Reproduced from Crop Science. Published by Crop Science Society of America. All copyrights reserved. CROP SCIENCE, VOL. 48, MARCH– APRIL 2008 517 RESEARCH I ncorporating durable disease resistance into susceptible host backgrounds has often been di fcult using conventional breeding methods. Conventional breeding programs have been limited to monogenic, race-specifc resistance genes since they are easy to introgress into susceptible backgrounds through simple backcrossing techniques (Kelly and Miklas, 1998). However, single gene resistance has often proven ephemeral and highly vulnerable to dynamic and diverse plant pathogen populations. Therefore, breeders are endeavoring to shift to breeding durable forms of resistance by pyramiding race-specifc genes into a single cultivar. Conventional gene pyramiding requires extensive disease screen- ings with several races of the pathogen due to the race speci fcity of many of these genes after each cycle of crossing. Further com- plicating a pyramiding efort is the frequent absence of an efective selection method due to a lack of di ferentiating races. Testcross- Pyramiding of Soybean Mosaic Virus Resistance Genes by Marker-Assisted Selection M. A. Saghai Maroof,* S. C. Jeong, I. Gunduz, D. M. Tucker, G. R. Buss, and S. A. Tolin ABSTRACT Soybean mosaic virus (SMV) causes a disease of soybean [Glycine max (L.) Merr.] that is preva- lent throughout the United States. The disease can be effectively managed through the deploy- ment of single-dominant resistance genes known as Rsv genes that confer resistance to different strains of SMV. Pyramiding respective Rsv genes from different loci (Rsv1, Rsv3, and Rsv4) through marker-assisted selection (MAS) is an ideal method for creating durable and wide spectrum resistance to all strains of SMV. In this study, simple sequence repeat markers were used to create isogenic lines of the susceptible cultivar Essex containing one, two, or three Rsv loci for observing background and epistatic effects of Rsv1, Rsv3, and Rsv4 on inoculation with six strains of SMV. Results indicate that an Essex background or modifer genes from the donor source had effects on reactions of Rsv3 and Rsv4 genes, causing the isogenic lines to be more susceptible than the Rsv donor parents. Two-gene and three-gene isolines of Rsv1Rsv3, Rsv1Rsv4 and Rsv1Rsv3Rsv4, acted in a comple- mentary manner, conferring resistance against all strains of SMV, whereas isolines of Rsv3Rsv4 displayed a late susceptible reaction to selected SMV strains. We demonstrate with MAS and three near-isogenic lines, each containing a dif- ferent SMV-resistance gene, that pyramided lines can be generated in a straightforward man- ner into two- or three-gene–containing lines with high levels of resistance to SMV. M.A. Saghai Maroof, D.M. Tucker, and G.R. Buss, Dep. of Crop & Soil Environmental Sciences, Virginia Tech, Blacksburg, VA 20461; S.C. Jeong, BioEvaluation Center, Korea Research Institute of Bioscience and Biotechnology, Chungbuk 363-883, Republic of Korea; I. Gun- duz, Phillip Morris USA, Research Center, Richmond, VA 23261; S.A. Tolin, Dep. of Plant Pathology, Physiology & Weed Science, Virginia Tech, Blacksburg, VA 24061. Received 19 Oct. 2007. *Corresponding author (smaroof@vt.edu). Abbreviations: BYDV, barley yellow dwarf virus; dpi, days postinoc- ulation; ELISA, enzyme linked immunosorbent assay; LS, late suscep- tible; MAS, marker-assisted selection, MLG, molecular linkage group; NIL, near-isogenic line; RFLP, restriction fragment length polymor- phism; SMV, soybean mosaic virus; SSR, simple sequence repeat. Published in Crop Sci. 48:517–526 (2008). doi: 10.2135/cropsci2007.08.0479 © Crop Science Society of America 677 S. Segoe 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.