The genus Neillia D. Don (Rosaceae) comprises twelve species of deciduous shrubs, primarily distributed in the temperate regions of East Asia from the Himalaya across China, Taiwan, Korea to Japan in the east, and south to Indonesia (Vidal, 1963; Schulze-Menz, 1964; Cullen, 1971; Oh, 2016; Wu et al., 2025). The genus Neillia can be characterized within Rosaceae by having ovate to lanceolate leaves with 3–5 lobes, acuminate to caudate apices, and persistent or deciduous stipules; racemose or paniculate inflorescences; a unicarpellate (rarely bicarpellate) gynoecium; and ovoid, shiny seeds with copious endosperm (Oh, 2015). Species of Neillia are typically found in moist places of forest margins and mountain slopes. Traditionally, Stephanandra Siebold & Zucc. was considered as a separate genus consisting of three species indigenous to China, Taiwan, Korea, and Japan (Ohwi, 1965; Yu and Ku, 1974; Lee, 1980). Molecular phylogenetic studies using DNA sequences of chloroplast and nuclear genes have suggested that Stephanandra should be merged into Neillia (Oh and Potter, 2003, 2005; Oh, 2013).

In Korea, two species of Neillia are recognized: N. uekii Nakai and N. incisa (Thunb.) S.-H. Oh. The former was previously thought to be endemic to Korea, but Gu and Alexander (2003) reported its distribution in Changdian (SE Liaoning, China), near the border with North Korea. The latter is widely distributed across China, Taiwan, Korea, and Japan.

These two species exhibit distinct morphological traits and ecological preferences (Oh and Potter, 2005; Oh, 2016). Neillia uekii can be easily distinguished from N. incisa by the presence of stellate trichomes on the inflorescence rachis, a unique (autapomorphic) feature for N. uekii in Neillia. Furthermore, the inflorescence of N. uekii is typically a raceme, rarely a panicle of racemes, measuring 3–7 cm in length, while N. incisa always has short panicles approximately 2–3 cm long. The hypanthium in N. uekii is campanulate, measuring 2.5–3 mm long in the flowers, and enlarges into a spherical shape, reaching approximately 1 cm in length. In contrast, the hypanthium in N. incisa is cupulate, about 1 mm long, and does not enlarge in the fruits. Capitate-glandular trichomes on the surface of the hypanthium and pedicel are present in N. uekii, which develop conspicuously in the fruits, making them sticky. Such trichomes are absent in N. incisa. Plants of N. uekii tend to occur in limestone areas up to 400 m in elevation and prefer to grow near streams and rivers. However, N. incisa is widespread, without an edaphic preference, and occupies not only moist habitats at low elevations but also dry habitats at high elevations.

During recent field studies of the population genetic structure of N. uekii, we identified five geographically isolated populations of unidentifiable plants of Neillia in Korea. They showed a unique combination of morphological traits, some of which were intermediate between N. uekii and N. incisa, while others were distinct from either species. Detailed morphological comparisons, along with ecological and geographic data, support the recognition of these populations as a new hybrid.

In this paper, we describe and illustrate the new hybrid of Neillia, provide the diagnostic characters that distinguish it from related taxa, and discuss its distribution, habitat, and possible origin in the context of species diversification and hybridization within the genus.

Morphological data were obtained from specimens collected in the field in 2024 and 2025. The specimens were preserved in 70% ethanol or pressed and deposited in the herbaria KB and TUT (acronyms follow Index Herbariorum: https://sweetgum.nybg.org/science/ih/, continuously updated). Measurements and descriptions of floral characters were based on both ethanol-preserved materials and dried specimens, and those of vegetative characters were primarily examined from dried herbarium specimens. The floral morphology was observed under an Optinity stereomicroscope. For comparison, morphological data of closely related species (N. uekii and N. incisa) were obtained from fresh materials and herbarium specimens and from the relevant literature (e.g., Oh, 2016). Approximately 70 specimens of N. uekii and 2,100 of N. incisa deposited in KB were examined (Appendix 1).

Neillia × jookpai S.-H. Oh & E. Eom, hybrid nov. (Fig. 1)––TYPE: KOREA. Chungcheongnam-do, Seosan-si, Haemi-eup, forest edge of the NW of the Sansu Reservoir, N-facing slope, elev. 80 m, 36°41′59″N, 126°33′45.2″ E, 20 May 2025, Oh and Eom 10001 (holotype: KB, isotype: KH, KWNU, TUT).

Korean name: jook-pa-gook-su-na-mu (죽파국수나무).

Putative hybrid between Neillia uekii and N. incisa; differing from N. uekii by its multi-branched panicles, absence of stellate trichomes on the inflorescence rachis, and weakly developing capitate-glandular trichomes on the outer surface of the hypanthium in fruits; differing from N. incisa by larger flowers with campanulate hypanthium, capitate-glandular trichomes on the surface of hypanthium and pedicel, reflexed sepals in flowering, and larger fruits.

Shrubs, to 3 m tall. Stems 3–10, erect to ascending, terminal branches spreading. Leaves 3–7 on the flowering branches, simple, alternate, deciduous; stipules caducous, 2 per node, ovate to lanceolate, 0.5–1.2 cm long, villous on margin; petioles 0.5–1.1 cm long, densely villous; blade ovate to lanceolate, 4.3–9.7(−12.4) × 3.0–6.2(−10.6) cm, shallowly 3–lobed near base, apex acuminate to caudate, base rounded or truncate to slightly cordate, margin doubly serrate; both surfaces moderately pilose. Inflorescences panicles, terminating lateral branches of the season, 20–120-flowered, 4.8–13.2 cm long, glabrous or moderately villous; bracts lanceolate, caducous at anthesis, 4–5 mm long. Pedicels 2.5–8.7 mm long, glabrous or moderately villous, sparsely pubescent with capitate-glandular trichomes in the upper part. Flowers bisexual, 0.8–1.0 cm in diam.; hypanthium campanulate, 2.3–4.4 mm long, 2.6–5.2 mm wide at the widest point, adaxial surface densely villous, abaxial surface glabrous or sparsely to moderately villous, sparsely pubescent with capitate-glandular trichomes developing after anthesis, free from carpel; sepals 5, triangular, recurved when flowering becoming erect after anthesis, persistent, 1.3–3.1 × 1.2–2.1 mm, apex acuminate, adaxial surface villous apically, abaxial surface glabrous or villous; petals 5, white, spreading, involute, obovate or oblanceolate, 1.9–3.7 × 1.9–3.2 mm; stamens 11–16, exserted, arising from the rim of hypanthium; filaments 1.0–1.5 mm long; anthers dorsifixed; carpel 1, 4.5–4.7 mm long; ovary superior, densely villous; style terminal, erect, villous at base; stigma capitate; ovules 2 per carpel. Fruits follicles enclosed by hypanthium and sepals; hypanthium campanulate or globose, 5–6 mm long, abaxial surface viscid, sparsely pubescent with capitate-glandular trichomes; capitate-glandular trichomes ca. 1 mm long; follicles light brown, 4–5 mm long, densely villous. Seeds 1 or 2, dark brown, shiny, ovoid, 2.2–2.6 × 1.8–1.9 mm.

Flowering: Late May to early June, fruiting: July to August.

Distribution and habitat: Neillia × jookpai is currently known only from Korea, with confirmed records in Gyeonggido (Gapyeong, Yeoncheon), Chungcheongnam-do (Seosan), and Chungcheongbuk-do (Chungju, Goesan [based on a photograph]). It occurs in partially shaded, moist margins of forest at low elevations, where its putative parental species, N. uekii and N. incisa, occur in sympatry (Fig. 2).

Etymology: The specific epithet × jookpai honors the late Korean botanist Woo Tchul Lee (1933–2022), who made significant contributions to the taxonomy and flora of Korea. “Jookpa” was his pen name, a traditional Korean name used by scholars. The name recognizes his lifelong dedication to Korean plant taxonomy and his influence on generations of botanists. The Latinized form jookpai is derived from “Jookpa” with the masculine genitive ending “-i” following Article 60.8 of the International Code of Nomenclature (Shenzhen Code). The putative hybridity of the species is indicated by the symbol × following Article H.3.1.

Conservation status: Based on our field study and examination of herbarium specimens, N. × jookpai exists in five localities in Korea. Due to its limited distribution, we consider its conservation status as Vulnerable (VU D1+2) according to International Union for Conservation of Nature criteria (IUCN, 2012).

Additional specimens examined: KOREA. Chungcheongbukdo: Chungju-si, Salmi-myeon, Seseong-ri 588-2, 27 Jun 2025, Y. G. Choi et al. 10097, 10098, 10099, 10100 (TUT); Chungcheongnam-do: Seosan-si, Haemi-myeon, Sansu 1-gil, 20 May 2025, S. H. Oh & Eom 9994, 9995, 9996, 9997, 9998, 9999, 10000 (TUT). Gyeonggi-do: Gapyeong-gun, Seorak-myeon, Songsan-ri, 18 May 2025, Y. G. Choi et a l. 9944, 9945, 9946, 9947, 9948, 9949, 9950, 9951, 9952, 9953 (TUT); Gapyeonggun, Seorak-myeon, Songsan-ri, 27 Jun 2025, Y. G. Choi et al. 10092, 10093, 10094 (TUT); Yeoncheon-gun, Jeongok-eup, Eundae-ri, 14 Jun 2025, D. H. Lee 10080, 10081 (TUT).

Morphological comparisons indicate that N. × jookpai is clearly different from its putative parents, N. uekii and N. incisa, and that it shows a mosaic of traits from both (Fig. 3, Table 1). The leaf size of N. × jookpai partly overlaps with N. uekii but is slightly larger than that of N. incisa. Overall, the shape and lobation patterns of N. × jookpai closely resemble those of N. uekii. The mosaic pattern in N. × jookpai is evident in its reproductive features (Fig. 3, Table 1). Neillia × jookpai combines the hypanthium shape, hypanthium indumentum, and sepal orientation from N. uekii, along with the inflorescence type and texture of the inflorescence rachis from N. incisa. While both N. × jookpai and N. incisa have panicle inflorescences, N. uekii mainly has racemose inflorescences (Fig. 3A–C). In N. × jookpai and N. uekii, the hypanthium is campanulate with capitate-glandular trichomes developing after anthesis, and the sepals are recurved during flowering (Fig. 3D, E). The sepal orientation changes as the flowers develops: it is erect in the floral bud, spreading as the flowers open, recurved at full bloom, and then returns to an erect position after anthesis. The flowers of N. uekii and N. × jookpai shown in Fig. 3D represent an early blooming stage. In N. incisa, the hypanthium is shallowly cup-shaped without capitate-glandular trichomes, and the sepals spread during flowering. Stellate trichomes on the surface of the inflorescence rachis are a key diagnostic feature for N. uekii (Fig. 4A). Neillia × jookpai lacks stellate trichomes (Fig. 4B), as seen in N. incisa.

The number of stamens in N. × jookpai is intermediate between N. uekii and N. incisa, with an irregular arrangement (Table 1). The stamens form an annular row or crown around the edge of the hypanthium (Fig. 5). Neillia incisa has ten stamens in one row: five are opposite the sepals, and five are opposite the petals. Neillia uekii has five to seven stamens in the inner row, usually opposite the sepals, and 15 stamens in the outer row, with five opposite the petals and ten alternating with the petals. The stamens in the inner row are shorter than those in the outer row, with filaments spreading toward the style and not curved or falcate. Neillia × jookpai has 11 to 16 stamens arranged in two rows. Four stamens develop in the inner row, opposite the sepals, while seven to twelve stamens are in the outer row, with five opposite the petals.

Fruit morphology provides the key diagnostic features (Fig. 3F–H, Table 1). The hypanthium encloses the follicle of N. × jookpai, as seen in N. uekii (Fig. 3F). The persistent hypanthium in the fruit of N. × jookpai is campanulate or globose, viscid, and sparsely pubescent with short, capitate-glandular trichomes, differing from the larger, globose, and densely pubescent hypanthia with long, capitate-glandular trichomes of N. uekii and the smooth, minute hypanthia of N. incisa (Fig. 3F, G). The hypanthium encloses the follicle of N. × jookpai, similar to N. uekii, and is clearly distinguished from the open hypanthium of N. incisa. The shape of the hypanthium in N. × jookpai varies among populations; plants in Goesan have globose hypanthia in the fruit, while those in other locations have campanulate hypanthia (personal observation). The follicles of N. × jookpai are ellipsoid, not inflated, and 4–5 mm long, sharply contrasting with the large, ovoid, slightly inflated follicles of N. uekii (8–10 mm) and the very small, oblate, non-inflated follicles of N. incisa (1–3 mm).

Neillia × jookpai is currently known from five locations in Korea: Gyeonggi-do (Gapyeong and Yeoncheon), Chungcheongnam-do (Seosan), and Chungcheongbuk-do (Chungju and Goesan). At all of these sites, hybrid individuals grow mixed with the parent populations, indicating that hybridization occurs naturally. The presence of the hybrid at multiple, geographically separate locations suggests that its formation occurred independently at each site as opposed to spreading from a single origin. This repeated, localized origin highlights the ongoing potential for hybridization between N. uekii and N. incisa in areas where they co-occur.

It suggests that repeated hybridization events may have happened whenever the two species co-occur. Interspecific hybridizations are frequently reported and play an important role in evolution within the Rosaceae family (Phipps, 2005; Joly et al., 2006; Morales-Briones et al., 2018; Vaezi et al., 2019; Liston et al., 2021). A well-known example is Prunus × nudiflora (Koehne) Koidz. (Rosaceae), where the hybrid originated from multiple hybridization events between P. itosakura var. ascendens (Makino) Makino and P. jamasakura Siebold ex Koidz./P. sargentii Rehder at Mt. Hallasan (Cho and Kim, 2019; Cho MS et al., 2014; Cho A et al., 2017; Baek et al., 2018). The repeated formation of hybrid species through allopolyploidization has been well documented in Tragopogon (Asteraceae) (Soltis and Soltis, 1989; Soltis et al., 2004; Chester et al., 2012). The tetraploid T. mirus Ownbey derives from hybridization between diploid ancestors T. dubius Scop. and T. pratensis L., and another tetraploid, T. miscellus Ownbey, formed independently from T. dubius and T. porrifolius L.

The chromosome numbers of N. incisa and N. uekii are both diploid, at 2n = 18 (Oh, 2015). The chromosome number of N. × jookpai is unknown, however. Cytogenetic studies should investigate whether the hybrid is a tetraploid, which would lead to reproductive isolation in sympatric populations, similar to Tragopogon, or if it is a diploid. It is also valuable to analyze the parentage of the hybrid and the genomic contributions of N. uekii and N. incisa using high-resolution genomic data (Park et al., 2023).

Populations of N. × jookpai from various origins may also show differences in morphology. Fruit shape variations serve as a good example in the hybrid. With regard to Tragopogon, the dimorphic ligules of the ray florets in T. miscellus result from the independent formation of different parent combinations, involving T. dubius and T. pratensis (Soltis et al., 2004). More detailed studies are needed to determine the parentage of N. × jookpai and to understand the genomic composition of the hybrid.