The evolutionary history of the Central Asian steppe-desert taxon Nitraria (Nitrariaceae) as revealed by integration of fossil pollen morphology and molecular data

The transition from a greenhouse to an icehouse world at the Eocene-Oligocene Transition (EOT) coincided with a large decrease of pollen from the steppe-adapted genus Nitraria. This genus, now common along the Mediterranean coast, Asia and Australia, has a proposed coastal origin and a geographically widespread fossil record. Here we investigated the evolution, taxonomic diversity and morphological disparity of Nitraria throughout the Cenozoic by integrating extant taxa and fossil palynological morphotypes into a unified phylogenetic framework based on both DNA sequences and pollen morphological data. We present the oldest fossil pollen grain of Nitraria, at least 53 Myr old. This fossil was found in Central Asian deposits, providing new evidence for its origin in this area. We found that the EOT is an evolutionary bottleneck for Nitraria, coinciding with retreat of the proto-Paratethys Sea, a major global cooling event and a turnover in Central Asian steppe vegetation. We infer the crown age of modern Nitraria spp. to be significantly younger (Miocene) than previously estimated (Palaeocene). The diversity trajectory of Nitraria inferred from extant-only taxa differs markedly from one that also considers extinct taxa. Our study demonstrates it is therefore critical to apply an integrative approach to fully understand the plant evolutionary history of Nitrariaceae.

Automated summary

This study investigates the evolution, taxonomic diversity, and morphological disparity of the genus Nitraria, using a unified phylogenetic framework based on both DNA sequences and pollen morphological data. The oldest Nitraria fossil pollen grain, at least 53 million years old, was found in Central Asian deposits, providing new evidence for its origin in this area. The Eocene-Oligocene Transition is identified as a crucial period for the evolution of Nitraria.