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).
