Rationalization of genes for resistance to Bremia lactucae in lettuce

Abstract

Lettuce downy mildew caused by Bremia lactucae is the most important disease of lettuce worldwide. Breeding for resistance to this disease is a major priority for most lettuce breeding programs. Many genes and factors for resistance to B. lactucae have been reported by multiple researchers over the past *50 years. Their nomenclature has not been coordinated, resulting in duplications and gaps in nominations. We have reviewed the available information and rationalized it into 51 resistance genes and factors and 15 quantitative trait loci along with supporting documentation as well as genetic and molecular information. This involved multiple rounds of consultation with many of the original authors. This paper provides the foundation for naming additional genes for resistance to B. lactucae in the future as well as for deploying genes to provide more durable resistance. Keywords

Nonhost wild lettuce as a donor for resistance to downy mildew in cultivated lettuce

“…Some L. saligna accessions also contain race-specific R genes (Parra et al 2016). However, R genes seem not essential for NHR, as the studies on the F2 and set of BILs from L. saligna CGN05271 indicated presence of quantitative resistance genes only (Fig.…”

“…So far, 28 downy mildew resistance (Dm) genes (R genes), 23 resistance (R) factors and 15 quantitative trait loci for resistance to downy mildew have been identified (Parra et al 2016). Resistance genes are denominated as Dm genes when they have been mapped to a single locus, whereas R factors have not (yet) been mapped.…”

“…Most of these resistances have been identified from L. sativa and L. serriola, but some originated from L. saligna and L. virosa. All 28 Dm genes that have been described are located in major resistance clusters, mainly on linkage group 1, 2 and 4 (Parra et al 2016). Until now, the sequence of only two of these Dm genes has been identified: Dm3 and Dm7, both in L. sativa (Shen et al 2002;Christopoulou et al 2015a).…”

“…In addition, a few L. saligna accessions are known to contain dominant monogenic race-specific R genes to B. lactucae (Parra et al, 2016). The race-specific effect of these R genes makes it unlikely that they are the main factor to explain NHR.…”

“…Since the 1970s more than 50 R genes have been deployed in lettuce breeding (Parra et al, 2016). Most of them originated from the primary genepool species L. sativa or L. serriola (Parra et al, 2016).…”