A phylogenomic analysis of Thesium using Angiosperms353
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 of the genus includes 138 samples representing 106 species (approximately 32% of the genus). Our results provide substantially improved resolution compared to previous studies that utilized only a few gene regions, with an almost fully supported backbone phylogeny. Thesium (including Austroamericium, Chrysothesium, Kunkeliella, and Thesidium) is confirmed as monophyletic and sister to a clade containing Lacomucinaea and Osyridicarpos. We recognize six monophyletic subgenera with full or strong support: T. subg. Hagnothesium, T. subg. Thesium, T. subg. Spinosum (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 described a new subgenus, T. subg. Spinosum, to ensure monophyly of all subgenera. We formally recognised 18 sections in Thesium, of which nine are newly described and two are raised from lower ranks. Our phylogenetic clades show a strong biogeographic signal and though morphological synapomorphies are sporadic most subgenera and sections can be distinguished by of combination of their distribution and morphology. We provide the first genetic evidence of hybridization 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.