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

数亿年前进化的生物体通常被认为是遗迹或“活化石”，因为它们与已灭绝的化石祖先相比似乎没有变化。在所有被认为是活的遗迹的古代分类群中，苏铁尤其引人注目，因为最近的证据表明，现存的苏铁可能根本不是遗迹，而是在新近纪（~20万年前）进化而来的。这一时期的特点是气温下降，大气中二氧化碳浓度下降，干旱地区扩大。这项研究询问新近纪气候变化是否推动了苏铁的多样化，因为它们在适应新环境的过程中进化了解剖学和生理学。这种从细胞到整个植物的整合方法将为苏铁的进化提供一个新的视角，并测试这一古老的谱系在苏铁起源后的数亿年里是否在多个大陆上经历了并行和平行的进化。苏铁是地球上最濒危的植物之一，70%的苏铁濒临灭绝，这主要是由于栖息地的破坏和偷猎。也许没有其他以如此惊人的速度消失的植物谱系能够提供一个独特的机会来研究种子植物进化的最早实验之一，我们的结果将直接为保护实践提供信息。该项目将为威斯康星大学麦迪逊分校和佛罗里达国际大学（美国最大的西班牙裔服务机构之一）的研究生和本科生提供研究经验，并为本科生提供研究驱动的实地课程。这项研究将揭示生态和进化过程，这些过程产生了最古老、最不寻常的种子植物之一苏铁的现存多样性。为了确定苏铁的新近系多样化是否通过适应性性状进化得以实现，该项目将：(A)表征苏铁解剖性状（气孔、束鞘伸展、输注组织和木质部解剖性状）的多样性，并测试其对非生物条件（如光、CO2、VPD、干旱）下气体交换和水力响应的影响；(B)测试这些解剖和生理性状是否与物种在气候空间中的分布有关；(C)确定性状进化的时间和顺序是否与新近纪气候变化相对应。利用苏铁不同寻常的解剖性状组合，本研究可能在植物生理、生态和进化方面产生深远的影响：(A)揭示了在其他植物类群（例如，束鞘延伸，输血组织）中也发现的功能鲜为人知的解剖学特征的重要性；(B)描述了在现存植物中非常罕见但在灭绝的分类群中常见的性状组合的功能(例如，管胞和同质坑)，从而使人们对灭绝谱系的功能有了新的认识，并更好地将维管植物的生理性状进化序列背景化，并且(C)验证了关于进化的可重复性和可预测性的长期假设。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

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.