A phylogenomic analysis and revised infrageneric classification of Thesium (Santalaceae)

Abstract

A phylogenomic analysis and revised infrageneric classification of Thesium (Santalaceae) is presented based on targeted enrichment using the Angiosperms353 probe set. This first targeted sequencing study includes 137 samples representing 106 species (approximately 32% of the genus). Our results provide substantially improved resolution compared to previous studies that utilised only a few gene regions, with an almost fully supported backbone topology. Thesium (including Austroamericium , Chrysothesium , Kunkeliella , and Thesidium ) is confirmed as monophyletic and sister to a clade containing Lacomucinaea and Osyridicarpos . We recognise six monophyletic subgenera with full or strong support: T.  subg. Hagnothesium , T.  subg. Thesium , T.  subg. Spinosa (newly described), T.  subg. Discothesium (with revised circumscription), T.  subg. Frisea , and T.  subg. Psilothesium . We found T.  subg. Discothesium as previously delineated to be paraphyletic and describe a new subgenus, T.  subg. Spinosa , to ensure monophyly of all subgenera. We formally recognise 18 sections in Thesium , of which 8 are newly described and 2 are raised from lower ranks. Our phylogenetic clades show a strong biogeographic signal and despite sporadic morphological synapomorphies, most subgenera and sections can be distinguished by a combination of their distribution and morphology. We provide the first genetic evidence of potential hybridisation in Thesium paving the way for further exploration of this long‐hypothesised compounding factor. This study provides a robust framework for future taxonomic, biogeographic, and evolutionary studies of this diverse and problematic genus.