Morphological and phylogenetic study identifies new Didiclis species in Thailand and China

Researchers described Didiclis pseudobisulcata as a new species in the Bisulcata group of Didiclis, a genus of lycophytes in the family Selaginellaceae. The species is known from Yunnan in southwestern China and Chiang Mai Province in northern Thailand. It is distinguished from its closest relatives by a combination of leaf, sporophyll and megaspore characters, while molecular analyses recovered the sampled populations as an independent lineage.

The study combined field collections, herbarium comparisons, spore imaging and phylogenetic analysis. Specimens were collected in China and Thailand between 2019 and 2024 and compared with material held at CDBI, KUN and PYU. The researchers also examined images from the Chinese Virtual Herbarium, the Global Biodiversity Information Facility and JSTOR Global Plants. Megaspores and microspores were examined by scanning electron microscopy at 25 kV, with spore terminology following established studies of Selaginellaceae.

The type specimen was collected on October 20, 2024, in Huanglianshan Mountain Nature Reserve, Lvchun County, Yunnan Province, at approximately 1,908 meters above sea level. Additional examined specimens came from Baoshan City and Honghe City in Yunnan and from Doi Chiang Dao Wildlife Sanctuary in Chiang Mai Province. The species grows beneath evergreen broad-leaved forests at elevations of 1,800–2,100 meters in tropical regions.

Didiclis pseudobisulcata is a terrestrial, evergreen, creeping plant with stems 15–20 centimeters long. Its main stems are pinnately branched from the base and measure 0.6–0.9 millimeters in diameter in the lower portion. The stems bear seven to 10 pairs of primary leafy branches, with each primary branch divided three to four times pinnately. The plant has solitary, terminal, flattened strobili 5.6–6.4 millimeters long, strongly dimorphic sporophylls and megasporangia in the basal portion of the lower side.

For phylogenetic analysis, the researchers used eight samples of the new species from different locations in China and Thailand within a dataset of 32 samples representing 19 Didiclis species. They sequenced the plastid rbcL gene and the nuclear ITS region, using Lycopodioides nipponica and Valdespinoa douglasii as outgroups. The concatenated alignment contained 2,160 characters, including 300 parsimony-informative sites. Maximum-likelihood and Bayesian analyses both used the TIM3+F+R2 substitution model selected by the Akaike information criterion.

Maximum-likelihood analysis placed all eight accessions of D. pseudobisulcata in an independent clade with a bootstrap value of 74. This clade was weakly supported as sister to a clade containing D. chiangdaoensis, D. soyauxii, D. obovata, D. bisulcata and D. pennata. Bayesian inference also recovered the new species as monophyletic, with a posterior probability of 0.69, but placed it as sister to D. obovata and D. bisulcata, again with weak support. The two analyses therefore agreed on an independent lineage for the sampled accessions but differed in its closest relatives and provided weak support for the sister relationships.

Morphologically, the new species most closely resembles D. bisulcata and D. obovata. Its dorsal leaves on branches have an arista extending from one-half to four-fifths of the leaf length, while those of D. bisulcata are short-aristate, with an arista one-half to three-quarters of the leaf length, and those of D. obovata are short-apiculate. The base of the dorsal sporophyll pteryx is densely shortly ciliolate in D. pseudobisulcata, sparsely ciliolate in D. bisulcata and subentire in D. obovata. Its axillary leaves are ovate, rather than ovate-lanceolate in D. bisulcata or elliptic to broadly elliptic in D. obovata.

The clearest diagnostic feature is the megaspore exospore. Didiclis pseudobisulcata has subreticulate ornamentation with regular white granules composed of a honeycombed structure, whereas the megaspores of D. bisulcata and D. obovata are described as smooth, and those of other known members of the Bisulcata group as nearly smooth. The species was described as the only member of that group known to bear subreticulate megaspore ornamentation. Its megaspores are white or brown, globose and 516.9–615.2 micrometers across, with laesurae extending nearly three-quarters of the distance to the equator. Yellowish-orange, hemispherical microspores measure 26.3–34.9 micrometers and have indistinct verrucate ornamentation with sparse small projections.

Sequence variation in ITS was low within D. pseudobisulcata, with two variable sites across five individuals, but greater between the new species and its relatives. The new species had 45 variable ITS sites relative to D. bisulcata and 42 relative to D. obovata. On the basis of this contrast, the researchers considered ITS a potentially useful DNA barcode for identifying members of the Bisulcata group. The current distribution is limited to the documented sites in Yunnan and Chiang Mai; the authors noted that occurrence in Myanmar, Vietnam or Laos cannot be ruled out because no specimens had yet been recorded from those adjacent countries. The name pseudobisulcata refers to its morphological resemblance to D. bisulcata.

Subject of Research: Description and phylogenetic analysis of the new lycophyte species Didiclis pseudobisulcata

Article Title: Didiclis pseudobisulcata ( Selaginellaceae ), a new species of the Bisulcata group from northern Thailand and southwest China

Article References: Gong, S.-R., Hu, Y.-P., Pollawatn, R., Chantanaorrapint, S., Wang, W., Zhang, L., & Zhou, X.-M. (2026). Didiclis pseudobisulcata (Selaginellaceae), a new species of the Bisulcata group from northern Thailand and southwest China. PhytoKeys, 278, 233-246. https://doi.org/10.3897/phytokeys.278.204624

Image Credits: AI Generated

DOI: 10.3897/phytokeys.278.204624

Keywords: Didiclis pseudobisulcata, Selaginellaceae, lycophytes, plant taxonomy, new species, phylogenetics, ITS barcode, megaspore morphology, Yunnan, Chiang Mai, Didiclis, pseudobisulcata