Reassessment of varietal status of Pavetta tomentosa var. tomentosa and P. tomentosa var. glabrescens (Rubiaceae) based on start codon targeted (SCoT) markers

The palaeotropical genus Pavetta is the largest genus of the tribe Pavetteae (Rubiaceae). In Thailand, 14 species are currently recognized based on morphology; however, the taxonomic status of some morphologically similar taxa remains uncertain. This study focuses on one problematic species, P. tomentosa, which is currently classified into two varieties: var. tomentosa and var. glabrescens. However, the taxonomic distinction between these two varieties remains questionable because it relies solely on indumentum characters (tomentose vs. sparsely puberulous or glabrescent) on various plant parts, with no clear boundaries between taxonomic character states. To reassess these varieties, a population genetic study using start codon targeted (SCoT) markers was conducted on 53 samples from seven populations of P. tomentosa to identify genetic groupings and examine their correspondence with traditional varietal classification. Amplification with four selected SCoT primers revealed a high level of polymorphism (100%). The analyses indicated moderate genetic diversity among the studied populations, with greater variation within populations than among them. This pattern is consistent with a relatively high gene flow (Nm = 1.6284), suggesting ongoing genetic exchange among populations. UPGMA clustering, principal coordinate analysis, and STRUCTURE analysis consistently revealed two genetic clusters within P. tomentosa, with individuals intermixed regardless of hair characters or geographical distribution. Thus, the considerable variation in hairiness, together with genetic evidence, suggests that P. tomentosa should not be separated into two varieties based solely on indumentum characters.