only do the petals differ in posture, but there are also significant morphological differences between H. lushuiensis and H. minor such as the number of calyx and leaf blade glands, and leaf size (Chen and Funston 2008; Dong et al. 2020). Their clustering together might be due to the limited genetic information from just the ITS region and more comprehensive molecular data will be needed in the future to explain this curious relationship.

Malpighiaceae is characterized by an unparalleled diversity of winged fruits in angiosperms (Anderson et al. 2006; Manchester and O’Leary 2010; Tan et al. 2018). Winged mericarps are an adaption for wind dispersal of fruits (Tan et al. 2018), suggesting that such morphological adaptations have significantly facilitated long-distance dispersal and speciation (Davis et al. 2001, 2002; Tan et al. 2018, 2019). Notably, the wings of H. stenopterum distinctly deviate from its phylogenetically proximate taxa, H. incurvatum. Moreover, these two taxa are partitioned by the Hengduan Mountains, implying that differing aerodynamic conditions, perhaps governed by localized wind patterns, might have driven morphological divergence. Furthermore, the type localities of H. stenopterum and H. incurvatum, encircled by high mountains and deep gorges, constitute a distinct isolated habitat, which restricts gene flow and facilitates speciation.

We thank the two anonymous reviewers and extend special thanks to the subject editor, Rafael Felipe de Almeida, for his thoughtful and constructive criticisms.