High-resolution mapping and new marker development for adult plant stripe rust resistance QTL in the wheat cultivar Kariega

Three major quantitative trait loci (QTL) contribute to the durable adult plant stripe rust resistance in the high-quality bread wheat cultivar Kariega; QYr.sgi-2B.1 and QYr.sgi-4A.1, and the pleiotropic resistance gene Lr34/Yr18/Sr57. While marker-assisted selection is currently being used to incorporate the Kariega stripe rust adult plant resistance into new South African wheat breeding lines, effective selection of the large QTL intervals remains a challenging task. In this study, we describe the development of expressed sequence tag (EST)-derived markers as an effective strategy for increasing the marker density in the selected QTL intervals and the conversion of Diversity Arrays Technology (DArT) and EST-derived markers to sequence tagged site (STS) markers to enable high-throughput screening. To reduce the QTL intervals, a high-resolution mapping population was developed, consisting of 1,020 F-2 individuals, from the cross Kariega x Avocet S. Through recombinant selection, QYr.sgi-2B.1 was shown to reside within a 6.1 cM interval on the short arm of chromosome 2B, between marker loci Xbarc55 and Xwmc344, a reduction from the previous interval size of 23 cM. Previously, QYr.sgi-4A.1 was poorly defined, but by mapping selected recombinants for the QYr.sgi-4A.1 interval, the QTL region was narrowed to a 16.3-cM interval between marker loci Xbarc78, Xwmc313 and Xwmc219 on the long arm of chromosome 4A. An EST-STS marker, Xufs1-4A, was also developed for QYr.sgi-4A.1, which improved selection for this slow-rusting QTL. The EST marker strategy was useful in increasing marker density and contributed to improved selection for the Kariega QTL in resistance breeding. In addition, BC4F2 families were developed to study the expression of QYr.sgi-2B.1 and QYr.sgi-4A.1 individually, and in combination with Lr34/Yr18/Sr57 in the common, stripe rust susceptible background of Avocet S.