Cycads studied to reconcile fossil and molecular evidence for evolutionary timescales

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Evolutionary biologists are trying to figure out the timescale of the tree of life. One way to do this is by using molecular dating, which relies on known fossil information. However, this is often difficult because lineages can be difficult to date accurately without fossils. Another way to do this is by using biogeographical or tectonic calibration. Dating involves associating evolutionary divergence events with geological or climatic events of known age. Scientists can do this by looking at the close tectonic and palaeogeological correlations between extant clades. Biogeographical events can be dated and give more accurate estimations than using single fossil calibrations. However, biogeographical calibrations need to justify several key assumptions: that the biogeographical event has had measurable evolutionary and genetic impacts and that the event can be dated independently. Liu and colleagues discuss how well biogeographical dating performs and how it can be implemented and interpreted carefully.

Cycads are considered the most ancient living seed plant lineage and the sister to all other cycads. Recent molecular dating suggests that the cycad family diversified post-Neogene. However, some palaeobotanists argue that Cycas diversified earlier than previously thought, based on multiple Palaeogene cycad fossils. However, other biogeographical analyses have yielded different results, raising the need for further research on this topic.

The section Wadeae of the Cycas genus is endemic to the closely associated islands of Culion and Palawan, which belong to the Palawan microcontinent in the Philippines. The Palawan microcontinent results from the rifting of North Palawan microcontinents from continental Eurasia since the Late Eocene. This process has been documented in many paleogeographical studies and has given rise to the ‘Palawan Arc’ hypothesis. The species from section Wadeae morphologically closely resemble those of sections Panzhihuaenses, Asiorientales and Stangerioides found in continental East Asia or along margins of the shallow sea basin of East Asia. This suggests the analogous ‘rafting Palawan’ hypothesis for the distribution of Wadeae.

Earlier work found that section Wadeae was close to section Stangerioides by cladistic analysis, but subsequent phylogenetic studies using different molecular markers have placed section Wadeae in a different position in the Cycas genus. The separation between Cycas from East Asia and Palawan provides a framework to test the utility of biogeographical evidence in the molecular dating of Cycas, which has not been done in any previous study. Plastid phylogenomics is a tool used to study the evolutionary relationships of organisms. Plastids are organelles outside the cellular nucleus that have their own DNA, such as chloroplasts and mitochondria. It has been used to resolve deep relationships of particularly recalcitrant lineages, such as those that have undergone recent radiations.

Liu and colleagues first reconstructed the phylogenetic framework of Cycadaceae using whole plastomic data and linked this phylogeny to the distribution of taxa. They then combined palaeogeographical and fossil evidence to establish different calibration schemes, use molecular dating to estimate the ages of major phylogenetic nodes under various calibration schemes, and test their robustness, among other effects. Comparing these results, they discuss which timeline based on calibration scenarios best explains the historical biogeography of Cycadaceae. By understanding the timescale and biogeography of extant Cycas, Liu and colleagues aim to settle debates on this group’s spatial and temporal origin and further discuss how to accommodate better the conflicts between clock and rock using the biogeographical calibrations.

The authors found recent dispersal cannot explain the distribution of Palawan Cycas. Despite this, the scenario including the tectonic calibration yielded a mean crown age of extant Cycadaceae of ~69–43 million years ago by different tree priors, consistent with multiple Palaeogene fossils assigned to this family. Biogeographical analyses incorporating fossil distributions revealed East Asia as the ancestral area of Cycadaceae. Their findings challenge the previously proposed Middle-Late Miocene diversification of cycads and an Indochina origin for Cycadaceae and highlight the importance of combining phylogenetic clades, tectonic events and fossils for rebuilding the evolutionary history of lineages that have undergone massive extinctions.

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Jian Liu, Anders J Lindstrom, Thomas E Marler, Xun Gong, Not that young: combining plastid phylogenomic, plate tectonic and fossil evidence indicates a Palaeogene diversification of Cycadaceae, Annals of Botany, https://doi.org/10.1093/aob/mcab118