Identification of Major Quantitative Trait Loci Controlling Field Resistance to Downy Mildew in Cultivated Lettuce (Lactuca sativa)

Abstract

Lettuce downy mildew caused by Bremia lactucae Regel is the most economically important foliar disease of lettuce (Lactuca sativa L.). The deployment of resistant cultivars carrying dominant resistance genes (Dm genes) plays a crucial role in integrated downy mildew disease management; however, high variability in pathogen populations leads to the defeat of plant resistance conferred by Dm genes. Some lettuce cultivars exhibit field resistance that is only manifested in adult plants. Two populations of recombinant inbred lines (RILs), originating from crosses between the field resistant cultivars Grand Rapids and Iceberg and susceptible cultivars Salinas and PI491224, were evaluated for downy mildew resistance under field conditions. One hundred and sixty RILs from the Iceberg × PI491224 and 88 RILs from the Grand Rapids × Salinas RIL populations were genotyped using genotyping by sequencing, which respectively generated 906 and 746 high quality markers that were used for quantitative trait locus (QTL) analysis. We found a QTL in Chromosome 4 that is present in both Grand Rapids × Salinas and Iceberg × PI491224 populations that has a major effect on field resistance. We also found two additional significant QTLs in Chromosomes 2 and 5 in the Iceberg × PI491224 RIL population. Marker-assisted gene pyramiding of multiple Dm genes in combination with QTLs for field resistance provides the opportunity to develop cultivars with more durable resistance to B. lactucae.

Genetic and physiological determinants of lettuce partial resistance to Impatiens necrotic spot virus

“…There was only a limited difference in the respective parental accessions in their rate of development and anthocyanin content. However, both populations segregated for the content of chlorophylls, the trait linked to qLG4.1 (Simko et al, 2016;Parra et al, 2021) that was also detected in the current study (Table 3, Figure 4). Three QTLs linked with INSV incidence were mapped on chr.…”

“…2, 3, 4 (three QTLs), 5 (two QTLs), 7 (two QTLs), 8, and 9 (two QTLs). One of the QTLs for the content of chlorophyll ( qLG4.1 ) was previously detected in multiple mapping populations ( Kwon et al., 2013 ; Simko et al., 2016 ; Mamo et al., 2019 ; Parra et al., 2021 , Simko et al., unpublished results), while the other three QTLs appeared to be located at the genomic regions not previously associated with the content of chlorophyll. Out of 12 QTLs for the content of anthocyanins, 7 ( qRLC2.1 , RLL3 , RLL2 , qRLC5.2 , qRLC7.2 , RLL4 , and ANS ) were found at or near QTLs and genes for red leaf color or anthocyanin content detected in previous studies ( Kwon et al., 2013 ; Mamo et al., 2019 ; Gurdon et al., 2021 ; Simko et al., 2022b , Simko et al., unpublished results, Zhang et al., 2017 ; Su et al., 2020 ; Wei et al., 2021 ; Wada et al., 2022 ), while the remaining five QTLs were new.…”

“…Iceberg and PI 491224 (Ice × PI). Both populations were previously used to identify QTLs for resistance to downy mildew ( Parra et al., 2021 ). All three mapping populations were genotyped with single-nucleotide polymorphism (SNP) markers anchored to the cv.…”

Genomics and Marker-Assisted Improvement of Vegetable Crops

“…A large number of loci for resistance to DM have been mapped for both race-specific and quantitative resistance phenotypes. Although the resistance loci have been located on all nine LGs except LG 6 (Paran et al, 1991;Paran and Michelmore, 1993;Maisonneuve et al, 1994;Jeuken and Lindhout, 2002;Jeuken et al, 2008;McHale et al, 2009;Zhang et al, 2009a;2009b;Simko et al, 2013;2015a;Parra et al, 2021), more than half of the loci were in major resistance clusters on LGs 1, 2 and 4 (Simko, 2013;Parra et al, 2016). These major resistance gene clusters span several Mb and frequently contain NBS-LRR proteins (Meyers et al, 1998a;Parra et al, 2016).…”