2626 WWW.CROPS.ORG CROP SCIENCE, VOL. 51, NOVEMBER– DECEMBER 2011 RESEARCH W hat happens to all of our familiar habitats and ecosystems during the Ice Ages? “…unless one can believe that the typical temperate species were pushed south of the Rio Grande and deep into peninsular Florida, the alternative is an extraordi- nary intermingling of boreal, temperate, and subtropical elements along the Gulf coast.” (Deevey, 1949) Edward Smith Deevey Jr. is largely responsible for converting the ield of paleol imnol- Natural Hybrids and Gene Flow between Upland and Lowland Switchgrass Yunwei Zhang, Juan Zalapa, Andrew R. Jakubowski, David L. Price, Ananta Acharya, Yanling Wei, E. Charles Brummer, Shawn M. Kaeppler, and Michael D. Casler* ABSTRACT Switchgrass (Panicum virgatum L.) is a perennial grass native to the North American tallgrass prai- rie and savanna habitats and is broadly adapted to the central and eastern United States. Upland and lowland ecotypes represent the two major taxa within switchgrass, which have distinct but overlapping distributions. The purpose of this study was to survey a broad array of putative upland and lowland accessions for the possible presence of natural hybrids or hybrid derivatives and evidence of historic gene low between the two ecotypes. All plants were classiied as upland, lowland, or intermediate based on visual assessment of phenotype, using large nurseries of known upland or lowland plants as controls. A total of 480 plants were surveyed for 19 simple sequence repeat (SSR) markers and sequenced using ive chloroplast DNA (cpDNA) segments. Genetic structure analysis revealed 21 individu- als with strong evidence for intertaxa hybrid origin and another 25 individuals with moder- ate evidence for intertaxa hybrid origin. All but two of these individuals originated from remnant populations of the central or eastern Gulf Coast or along the Atlantic Seaboard, a region that is populated with signiicant quantities of both upland and lowland ecotypes. We propose the central and eastern Gulf Coast glacial refuge as the primary center of origin and diversity for switchgrass, with the western Gulf Coast as the secondary center of origin and diversity. Much of this diversity appears to have been preserved along one of the major northward postglacial migration routes, the Atlantic Seaboard. Y. Zhang, Grassland Institute, China Agricultural Univ., No. 2 Yuan- mingyuan Xilu, Beijing, China 100193; J. Zalapa, USDA-ARS, Veg- etable Crops Research Unit, Dep. of Horticulture, Univ. of Wisconsin, 1575 Linden Dr., Madison, WI 53706; A.R. Jakubowski, D.L. Price, and S.M. Kaeppler, Dep. of Agronomy, Univ. of Wisconsin, 1575 Linden Dr., Madison, WI 53706; A. Acharya and Y. Wei, Center for Applied Genetic Technologies, Crop and Soil Science Dep., Univ. of Georgia, 111 Riverbend Rd., Athens, GA 30602; E.C. Brummer, The Samuel Roberts Noble Foundation, 2510 Sam Noble Pkwy., Ardmore, OK 73401; M.D. Casler, USDA-ARS, U.S. Dairy Forage Research Cen- ter, 1925 Linden Dr., Madison, WI 53706-1108. Y. Zhang, J. Zalapa, and A.R. Jakubowski contributed equally to the work described in this manuscript. S.M. Kaeppler and M.D. Casler are ailiated with the DOE Great Lakes Bioenergy Research Center (GLBRC), Univ. of Wiscon- sin, Madison, WI; J. Zalapa was previously ailiated with GLBRC. A. Acharya and E.C. Brummer are ailiated with the DOE BioEnergy Sci- ences Center (BESC), Univ. of Georgia, Athens, GA. Received 24 Feb. 2011. *Corresponding author (michael.casler@ars.usda.gov). Abbreviations: cpDNA, chloroplast DNA; PCR, polymerase chain reaction; p H , probability of any type of hybrid ancestry; PIC, polymor- phism information content; ROX, carboxy-X-rhodamine; SSR, simple sequence repeat. Published in Crop Sci. 51:2626–2641 (2011). doi: 10.2135/cropsci2011.02.0104 Published online 12 Aug. 2011. © Crop Science Society of America | 5585 Guilford Rd., Madison, WI 53711 USA All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permission for printing and for reprinting the material contained herein has been obtained by the publisher.