Identification of QTLs for Liriomyza trifolii resistance and two agronomic traits in lettuce

Lettuce (Lactuca sativa L.) is an important salad crop around the world especially in the western countries. The serpentine leafminer (Liriomyza trifolii Burgess) is an economically important pest of lettuce in the US. Historically, lettuce growers in Florida have relied on insecticides to control the pest. However, L. trifolii developed resistance to frequently used insecticides within a few years. Host plant resistance (HPR) is an ecologically sound and economically viable strategy to control insect pests. The identification of Quantitative Trait Loci (QTLs) associated with L. trifolii resistance-related traits was the primary goal of this project. The study was conducted on a population of 125 F2 individuals from a cross between cv. Valmaine (resistant) and cv. Okeechobee (susceptible) romaine cultivars. The plants were grown in greenhouse conditions and were moved to the insectarium for testing. Progeny testing on F2 individuals exhibited continuous variation in damage scores among the F3 families. A total of 251 SNP, 86 AFLP, and 6 SSR markers were used to genotype the F2 individuals. QTL mapping revealed two major novel QTLs on LG1 and LG2 for damage score, which together explained over 85% of total phenotypic variation. It detected one major novel QTL on LG1 for survival of adult L. trifolii, which explained 39% of total phenotypic variation. The QTLs for both traits on LG1 were flanked by SNP188 and SNP187.;Further, an attempt was made to dissect the genetic architecture of two important developmental traits for lettuce growers; flowering time and plant height. The composite interval mapping (CIM) identified two novel QTLs for flowering time on LG2 at 34.0 cM and 120.0 cM. These two QTLs explained over 28% of total phenotypic variation. However, the multiple interval mapping detected only one QTL flanked by SNP246 and SNP247 on LG2 at 120.7 cM (LOD ∼5.4) that explained over 11% of total phenotypic variation. Plant height was measured for each F3 family at seed maturity stage. The MIM method detected two major novel QTLs on LG1 at 208.2 cM and LG5 at 52.8 cM, which together explained over 74% of total phenotypic variation.