CROP SCIENCE, VOL. 50, JANUARY– FEBRUARY 2010 59 RESEARCH L eaf rust, caused by Puccinia triticina Eriks., can cause yield losses up to 40% in susceptible wheat cultivars (Knott 1989) and is one of the most important diseases of wheat ( Triticum aestivum L.) worldwide (Kolmer, 1996). One of the most effective approaches for minimizing losses due to leaf rust is the use of resistant culti- vars. However, race-specific resistance is often a temporary solution because it can be overcome by a shift in the pathogen population. This has prompted a continuous search for new sources of resistance. More than 50 leaf rust resistance genes have been reported in wheat and its relatives. Many leaf rust resistance genes are derived from wheat wild relative Aegilops tauschii Coss., including Lr21 (located on wheat chromosome 1DS), Lr22a (2DS), Lr32 (3D), Lr39 (2DS), Lr41 (2DS), and Lr42 (1DS) (Rowland and Kerber, 1974; Gill et al., 1991; Kerber, 1987; Cox et al., 1994). It has been reported that recombination between the corresponding chromosomes of A. tauschii and the D genome of T. aestivum occurs at a level similar to that within the cultivated hexaploid species (Fritz et al ., 1995). This allows gene introgression from A. tauschii with minimal linkage drag. Lr42, a race-specific gene introgressed from A. tauschii, was located on wheat chromosome 1DS in an earlier genetic study (Cox et al., 1994). Germplasm lines containing Lr42 have been utilized by several U.S. and international breeding programs (Bacon et al., Molecular Mapping of Wheat Leaf Rust Resistance Gene Lr42 Xiaochun Sun, Guihua Bai,* Brett F. Carver, and Robert Bowden ABSTRACT Leaf rust, caused by Puccinia triticina Eriks., is an important foliar disease of wheat (Triticum aestivum L.) worldwide. Leaf rust resistance gene Lr42 from Aegilops tauschii Coss. has been used as a source of rust resistance in breeding programs. To identify molecular markers closely linked to Lr42, a segregating population of near- isogenic lines contrasting for the presence of Lr42 was developed in the hard winter wheat cul- tivar Century background and evaluated for rust infection type at both seedling and adult-plant stages. Simple sequence repeat (SSR) markers were screened using bulked-segregant analysis. Two markers closely linked to Lr42 were identi- fied on chromosome 1DS. The closest marker, Xwmc432, is about 0.8 cM from Lr42. Physical mapping of both SSR markers using Chinese Spring nullitetrasomic and ditelosomic genetic stocks confirmed that the markers linked to Lr42 were on 1DS. Markers for Lr42 were highly poly- morphic between parents and among a diverse set of wheat germplasm collected from several countries, indicating that these markers are use- ful for marker-assisted selection for Lr42. X. Sun, Dep. of Agronomy, Kansas State Univ., Manhattan KS 66506; G. Bai and R. Bowden, USDA-ARS, Hard Winter Wheat Genetics Research Unit, Manhattan, KS 66506; B.F. Carver, Dep. of Plant and Soil Sciences, Oklahoma State Univ., Stillwater, OK 74078. Received 27 Jan. 2009. *Corresponding author (guihua.bai@ars.usda.gov). Abbreviations: IT, infection type; MAS, marker-assisted selection; NIL, near-isogenic line; PCR, polymerase chain reaction; SSR, simple sequence repeat. Published in Crop Sci. 50:59–66 (2010). doi: 10.2135/cropsci2009.01.0049 © 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.