Dissertation Research: The Geological History of Polyploidy in Woody Angiosperms

多倍体是指生物体具有两套以上完整染色体的情况，是开花植物（被子植物）中一种常见而重要的进化现象，但对多倍体的历史发展知之甚少。如果能够利用植物化石来确定被子植物多倍体的历史，那么关于植物多倍体的基本染色体数目（对确定多倍体的程度很重要）和多倍体起源的许多问题都将得到解决。多倍体的一个常见物理效应是细胞体积和保护细胞（控制植物气体交换孔或气孔的打开和关闭的叶表皮细胞）的大小普遍增加。至少有两个先前的研究使用保护细胞测量来估计红杉和水杉化石分类群的倍性水平。菲尔德博物馆和芝加哥大学的古植物学家彼得·克兰博士将指导研究生简·马斯特森对四个木本被子植物科的多倍体历史进行分析：Platanaceae（槭树）、Magnoliaceae（木兰科）、Juglandaceae（核桃）和Lauraceae（月桂）。选择这些群体是因为它们都有很长的化石遗迹记录；(2)保存良好的叶片角质层样品，在不同生长阶段具有可测量的保护细胞；(3)现存物种的多倍性水平；(4)初步研究表明细胞大小与倍性水平呈正相关，包括使用显微分光光度法测量染色体中的DNA含量。这项工作将确定几个被子植物科多倍体的古老，并划定可能促进多倍体的古生态条件。它将为围绕开花植物基本染色体数目的争论提供经验数据，并为植物进化多样化（谱系中的物种数量）和基因组大小（染色体数量和DNA数量）之间假设的反比关系提供第一个测试。

Polyploidy, the condition in an organism of having more than two complete sets of chromosomes, is a common and important evolutionary phenomenon in flowering plants (angiosperms), but the historical development of polyploidy is poorly understood. Many questions about the base chromosome number of angiosperms (important for establishing the extent of polyploidy) and the origin of polyploidy could be addressed if fossil plants could be used to determine its history. One of the common physical effects of polyploidy is a general increase in cell volume and in the size of guard cells, the leaf epidermal cells that control the opening and closing of the gas exchange pores or stomates of plants. At least two previous studies have used guard cell measurements to estimate ploidy levels in fossil taxa of Sequoia and Metasequoia. The paleobotanist Dr. Peter Crane of the Field Museum and University of Chicago will supervise graduate student Jane Masterson in an analysis of the history of polyploidy in four woody angiosperm families: Platanaceae (sycamores), Magnoliaceae, Juglandaceae (walnuts), and Lauraceae (laurels). These groups were chosen because all have (1) a long record of fossil remains; (2) well preserved leaf cuticle samples with measurable guard cells at a variety of growth stages; (3) multiple ploidy levels presently known for living species; and (4) a positive correlation between cell size and ploidy level based on preliminary study, including measurement of DNA content in chromosomes using microspectrophotometry. The work will establish the antiquity of polyploidy in several angiosperm families and circumscribe the paleoecological conditions that may have promoted polyploidy. It will contribute empirical data to the debate surrounding the base chromosome number of flowering plants, and provide the first test in plants of the hypothesized inverse relation between evolutionary diversification (numbers of species in a lineage) and genome size (numbers of chromosomes and DNA amounts).