Hydrangea is one of the most important understory components of warm-temperate and tropical forests in East Asia, encompassing shrubs, herbs, and climbing shrubs. Early studies recognized roughly 70 species distributed from East and Southeast Asia to the Americas, but molecular work revealed that the genus as traditionally circumscribed was not monophyletic, meaning it did not contain all descendants of a single common ancestor. To restore a natural classification, researchers proposed a broadened concept of Hydrangea sensu lato, absorbing former genera such as Dichroa, Schizophragma, Pileostegia, Cardiandra, Decumaria, Deinanthe, Platycrater, and Broussaisia as sections within the genus. This broad concept has since been supported by subsequent phylogenetic studies, making every newly discovered lineage a potential test of how the genus is organized.

The new species grows as a shrub 1 to 1.5 meters tall, with papery elliptic or ovate-elliptic leaves measuring 5.8 to 13 centimeters long and 2.2 to 7 centimeters wide. Its inflorescence is a terminal corymbose cyme 7 to 13 centimeters across, bearing showy sterile flowers with pale pink-purple to pale blue-purple sepals and minute blue petals. Fertile flowers carry a subglobose calyx tube with a constricted opening, triangular calyx teeth 0.6 to 1 millimeters long, and blue oblong-ovate petals 2.5 to 3.5 millimeters long. Stamens number 8 to 10 and are unequal in length, while the ovary is more than four-fifths inferior, nearly completely embedded in the floral tube. The capsule is subglobose and glabrous, roughly 5 millimeters long, and the brownish seeds are elliptic to oblong, 1.0 to 1.2 millimeters long, with long reticulate venation and a very narrow wing around the margin.

Morphologically, Hydrangea medogensis is most similar to H. stylosa, a species described by Hooker and Thomson in 1858. The two share a shrub habit of about 1 to 1.5 meters, grayish-white second-year branchlets, papery elliptic leaves with shortly acuminate apices and cuneate to subrounded bases, and corymbose cymes with truncate apices. However, the new species differs in several diagnostic characters: its adaxial leaf surface is covered with two types of soft hairs, long and short, whereas H. stylosa is nearly glabrous or glabrous on both surfaces; its ovary is more than four-fifths inferior, compared with roughly two-thirds inferior in H. stylosa; its calyx tube is subglobose with a constricted opening, rather than campanulate with a tubular opening; its calyx teeth are triangular and 0.6 to 1 millimeters long, rather than ovate to suboblong and 1 to 1.5 millimeters long; and its seeds are narrowly winged around the margin, rather than minutely winged at one or both ends.

To test whether these differences reflect a genuinely distinct lineage, the researchers sequenced fresh leaf material of the new species on an Illumina NovaSeq platform, obtaining raw data at approximately 5× genome coverage. Because sequencing depth was low, they adopted a reference-guided assembly strategy for the internal transcribed spacer (ITS) region: congeneric ITS sequences were downloaded from NCBI as reference baits, raw reads were screened and mapped using BLASTN, positive reads were extracted and merged with Seqtk, and contigs were assembled de novo with the SPAdes genome assembler. The complete ITS region was identified and extracted using ITSx, aligned with reference sequences, and manually verified in Geneious Prime. In total, ITS sequences of 55 samples were analyzed, with five species of Philadelphus and Deutzia serving as outgroups to root the tree.

Both Bayesian and maximum likelihood analyses produced similar topologies. The results divided Hydrangea into two major clades that are reciprocal sister groups: Clade 1 mainly includes Sect. Petalanthae, while Clade 2 includes Sect. Heteromallae, Sect. Calyptranthe, Sect. Cornidia, and Sect. Hydrangea. The two samples of the new species formed a distinct terminal clade within Clade 1, supporting their conspecific status, and Hydrangea medogensis was placed as sister to the remainder of Clade 1. This phylogenetic distance from H. stylosa, despite their morphological similarities, suggests that the shared characters likely represent plesiomorphic traits retained in both lineages or convergent evolution in vegetative morphology adapted to similar understory habitats.

The researchers also assembled the complete chloroplast genome of the new species, a circular molecule of 156,946 base pairs exhibiting the typical quadripartite structure of angiosperm plastomes, comprising a large single-copy region of 86,545 base pairs, a small single-copy region of 18,693 base pairs, and two inverted repeat regions of 25,854 base pairs each. The genome encodes 132 genes, including 87 protein-coding genes, 37 tRNA genes, and eight rRNA genes, with 17 genes duplicated in the inverted repeats. Twenty-two genes contain introns, with trnK-UUU harboring the largest intron, which includes the matK gene, a typical feature of angiosperm chloroplast genomes. Notably, Hydrangea medogensis possesses the shortest total genome length and the smallest large single-copy region among the analyzed Hydrangea species, while the inverted repeat length is similar to that of its relatives.

Comparison of the inverted repeat/single-copy boundary regions with six other Hydrangea species showed that the junction structures are generally conserved across the analyzed taxa but exhibit lineage-specific variations in gene cross-region lengths and precise junction positions. In the new species, the LSC/IRb junction falls within the rps19 gene, the SSC/IRb junction within ndhF, and the SSC/IRa junction within ycf1, with a portion of the ycf1 fragment duplicated in the adjacent inverted repeat. The boundary positions of the two sampled individuals were almost identical, with LSC and SSC lengths differing by only 1 to 2 base pairs, confirming high intraspecific stability of these boundaries.

Despite the phylogenetic distinctness of the new species, assigning it to a section proved difficult. According to the grouping key of Hydrangea in Flora of China, the new species possesses general characteristics of Hydrangea sensu stricto, but its ovary position, more than four-fifths inferior, falls in a transitional state between the two primary branches of the key and cannot be accurately assigned to either the completely inferior or the one-third to two-thirds superior category. Under the superior branch, its truncate petal bases and reticulate seed venation with a very narrow wing do not match Sect. Petalanthae or Sect. Heteromallae; under the inferior branch, its shrub habit with free petals that do not fall off during anthesis, seeds with long reticulate venation and a very narrow marginal wing, and absence of bracts are inconsistent with Sect. Calyptranthe and Sect. Hydrangea. The species is therefore provisionally assigned to Hydrangea without sectional affiliation, pending more comprehensive sampling and genomic data.

Hydrangea medogensis is currently known from only three localities in Medog County, growing in the understory of broad-leaved forests and in shrub thickets at forest margins at elevations of 2200 to 2500 meters. Each population comprises approximately 20 to 30 individuals, and the habitats are susceptible to disturbance or destruction. Based on the three known occurrence points, the extent of occurrence was estimated at roughly 13 square kilometers and the area of occupancy at roughly 8 square kilometers using a 2 by 2 kilometer grid. However, because further field surveys are needed to assess the exact distribution, the species is provisionally assessed as Data Deficient under the IUCN Red List Categories and Criteria. The researchers note that only ITS sequences were used in the phylogenetic analysis, constrained by the extremely limited molecular data available for closely related species, particularly H. stylosa, which is represented in public databases by only a few short and fragmentary plastid sequences. Future studies employing whole chloroplast genomes and additional nuclear markers will be necessary to more precisely resolve the phylogenetic relationships of the new species within Hydrangea.

Subject of Research: Description of a new hydrangea species from Xizang, China, based on morphology and molecular phylogenetics

Article Title: Hydrangea medogensis ( Hydrangeaceae ), a new species from Xizang (Tibet), China

Article References: He, W.-Q., Zheng, W.-L., Zhao, F.-Y., Yin, K.-X., Li, L.-Q., Zhao, K.-H., Zhang, F.-Y., Liao, Z.-H., & Lan, X.-Z. (2026). Hydrangea medogensis (Hydrangeaceae), a new species from Xizang (Tibet), China. PhytoKeys, 279, 51-68. https://doi.org/10.3897/phytokeys.279.203326

Image Credits: AI Generated

DOI: 10.3897/phytokeys.279.203326

Keywords: Hydrangea medogensis, new species, Xizang, Tibet, Medog County, Hydrangeaceae, phylogenetics, chloroplast genome, ITS sequences, taxonomy, conservation, Data Deficient