Nuclear phylogenomics of Asteraceae with increased sampling provides new insights into convergent morphological and molecular evolution

Convergent morphological evolution is widespread in flowering plants,and understanding this phenomenon relies on well-resolved phylogenies.Nuclear phylogenetic reconstruction using transcriptome datasets has been successful in various angiosperm groups,but it is limited to taxa with available fresh materials.Asteraceae,which are one of the two largest angiosperm families and are important for both ecosystems and human livelihood,show multiple examples of convergent evolution.Nuclear Asteraceae phylogenies have resolved relationships among most subfamilies and many tribes,but many phylogenetic and evolutionary questions regarding subtribes and genera remain,owing to limited sampling.Here,we increased the sampling for Asteraceae phylogenetic reconstruction using transcriptomes and genome-skimming datasets and produced nuclear phylogenetic trees with 706 species representing two-thirds of recognized subtribes.Ancestral character reconstruction supports multiple convergent evolutionary events in Asteraceae,with gains and losses of bilateral floral symmetry correlated with diversification of some subfamilies and smaller groups,respectively.Presence of the calyx-related pappus may have been especially important for the success of some subtribes and genera.Molecular evolutionary analyses support the likely contribution of duplications of MADS-box and TCP floral regulatory genes to innovations in floral morphology,including capitulum inflorescences and bilaterally symmetric flowers,potentially promoting the diversification of Asteraceae.Subsequent divergences and reductions in CYC2 gene expression are related to the gain and loss of zygomorphic flowers.This phylogenomic work with greater taxon sampling through inclusion of genome-skimming datasets reveals the feasibility of expanded evolutionary analyses using DNA samples for understanding convergent evolution.

Support for the predictions of the pollinator-mediated stabilizing selection hypothesis

Aims Floral traits are frequently used in traditional plant systematics because of their assumed constancy.One potential reason for the apparent constancy of flower size is that effective pollen transfer between flowers depends on the accuracy of the physical fit between the flower and pollinator.Therefore,flowers are likely to be under stronger stabilizing selection for uniform size than vegetative plant parts.Moreover,as predicted by the pollinator-mediated stabilizing selection(PMSS)hypothesis,an accurate fit between flowers and their pollinators is likely to be more important for specialized pollination systems as found in many species with bilaterally symmetric(zygomorphic)flowers than for species with radially symmetric(actinomorphic)flowers.Methods In a comparative study of 15 zygomorphic and 13 actinomorphic species in Switzerland,we tested whether variation in flower size,among and within individuals,is smaller than variation in leaf size and whether variation in flower size is smaller in zygomorphic compared to actinomorphic species.Important findings Indeed,variation in leaf length was significantly larger than variation in flower length and width.Within-individual variation in flower and leaf sizes did not differ significantly between zygomorphic and actinomorphic species.In line with the predictions of the PMSS,among-individual variation in flower length and flower width was significantly smaller for zygomorphic species than for actinomorphic species,while the two groups did not differ in leaf length variation.This suggests that plants with zygomorphic flowers have undergone stronger selection for uniform flowers than plants with actinomorphic flowers.This supports that the relative uniformity of flowers compared to vegetative structures within species,as already observed in traditional plant systematics,is,at least in part,a consequence of the requirement for effective pollination.