Overview
This study provides the first multi-locus phylogenetic framework for Premna species in Thailand, using four chloroplast markers. The results corroborate a monophyletic Premna within Premnoideae and its sister relationship to Gmelina L., while highlighting unresolved nodes within the Thai Premna lineage that require deeper genomic analyses.
Phylogenetic framework and key findings
Using four chloroplast markers concatenated in two main clades, the Thai Premna taxa split into two major groups. Clade I includes P. mekongensis, P. punicea, and P. tenii from China and Vietnam, sharing a dense cymose inflorescence and distinctly 5-lobed calyces. This morphology-guided signal aligns with previous Flora of China classifications and supports pursuing a broader test of monophyly for species with similar traits across adjacent regions.
Clade II contains several subclades (I–III) with strong support, yet relationships among P. serrata and P. tomentosa remain unresolved. These two species exhibit clear morphological departures from most Thai congeners, underscoring why more markers or genome-scale data are needed to resolve deep relationships within Premna. The study notes the common occurrence of polytomies in Lamiaceae Premna and related genera, which likely stems from limited chloroplast markers in prior work.
Subclades and morphology-guided inferences
Subclade I
Nine Premna species, largely shrubs or woody climbers from Myanmar, China, and Thailand, fall here. A subset (P. bracteata, P. garrettii, P. interrupta, P. scandens, P. trichostoma) share clavoid fruits and a thin pericarp, supporting a morphology-based infrageneric grouping. P. cordifolia, long viewed as a synonym of P. serratifolia in some treatments, clusters within this subclade and gains support from integrative morphology and ecology, reinforcing its status as a distinct entity in Thailand.
Subclade II
This subclade comprises two herbaceous species, P. herbacea and P. nana, with a dry-forest distribution in Southeast Asia. Molecular data corroborate their close relationship, while P. nana’s recognition as distinct from P. herbacea is upheld by both morphology and DNA evidence, consistent with earlier revisions.
Subclade III
Vastly morphologically diverse, this group contains shrubs, climbers, and some tree species (P. mollissima, P. rabakensis, P. szemaoensis). It includes endemics such as P. annulata (Thailand) and P. vietnamensis (Vietnam). Within this subclade, P. serratifolia samples from peninsular Thailand form a monophyletic unit, while P. punctulata—previously synonymized with P. serratifolia—exhibits a distinct lineage with a longer branch, though not fully resolved, supporting its partial species status in the Thai context.
Taxonomic notes: Clarifying species status in Thailand
The study undertakes a taxonomic revision focused on the P. serratifolia complex in Thailand, reinstating several taxa that had been ambiguously treated as synonyms. Notably, P. cordifolia (sensu Roxburgh) is validated as distinct from P. serratifolia based on leaf bases, ovary pubescence, corolla patches, and endocarp morphology, as well as ecological preferences. P. paniculata is reinstated as a separate species endemic to Thailand, differing from P. serratifolia in interpetiolar ridge presence, leaf venation, corolla tube pubescence, and endocarp type. P. punctulata is restored as distinct from P. serratifolia, supported by morphological differences and habitat separation. DNA data, though limited for some taxa due to extraction challenges, align with these distinctions and reinforce the need for focused sampling and young-leaf material to optimize sequencing success.
Other taxonomic notes
In Thailand, several names previously conflated with P. serratifolia—such as P. octonervia and P. coriacea variants—are reinterpreted. The evidence suggests P. mollissima aligns with former P. octonervia, while P. coriacea variants are restricted to the Indian subcontinent and do not occur in Thailand. These clarifications support a more accurate Flora of Thailand treatment and better understanding of regional Premna diversity.
Implications and future directions
The Thai Premna study highlights the value of integrating molecular data with morphological and ecological observations to resolve species limits in a complex tropical genus. It also identifies gaps, especially for P. serratifolia, P. serratifolia complex, and P. tomentosa, where genome-scale sequencing or plastid genomics could untangle persistent polytomies. Future work should prioritize collecting young-leaf material for challenging taxa and expanding geographic sampling to capture infraspecific variation across the Southeast Asian region.
