Evolutionary history of the arid climate-adapted Helichrysum (Asteraceae: Gnaphalieae): Cape origin and association between annual life-history and low chromosome numbers

© 2018 Institute of Botany, Chinese Academy of Sciences The basal grade of the large, widely-distributed Helichrysum-Anaphalis-Pseudognaphalium (HAP) clade (Asteraceae, Gnaphalieae) comprises exclusively southern African taxa. These species possess unusual trait combinations relative to the remaining species (a high proportion of annuals, unusual capitulum arrangement, and low base chromosome numbers). A time-proportional Bayesian phylogenetic hypothesis is generated from nuclear ribosomal sequences from 110 accessions. Ancestral area, life history, and base chromosome number are reconstructed using maximum likelihood, and correlations between life-history and chromosome number are tested in a phylogenetic framework. The results show that the HAP clade probably originated and experienced initial diversification in the Greater Cape Floristic Region in the Early to Middle Miocene. The ancestor of the HAP clade is inferred to have been perennial with x = 7 base chromosome number. Several independent acquisitions of the annual life-history are inferred, accompanied by reductions to x = 4 and 5. A single reversal to perennial life history is associated with a subsequent change back to the state of x = 7. Origin and early diversification within the HAP clade follows the pattern of multi-area seeded radiations within southern Africa, with subsequent migrations to the rest of Africa and the Northern Hemisphere. Occupation of drier habitats with shorter growing seasons may select for the acquisition of a shorter life-cycle, and our results indicate a strong association between short life-cycle and reduced chromosome number.