Table 2.

Summary of QTL studies conducted in lettuce, organised by trait category.

Trait(s)	Description	Population(s)	Reference(s)
Abiotic stress
Various	Applied nutrient, drought, salt and competition stress and identified QTL for above ground dry weight	L. sativa cv. Salinas and L. serriola acc. US96UC23 RIL population	Hartman et al., 2014
Drought stress	QTL for root architecture and water acquisition after mild drought stress	L. sativa cv. Salinas and L. serriola acc. US96UC23 F2 population	Johnson et al., 2000
Resource utilisation	Identified significant marker-trait associations for water and nitrate capture below-ground and above-ground accumulation	Collection of cultivars for association analysis	Kerbiriou et al., 2016
Tipburn	QTL for tip burn incidence and severity and other temperature stress-related physiological disorders	L. sativa cvs. “Emperor” x “El Dorado” RIL population	Jenni et al., 2013
	QTL for tipburn severity mapped alongside head lettuce morphological traits	Seven RIL populations, including L. sativa cv. Salinas and L. serriola acc. US96UC23 RIL population and crosses between several crisphead, butterhead, batavia and leaf lettuce types	González et al., 2019
Biotic stress
Downy mildew	QTL for downy mildew resistance	L. saligna (CGN 5271) x L. sativa cv. “Olof” F2 population	Jeuken and Lindhout, 2002
	QTL for downy mildew resistance and QTL fine-mapping reported	Backcross inbred lines (BC4) developed by crossing lines of an L. saligna (CGN 5271) x L. sativa cv. “Olof” F2 population to the L. sativa parent	Jeuken et al., 2008; Zhang et al., 2009a, 2009b den Boer et al., 2013
	QTL for downy mildew resistance	L. sativa cvs. “Iceberg” x “Grand Rapids” RIL population	Simko et al., 2013
	QTL for downy mildew resistance	L. sativa cv. Salinas and L. serriola acc. US96UC23 RIL population	Simko et al., 2014
	QTL for downy mildew resistance	L. sativa cvs. “Salinas 88” x “La Brillante” RIL population	Simko et al., 2015
Lettuce dieback	QTL for lettuce dieback resistance	L. sativa cvs. “Salinas” x “Iceberg” RIL population	Grube et al., 2005
	QTL for lettuce dieback resistance	L. sativa cvs. “Salinas” x “Iceberg” RIL population for linkage mapping and a collection of cultivars for association analysis	Simko et al., 2009
Verticillium dahliae	Identified a QTL for Verticillium dahliae resistance	L. sativa cvs. “Salinas 88” x “La Brillante” RIL population	Hayes et al., 2011
Lettuce drop	QTL for lettuce drop resistance	L. sativa cvs. “Eruption” x “Reine des Glaces” RIL population	Mamo et al., 2019
Bacterial leaf spot	Significant marker-trait associations for bacterial leaf spot resistance	Diversity panel of cultivars representing the major leaf types	Lu et al., 2014
	QTL for bacterial leaf spot resistance	L. sativa cvs. “Eruption” x “Reine des Glaces” and “Delsay” x “Reine des Glaces” RIL populations	Sandoya et al., 2019
Root rot	QTL for race 2 major resistance locus	L. sativa cvs. “VP1013” x “Patriot” F2 population	Aruga et al., 2012
Lettuce aphid	Identified significant marker-trait associations for aphid resistance	Diverse panel of wild and cultivated lettuce accessions, representing all major leaf types and of worldwide origin	Walley et al., 2017
Post-harvest
Shelf life	Identification of multiple QTL for shelf life and association physiological, biochemical and biophysical traits, trait data collected in two field trials in two geographic locations	L. sativa cv. Salinas and L. serriola acc. US96UC23 RIL population	Zhang et al., 2007
	Identification of three QTL for shelf life from five field trials	L. sativa cvs. “Salinas 88” x “La Brillante” RIL population	Hayes et al., 2014
	Significant marker-trait associations identified for shelf life and developmental rate	Diversity panel of cultivars, advanced breeding lines, individuals from RIL populations and plant introductions	Sthapit Kandel et al., 2020
Discolouration	Identified six QTL for discolouration traits in fresh-cut lettuce in two geographic locations	L. sativa cvs. “Saladin” x “Iceberg” RIL population	Atkinson et al., 2013
Nutrition	QTL for antioxidant and chlorophyll content detected at two different stages of maturity and several production cycles	L. sativa cvs. “Diplomat” x “Margarita” RIL population	Hayashi et al., 2012
	Multiple QTL for antioxidant content, total phenolics and individual secondary metabolites, chlorophyll and carotenoids measured in two trials	L. sativa cv. Salinas and L. serriola acc. US96UC23 RIL population	Damerum et al., 2015
Seed traits
Seed germination/ longevity	Identified QTL for seed germination under different temperature and light conditions	L. sativa cvs. “Diplomat” x “Margarita” RIL population	Hayashi et al., 2008
	QTL for germination under high and low temperature and hormone treatment, seedling quality	L. sativa cv. Salinas and L. serriola acc. US96UC23 RIL population	Argyris et al., 2005, 2008
	QTL for seed longevity following different storage conditions and after seed priming	L. sativa cv. Salinas and L. serriola acc. US96UC23 RIL population	Schwember and Bradford, 2010a
	Identified nine significant marker-trait associations for horticultural traits such as leaf undulation, leaf and stem anthocyanin content and seed colour	Diversity panel of homozygous accessions collected worldwide	Kwon et al., 2013a
General horticultural/ domestication traits	QTL detected for 37 fitness-related traits including plant morphology, flowering time, seed output and numerous other domestication traits	L. sativa cv. Salinas and L. serriola acc. US96UC23 RIL population	Hartman et al., 2013
	QTL for plant height, fresh and dry weight and relative moisture content identified	A first and second generation of backcrosses, with L. serriola backcrossed to L. sativa cv. Dynamite	Uwimana et al., 2012
	QTL for 10 fitness-related traits including germination, biomass, flowering time and seed output	L. sativa cv. Salinas and L. serriola acc. US96UC23 RIL population	Hartman et al., 2012
	QTL for leaf morphology and rubber content	L. serriola F2 population	Bell et al., 2015