Abstract
Molecular markers allowed breeders to mendelize quantitative trait loci (QTL) providing another demonstration that quantitative traits are governed by the same principles as single qualitative genes. This research extends the QTL analysis to two and three QTL and tests our ability to mendelize an oligogenic trait. In tomato, agricultural yield is determined by the weight of the fruits harvested per unit area and the total soluble solids (% Brix)–sugars and acids. The current study explores the segregation of multiple independent yield-related QTL that were identified and mapped using introgression lines (IL) of Solanum pennellii in cultivated processing tomato (S. lycopersicum). We screened 45 different double and triple IL-QTL combinations for agricultural yield, to identify QTL pyramids that behaved in an additive manner and were suitable substrate for mendelizing an oligogenic trait. A pyramid of three independent QTL that significantly improved Brix∗Yield (BXY - the soluble solids output per unit area) compared to M82 was selected. In the progenies of the tri-hybrid we bred using markers a nearly isogenic ‘immortalized F2.’ While the common mode of QTL–QTL interactions across the 45 IL-QTLs combinations was less than additive, the three QTLs in the selected triple-stack performed in an additive manner which made it an exceptional material for breeding. This study demonstrates that using the phenotypic effect of all 27 possible QTL-alleles combinations it is possible to make reliable predictions about the genotypes that will maximize the yield.
Highlights
Yield is a key trait for commercially grown crop plants
By comparing the yield of the independent ILHs with the values of the double ILHs, we were able to determine whether there is a deviation from additivity indicative of epistasis
The S. pennellii introgression lines (IL) population (Eshed and Zamir, 1995) was exploited to address two components related to tomato genetics and breeding: (1) introduction of unused favorable trait alleles that were neglected during tomato domestication and (2) creation of an effective framework for detection and mapping of quantitative trait loci (QTL) for complex traits, such as yield
Summary
The challenge in breeding and genetic analysis of yield is posed by the biological complexity of this trait, since yield reflects the cumulative effects of multiple factors over time and across plant organs and field environments. While the interaction of quantitative trait loci (QTL) with environmental conditions can be controlled in genetic studies, epistasis which is a major force shaping the phenotype remains a difficult component to quantitate (Carlborg and Haley, 2004; Mackay, 2014). In the case of quantitative traits, epistasis refers to the deviation of a phenotype from its predicted value based on additivity between the effects of the underlying independent QTL. The role and importance of epistasis in the genetic architecture of quantitative traits remains controversial (Carlborg and Haley, 2004; Hill et al, 2008)
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