Introduction
This article summarizes the molecular phylogeny and taxonomic notes for Premna (Lamiaceae) in Thailand, based on a multi-locus chloroplast approach. Utilizing four chloroplast markers and concatenated datasets, the study delineates phylogenetic relationships among Thai Premna species, highlights unresolved polytomies, and revisits trusted species concepts within the P. serratifolia complex.
Phylogenetic reconstruction
Using a multi-locus strategy with four chloroplast markers, the Thai Premna dataset resolves the genus as monophyletic within Premnoideae and identifies a sister relationship with Gmelina. While this aligns with prior chloroplast- and plastome-based studies, certain species (e.g., P. ligustroides, P. serrata, P. tomentosa) retain unresolved placements due to polytomies, a pattern also seen in other Lamiaceae genera. The authors note that expanding data, through plastid genomes, target enrichment, or whole-genome approaches, could help resolve these relationships.
In the present analysis, two major clades emerge from four concatenated chloroplast markers:
CLADE I
This well-supported clade includes Chinese and Vietnamese taxa such as P. mekongensis, P. punicea, and P. tenii. These species share morphological synapomorphies—dense capitate inflorescences and distinctly 5-lobed calyces—and the clade is consistent with morphology-based classifications. The study suggests further molecular testing to determine whether other taxa with similar traits in Flora of China and neighboring regions form a monophyletic group.
CLADE II
This clade comprises subclades I–III (red-labelled in Fig. 1). The relationships among P. serrata and P. tomentosa remain unresolved within this clade. P. serrata features serrate leaf margins and characteristic calyx lobes, while P. tomentosa is distinguished by stellate or dendritic leaf pubescence on both surfaces. The CLADE II grouping also includes several taxa not previously analyzed in depth, such as P. annulata, P. cordifolia, P. repens, P. scandens, P. serrata, P. tenii, P. tomentosa, and P. trichostoma.
Three unresolved subclades (I–III) sit within CLADE II, incorporating Thai species and members of the P. serratifolia complex. Subclade I contains nine species, primarily shrub or woody climbers from Myanmar, China, and Thailand, with a monophyletic group of P. bracteata, P. garrettii, P. interrupta, P. scandens, and P. trichostoma associated with synapomorphies in fruit and endocarp morphology. Subclade II includes two herbaceous species (P. herbacea and P. nana), the latter once discounted as a synonym but now recognized as distinct in light of molecular and morphological evidence. Subclade III holds diverse life forms and habitats, including several endemics like P. annulata in Thailand and P. szemaoensis in China. P. serratifolia itself appears widely distributed in Southeast Asia, and its Thai accessions reveal a monophyletic grouping with the complex, though P. punctulata shows distinct lineage patterns that echo prior taxonomic debates.
Taxonomic notes: P. serratifolia complex in Thailand
The study revisits several names within the P. serratifolia umbrella in Thailand—P. cordifolia, P. paniculata, and P. punctulata—and supports recognizing them as distinct species rather than synonyms of P. serratifolia. Notably, P. serratifolia occupies coastal habitats, while the other taxa favor evergreen forest interiors. The combined morphological, ecological, and chloroplast data justify reinstating P. cordifolia and P. paniculata as separate species and clarify the status of P. punctulata.
Premna cordifolia
Distinguishing features include cordate or rounded leaf bases, interpetiolar ridges on stems, a longer inflorescence, and a corolla with a yellow patch on the middle lobe. Endocarp differences (type II protrusion in P. cordifolia vs type I in P. serratifolia) further separate the taxa. Distribution spans Thailand and Vietnam with habitat preferences in primary and secondary evergreen forests.
Premna paniculata
Previously lumped with P. serratifolia, P. paniculata is reinstated as distinct based on the absence of an interpetiolar ridge, a glabrous corolla tube externally, shorter inflorescences, and a 3-lobed posterior calyx lip. Endocarp morphology (type II protrusion) also supports separation. Molecular analyses were hampered by DNA extraction issues from mature leaves; future work should target young material for definitive placement.
Premna punctulata and P. serratifolia
P. punctulata shows a longer corolla, yellow patches on the middle lobe, and a hairy ovary, with a distinct habitat in evergreen or dry evergreen forests. P. serratifolia, by contrast, lacks these traits and occupies coastal/mangrove environments. Phylogenetically, P. punctulata maintains a relatively long branch and is placed in an unresolved position relative to P. serratifolia, reinforcing the need for broader geographic sampling to clarify species limits.
Conclusions
The Thai Premna study advances our understanding of species boundaries in a region where taxonomy has been fraught with synonymy. The integration of morphology, ecology, and chloroplast phylogenetics yields a clearer framework for recognizing P. cordifolia and P. paniculata as distinct from P. serratifolia, while reaffirming the distinctness of P. punctulata and its relationship to P. serratifolia. Ongoing genomic approaches promise to resolve the remaining polytomies and refine infrageneric classification within Premna in Thailand.
