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How Old Are Sunflowers? A Molecular Clock Analysis of Key Divergences in the Origin and Diversification of Helianthus (Asteraceae)

Premise of research. For many questions in evolutionary ecology, it is quite valuable to have an estimate of the span of geologic time over which a group of species has arisen and diversified. This study uses a molecular clock approach to generate a first estimate of key divergences in the evolutionary history of the genus Helianthus, which to date has lacked such an estimate. Methodology. Divergence time analysis was performed with well-resolved phylogenies of Asteraceae and Helianthus in a stepping-stone manner using the RelTime maximum likelihood method with a variety of available macrofossil, fossil pollen, and molecular clock calibrations derived from the literature. Pivotal results. Mean estimates from nine individual calibration scenarios time the divergence of Helianthus from the sister genus Phoebanthus at between 2.47 and 5.41 Ma. Composite calibration incorporating all literature estimates time this divergence at 3.63 Ma, with a 95% confidence interval spanning 0–8.26 Ma. Subsequent diversification within Helianthus is therefore confined to the last few million years. Conclusions. These findings place the origin and diversification of Helianthus into the temporal context of the Pliocene expansion of open habitats across North America and the subsequent vegetation fluctuations of the Pleistocene glacial-interglacial cycles.