Phylogeny-based morphological evolution and species complex circumscription in <i>Rosa</i> sections <i>Synstylae</i> and <i>Chinenses</i> (Rosaceae), with description of one new species

Abstract The genus Rosa presents significant challenges in species identification and phylogenetic research due to phenotypic similarities, rapid diversification, hybridization, and incomplete lineage sorting. This study investigates the phylogenetic relationships and morphological evolution of 56 taxa (53 Synstylae and three Chinenses) using six nuclear regions and 126 chloroplast genomes, resulting in 1142 sequences. We found that these taxa, including 15 newly sampled, were assigned to 12 species complexes based on molecular, morphological, and geographical features. Nuclear markers offer a more reliable phylogenetic backbone than chloroplast genomes, aligning more closely with morphological classifications. A new species, Rosa austrotibetana sp. nov., from Tibet, China, is described, and R. mulliganii is identified as a hybrid of R. luciae and R. filipes; six morphological characters were analysed in identifying synapomorphic traits among species complexes; leaflet evolution exhibits homoplasy; and stipule margin shape, petal hairs, and style hairs are key traits for circumscribing species series and complexes, whereas leaflet texture, leaflet number, and abaxial leaflet hairs are key for species delimitation within complexes.