Ploidy differentiation and floral scent divergence in Buddleja macrostachya (Scrophulariaceae) complex

Abstract Floral scent has been suggested to be a key factor both in the diversification of the angiosperms, and the evolution and intra-specific differentiation of plants. In this study, we determined the ploidy and nuclear DNA content of B. macrostachya plants from 14 different populations across Yunnan using flow cytometry. The components of the floral scent were also identified using gas chromatography and mass spectrometry (GC-MS). Within these 14 populations, two new ploidy levels, tetraploid (2n = 4x) and octaploid (2n = 8x) were detected and identified for the first time here. The mean 2C DNA content of the tetraploids ranged from 1.81 ± 0.01 to 1.92 ± 0.02 G, but was 3.52 ± 0.15 to 3.73 ± 0.04 G in the octaploids. In addition, thirty-two scent compounds were identified, most of which could be categorized into four major classes: fatty acid derivatives (49.60 ± 7.45%); benzenoids (21.24 ± 3.96%); monoterpenes (25.28 ± 3.70%) and sesquiterpenes (3.88 ± 1.05%). Both tetraploids and octaploids emitted large relative amounts of 4-hydroxy-4-methyl-2-pentanone (42.21 ± 3.52% and 21.57 ± 3.10%, respectively) and benzadehyde (20.42 ± 2.35% and 28.45 ± 2.37%, respectively). Multivariate analysis of floral scent chemistry revealed marked dissimilarity (49.49%) between tetraploids and octaploids, and 4-hydroxy-4-methyl-2-pentanone and benzadehyde contributed greatly to this divergence in chemical scent profile. Our results suggest that B. macrostachya is a species complex with four different ploidy levels (2n = 4x, 6x, 8x, 12x). We believe the complex is in a process of pollinator-mediated rapid divergence.