Collaborative Research: Did the Neogene aridification drive adaptive ecological radiation in an ancient plant lineage?

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Living organisms that evolved hundreds of millions of years ago are often considered relics or "living fossils" because they seem unchanged from their extinct, fossilized progenitors. Of all ancient taxa regarded as living relics, cycads are particularly notable because recent evidence suggests extant cycads may not be relics at all, but instead evolved during the Neogene period (~20My ago). This time period was characterized by declining temperatures, atmospheric CO2 concentrations, and the expansion of arid regions. This research asks whether Neogene climate change drove the diversification of cycads as they evolved their anatomy and physiology in response to the new conditions. This integrative approach that spans from cells to whole plants will provide a novel perspective on cycad evolution and test whether this ancient lineage experienced concurrent and parallel evolution on multiple continents hundreds of millions of years after the origin of cycads. The proposed work is especially critical because cycads are among the most endangered plant groups on Earth, with ~70% of species threatened with extinction, primarily due to habitat destruction and poaching. Perhaps no other lineage of plants disappearing at such an alarming rate can provide as unique an opportunity to examine one of the earliest experiments of seed plant evolution, and our results will directly inform conservation practices. This project will provide research experiences for graduate students and undergraduates at University of Wisconsin-Madison and Florida International University, one of the nation's largest Hispanic Serving Institutions, as well as research-driven field courses for undergraduates. This research will uncover the ecological and evolutionary processes that have generated the extant diversity of one of the oldest and most unusual groups of seed plants, the cycads. To determine whether Neogene diversification of cycads was enabled by adaptive trait evolution, the project will: (A) characterize the diversity of cycad anatomical traits (e.g. anatomical traits of stomata, bundle sheath extensions, transfusion tissue, and xylem) and test their impacts on gas exchange and hydraulic responses to abiotic conditions (e.g. light, CO2, VPD, drought), (B) test whether these anatomical and physiological traits are linked to species’ distributions in climate space, and (C) determine whether the timing and order of trait evolution corresponded to Neogene climate change. By leveraging the unusual anatomical trait combinations exhibited by cycads, this research may have far reaching impacts in plant physiology, ecology, and evolution by: (A) revealing the significance of anatomical traits with poorly known function(s) that are also found in other plant groups (e.g., bundle sheath extensions, transfusion tissue), (B) characterizing the function of trait combinations that are exceptionally rare among extant plants but commonly found in extinct taxa (e.g., tracheids and homogeneous pits) thus allowing new insight into the function of extinct lineages and better contextualizing the sequence of physiological trait evolution in vascular plants, and (C) testing long-standing hypotheses about the repeatability and predictability of evolution.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.