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上罕见的现象。马尾蕨科热带蕨类植物是一种古老的植物谱系，其化石可以追溯到大约3亿年前。相对于其他植物，热带蕨类植物在进化过程中DNA序列变化的速度非常慢。这些蕨类植物通常被称为传统形态意义上的“活化石”：活体植物与一些古代化石几乎难以区分。这在植物群中偶尔会被观察到，然而，从理论上讲，马蹄类蕨类植物不同于在其他活化石群中观察到的传统分子进化模式，因为它也具有随时间变化的低序列变化率，使这些蕨类植物成为分子活化石。本研究为软体蕨类植物的现存代表建立了一个广泛的脱氧核糖核酸序列文库，以：(1)评估分子活化石假说是否在仔细的仔细检查下成立，(2)定位基因组中发生这种现象的区域(S)，(3)分离限制脱氧核糖核酸序列突变的潜在原因，以及(4)更好地理解这个独特的植物科的进化史。为了实现这些目标，将使用DNA序列和形态特征来创建马蹄类蕨类植物的家系树或“系统发育”，并将结合已知的化石日期使用统计方法，以评估这一类植物随时间的变化率。使用基因组不同区域的DNA序列可以检测基因组内的速率差异，告诉我们假设的分子活化石现象在哪里起作用，强度有多大。虽然生物学家早就知道生命树的树枝随着时间的推移呈现出不同的变化速度，但人们对这种变化速度差异背后的机制知之甚少。大量的研究都在研究那些似乎进化得比其他人更快的群体，但很少有人从相反的角度来研究这个问题，研究那些似乎几乎不会随着时间的推移而改变的群体。了解限制DNA序列突变率的机制是当前的一个重要研究领域，这项研究是朝着这个方向迈出的第一步，并为未来在这一领域的探索奠定了基础。这个项目将通过提供一组重要但研究较少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洞察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.