Unraveling the taxonomy of Asian Buddleja: Species discovery and delimitation through morphological comparison, ploidy analysis and phylogenomic insights

Abstract Hybridization and polyploidization have given rise to complex cytonuclear conflicts and morphological continuity within the Asian Buddleja group, posing challenges in species delimitation and the assignment of conservation efforts. This study employed an integrative approach, utilizing morphological comparisons, ploidy estimations and phylogenomic insights to investigate the taxonomy of B. purdomii , previously treated as a synonym of B. brachystachya , and to describe two new species, B. atropurpurea and B. calostachya . A comprehensive phylogenetic analysis was conducted using whole plastid genomes and a large number of low‐copy nuclear genes assembled from genome skimming data from 61 samples, covering all the known Asian Buddleja species. Additionally, a population genetic analysis of B. lindleyana and B. atropurpurea was performed to further confirm whether B. atropurpurea is a distinct taxon from B. lindleyana . The ploidy level of each of the three species studied in depth was determined through chromosome counts, and morphological comparisons were made between the two putative new species with their sister relatives. The time of divergence of each species was estimated by dating of the maternally inherited plastomes. Phylogenetic reconstruction combined with morphological measurements confirmed the distinct taxonomic status of B. purdomii , B. atropurpurea , and B. calostachya. The times of divergence between these putative species and their sister taxa are estimated to be 10.7 Ma, 3.6 Ma, and 2.1 Ma, which may correspond to the intensification of the East Asian Summer Monsoon (EASM), the Qingzang movement and the Quaternary climate upheaval, respectively. This study sheds new light on species delimitation and the speciation process in Asian Buddleja , and also provides a robust taxonomic framework for the conservation and utilization of these species.