Dissertation Research: Systematics and Molecular Evolution of Marattioid Ferns (Marattiaceae)

最近对陆地植物DNA序列的研究揭示了一种在生命树中罕见的现象。Marattiaceae家族中的热带蕨类植物是一种古老的植物谱系，化石可以追溯到大约3亿年前，相对于其他植物，它们在进化过程中的DNA序列变化速度非常慢。这些蕨类植物通常被称为传统形态学意义上的“活化石”：活着的植物几乎无法与一些古老的化石区分开来。这偶尔在植物群中观察到，但marattioid蕨类植物理论上与其他活化石群中观察到的分子进化模式不同，它们也具有随时间推移的低序列变化率，使这些蕨类植物成为“分子活化石”。这项研究建立了一个广泛的DNA序列库，用于marattioid蕨类植物的活体代表：（1）通过彻底的采样，评估分子活化石假说是否在仔细审查下成立，（2）在基因组内定位发生这种现象的区域，（3）分离限制DNA序列突变的潜在因果因素，以及（4）更好地了解这个独特的植物家族的进化历史。为了实现这些目标，DNA序列和形态特征将被用来创建一个谱系树或“系统发育”的marattioid蕨类植物和统计方法将被用于结合已知的化石日期，以允许评估率的变化，通过时间为这个家庭的植物。使用来自基因组不同区域的DNA序列将允许检测基因组内的速率差异，从而告诉我们假设的分子活化石现象在哪里起作用以及强度如何。虽然生物学家早就知道生命之树的分支随着时间的推移表现出不同的变化率，但对这些变化率差异的机制知之甚少。大量的研究都集中在研究那些似乎比其他群体进化得更快的群体上，但很少有人从相反的角度来研究那些似乎几乎没有变化的群体。了解限制DNA序列突变率的机制是目前一个重要的研究领域;这项研究是朝着这个方向迈出的第一步，为该领域的未来探索奠定了基础。该项目将通过提供一组重要但研究不足的植物的进化关系的细节来执行系统研究的传统功能，并将通过提供对分子进化的基本机制的见解来扩展系统学的边界。

Recent studies of DNA sequences of land plants have uncovered a phenomenon that is rare in the tree of life. Tropical ferns in the family Marattiaceae, an ancient lineage of plants with fossils dating back roughly 300 million years, are hypothesized to have extremely slow rates of DNA sequence change through evolutionary time, relative to other plants. These ferns have often been called "living fossils" in the traditional morphological sense: living plants are nearly indistinguishable from some ancient fossils. This is occasionally observed in plant groups, however marattioid ferns are theorized to differ from conventional patterns of molecular evolution observed in other living fossil groups by also having low rates of sequence change through time, making these ferns "molecular living fossils". This research builds an extensive library of DNA sequences for living representatives of the marattioid ferns to: (1) assess if the molecular living fossil hypothesis holds up under close scrutiny with thorough sampling, (2) locate the area(s) within the genome where this phenomenon is taking place, (3) isolate potential causal factors for constraints on DNA sequence mutation, and (4) better understand the evolutionary history of this unique family of plants. In order to achieve these goals, DNA sequences and morphological characters will be used to create a family tree or "phylogeny" of the marattioid ferns and statistical methods will be used in combination with known fossil dates to allow assessment of rates of change through time for this family of plants. Using DNA sequences from different areas of the genome will allow detection of rate differences within the genome telling us where the hypothesized molecular living fossil phenomenon is acting and with what strength. While it has long been understood by biologists that branches of the tree of life exhibit different rates of change through time, little is known about the mechanisms that underlie these rate differences. Significant amounts of research have gone into studying groups that appear to evolve more quickly than others, yet few have approached this problem from the opposite angle by studying groups that appear to hardly change at all through time. Understanding mechanisms that constrain the rate of mutation in DNA sequence is currently an important area of study; this research is a first step in this direction and lays the groundwork for future explorations in this area. This project will perform the traditional function of a systematic study by providing details of the evolutionary relationships of an important but poorly studied group of plants, and will extend the boundaries of systematics by providing insights into the underlying machinery of molecular evolution.