Identification and mapping of new genes for resistance to downy mildew in lettuce
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ORIGINAL ARTICLE
Identification and mapping of new genes for resistance to downy mildew in lettuce Lorena Parra1 · Kazuko Nortman1 · Anil Sah1 · Maria Jose Truco1 · Oswaldo Ochoa1 · Richard Michelmore1 Received: 31 July 2020 / Accepted: 16 October 2020 © The Author(s) 2020
Abstract Key message Eleven new major resistance genes for lettuce downy mildew were introgressed from wild Lactuca species and mapped to small regions in the lettuce genome. Abstract Downy mildew, caused by the oomycete pathogen Bremia lactucae Regel, is the most important disease of lettuce (Lactuca sativa L.). The most effective method to control this disease is by using resistant cultivars expressing dominant resistance genes (Dm genes). In order to counter changes in pathogen virulence, multiple resistance genes have been introgressed from wild species by repeated backcrosses to cultivated lettuce, resulting in numerous near-isogenic lines (NILs) only differing for small chromosome regions that are associated with resistance. Low-pass, whole genome sequencing of 11 NILs was used to identify the chromosome segments introgressed from the wild donor species. This located the candidate chromosomal positions for resistance genes as well as additional segments. F 2 segregating populations derived from these NILs were used to genetically map the resistance genes to one or two loci in the lettuce reference genome. Precise knowledge of the location of new Dm genes provides the foundation for marker-assisted selection to breed cultivars with multiple genes for resistance to downy mildew.
Introduction Downy mildew, caused by the oomycete Bremia lactucae, is the most important disease in lettuce (Lactuca sativa L.) worldwide. B. lactucae is a biotrophic pathogen that primarily infects the foliar tissue, reducing yield and decreasing the quality of the marketable portion of the crop. The use of cultivars carrying dominant resistant genes (Dm genes) is the most effective strategy to control this disease. However, B. lactucae rapidly evolves to new virulence phenotypes that defeat individual Dm genes. For this reason, lettuce breeding programs continually seek new sources of resistance to downy mildew and new resistance genes (Crute 1992; Lebeda et al. 2014; Parra et al. 2016). Communicated by Hong-Qing Ling. Electronic supplementary material The online version of this article (https://doi.org/10.1007/s00122-020-03711-z) contains supplementary material, which is available to authorized users. * Richard Michelmore [email protected] 1
The Genome Center, Department of Plant Sciences, University of California, Davis, CA 95616, USA
For over fifty years, breeding for resistance to B. lactucae has relied on the introgression of new genes from wild species (Crute 1992; Beharav et al. 2006; Parra et al. 2016). Of the 100 wild Lactuca species described thus far, only 14 are known to be natural hosts of B. lactucae (Lebeda et al. 2002). L. serriola and L. saligna have been used as sources of resistance to downy mildew in multiple breeding pr
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