A Thorny Tale: The Origin and Diversification of Cirsium (Compositae)

Abstract

Widely distributed plant genera offer insights into biogeographic processes and biodiversity. The Carduus-Cirsium group, with over 600 species in eight genera, is diverse across the Holarctic regions, especially in the Mediterranean Basin, Southwest Asia, Japan, and North America. Despite this diversity, evolutionary and biogeographic processes within the group, particularly for the genus Cirsium, remain underexplored. This study examines the biogeographic history and diversification of the group, focusing on Cirsium, using the largest molecular dataset for the group (299 plants from 251 taxa). Phylogenomic analyses based on 350 nuclear loci, derived from target capture sequencing, revealed highly resolved and consistent phylogenetic trees, with some incongruences likely due to hybridization and incomplete lineage sorting.Ancestral range estimations suggest that the Carduus-Cirsium group originated during the Late Miocene in the Western Palearctic, particularly in the Mediterranean, Eastern Europe, or Southwest Asia. A key dispersal event to tropical eastern Africa around 10.7 million years ago led to the genera Afrocarduus and Afrocirsium, which later diversified in the Afromontane region. The two subgenera of Cirsium—Lophiolepis and Cirsium—began diversifying around 7.2-7.3 million years ago in the Western Palearctic. During the Early Pliocene, diversification rates increased, with both subgenera dispersing to Southwest Asia, where extensive in situ diversification occurred. Rapid radiations in North America and Japan during the Pleistocene were triggered by jump-dispersals from Asia. These events were likely driven by geographic and ecological speciation processes, with taxonomic implications for the complex classifications within Cirsium.

Keywords: Biogeography, Carduinae, Cirsium, Diversification, Target-enrichment, Taxonomy