Dissertation Research: The Geological History of Polyploidy in Woody Angiosperms

多倍体是开花植物(被子植物)中普遍而重要的进化现象，是生物体中具有两组以上完整染色体的状态，但对多倍体的历史发展知之甚少。如果植物化石可以用来确定被子植物的历史，许多关于被子植物的基本染色体数目(对确定多倍体的程度很重要)和多倍体起源的问题都可以得到解决。多倍体的常见生理效应之一是细胞体积和保卫细胞的大小普遍增加，保卫细胞是控制植物气体交换气孔或气孔打开和关闭的叶表皮细胞。之前至少有两项研究使用保卫细胞测量来估计红杉和水杉化石分类群的倍性水平。芝加哥大学菲尔德博物馆的古植物学家Peter Crane博士将指导研究生Jane Masterson分析四个木本被子植物家族的多倍体历史：梧桐科(梧桐科)、木兰科(Magnoliaceae)、胡桃科(Juglandaceae)和樟科(Lauraceae)。选择这些群体是因为它们都有(1)化石遗迹的长期记录；(2)保存完好的叶片角质层样本，在不同的生长阶段具有可测量的保卫细胞；(3)目前已知的活物种的多重倍性水平；(4)在初步研究的基础上，细胞大小与倍性水平呈正相关，包括用显微分光光度法测量染色体中的DNA含量。这项工作将在几个被子植物家族中确立多倍体的古老，并限制可能促进多倍体的古生态条件。它将为围绕开花植物的基本染色体数量的辩论提供经验数据，并首次在植物中测试进化多样性(谱系中的物种数量)和基因组大小(染色体数量和DNA数量)之间的假设相反关系。

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).