Shrink teacost (Isodon attenuatus), Gongshan teacost (Isodon gongshanensis) and Su-Kung teacost (Isodon sukungii)

NEWS - Three new species of Isodon (Schrad. ex Benth.) Spach (Ocimeae, Nepetoideae, Lamiaceae) are reported based on the latest morphology and phylogenomics of shrink teacost (Isodon attenuatus), Gongshan teacost (Isodon gongshanensis) and Su-Kung teacost (Isodon sukungii) from Mount Fanjing and Hengdua Mountains in China.

Isodon is a genus with about 140 species and is mostly distributed in subtropical to tropical Asia with two separate species endemic to Africa. The genus is most diverse in southwest China especially in the dry valleys of the Hengduan Mountains.

The genus can be distinguished from other genera of the family Ocimeae by petioles and bracteolates, calyces slightly or very 2-lipped (3/2), very 2-lipped (4/1) corollas and free filaments inserted at the base of the corolla tube. DNA loci or plastome sequencing are difficult due to the rapid radiation of the genus.

Recently, researchers from the Kunming Institute of Botany in Kunming and The Royal Botanic Gardens in Richmond reconstructed a robust phylogeny for 126 Isodon taxa using transcriptome and genome re-sequencing data.

Except for four clades (Clade I–Clade IV) found consistently in previous molecular phylogenetic studies, four subclades (Clade IVa–Clade IVd) were further recognized within the largest Clade IV that includes about 80% of the species of the genus. Meanwhile, three specimens collected during 2018–2020 are new species.

I. attenuatus is most closely related to I. villosus, but differs in having a subglabrous lamina (vs. densely to sparsely villous) with a descending base (vs. not descending), densely pubescent and glandularly pubescent inflorescences (vs. densely villous and glandularly pubescent), longer pedicels (4-6 mm vs. 2.5-4 mm), triangular calyx teeth (vs. oval) and a corolla tube that tapers towards the throat (vs. not tapers).

This species is known only from the Fanjing Mountains in Guizhou Province, China, and grows in evergreen broadleaf forests at elevations of 1600-2000 m. The specific epithet refers to the corolla tube that tapers towards the throat. The Chinese name is jiàn xiá xiāng chá cài (渐狭香茶菜).

I. gongshanensis is similar to I. rosthornii, but differs in having lanceolate (vs. triangular) and longer (c. 1 mm vs. c. 0.5 mm) teeth on the posterior petal lip, longer corolla (6-7 mm vs. c. 5 mm) with a pink posterior lip (vs. white), but without spots (vs. with reddish-purple spots) and an ovary without glands (vs. with glands).

This species is known only from Gongshan County in southwest China's Yunnan Province and occurs in mixed coniferous and broadleaf forests at elevations of 2700-2900 meters. The specific epithet is derived from the location of the new species, Gongshan County. The Chinese name is gòng shān xiāng chá cài (贡山香茶菜).

I. sukungii is similar to I. tenuifolius but differs by having longer laminae (1-4 cm vs. 0.5-1 cm long) with serrated margins (vs. entire), panicle-forming sympes (vs. single sympes not forming thirsus or panicles) and longer corollas (6-7 mm vs. 4 mm long) with straight anterior lip (vs. strongly folded).

This species is widely distributed in dry valleys along the Min River, a tributary of the Yangtze River, in Sichuan Province, southwest China and grows on dry open slopes with savanna vegetation at elevations of 1600-2700 meters. Specific name according to the late Prof. Su-Kung Wu. Chinese name is sù gōng xiāng chá cài (素功香茶菜).

Original research

Chen Y-P, Peng H, Paton AJ, Xiang C-L (2024). Three new species of Isodon (Nepetoideae, Lamiaceae) from China. PhytoKeys 246: 283-293, DOI:10.3897/phytokeys.246.130432

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