Abstract
Key messageEleven new major resistance genes for lettuce downy mildew were introgressed from wild Lactuca species and mapped to small regions in the lettuce genome.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. F2 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.
Highlights
Downy mildew, caused by the oomycete Bremia lactucae, is the most important disease in lettuce (Lactuca sativa L.) worldwide
Salinas that was used as a recurrent parent for the generation of the near-isogenic lines (NILs) was sequenced to assemble the reference genome (Reyes-Chin-Wo et al 2017)
This facilitated the identification of introgressed regions in the NILs by visualizing chromosomal regions with high densities of Single nucleotide polymorphisms (SNPs) when comparing reads of the NILs aligned against the reference genome
Summary
Downy mildew, caused by the oomycete Bremia lactucae, is the most important disease in lettuce (Lactuca sativa L.) worldwide. 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). 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). The knowledge of the position of these new genes for resistance to lettuce downy mildew will facilitate marker-assisted selection to pyramid multiple Dm genes into cultivars, which will increase the evolutionary hurdle for the pathogen to become virulent and provide more durable disease resistance
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