Genetics of drought tolerance during seed germination in tomato: Inheritance and QTL mapping

A BC₁ population (N = 1000) of an F₁ hybrid between a stress-sensitive Lycopersicon esculentum breeding line (NC84173; maternal and recurrent parent) and a germination stress-tolerant Lycopersicon pimpinellifolium accession (LA722) was evaluated for seed germination rate under drought stress (DS) (14% w/v polyethyleneglycol-8000, water potential approximately -680 kPa), and the most rapidly germinating seeds (first 3% to germinate) were selected. The 30 selected BC₁ seedlings were grown to maturity and self pollinated to produce BC₁S₁ progeny seeds. Twenty of the 30 selected BC₁S₁ progeny families were evaluated for germination rate under DS and their average performance was compared with that of a "nonselected" BC₁S₁ population of the same cross. Results indicated that selection for rapid germination under DS significantly improved progeny germination rate under DS (selection gain = 19.6%), suggesting a realized heritability of 0.47 for rate of germination under DS in this population. The 30 selected BC₁ plants were subjected to restriction fragment length polymorphism (RFLP) analysis, and marker allele frequencies for 119 RFLP markers which spanned 1153 cM of the 12 tomato chromosomes were determined. A distributional extreme marker analysis, which measures statistical differences in marker allele frequencies between a selected and a nonselected population, detected four quantitative trait loci (QTLs) for rate of germination under DS in this population. Of these, two QTLs, located on chromosomes 1 and 9, were contributed by the L. pimpinellifolium donor parent and had larger effects than the other two QTLs, located on chromosomes 8 and 12, which were contributed by the L. esculentum recurrent parent. A few BC₁S₁ families were identified with all or most of the identified QTLs and with germination rates comparable with that of LA722. These families should be useful for the development of germination drought-tolerant tomato lines using marker-assisted selection (MAS). The overall results indicate that drought tolerance during seed germination in tomato is genetically controlled and potentially could be improved by directional phenotypic selection or MAS.