LESS-THAN-ADDITIVE EPISTATIC INTERACTIONS OF QUANTITATIVE TRAIT LOCI IN TOMATO

Epistasis plays a role in determining the phenotype, yet quantitative trait loci (QTL) mapping has uncovered little evidence for it. To addr ess this apparent contradiction, we analyzed interactions between indi vidual Lycopersicon pennellii chromosome segments introgressed into an otherwise homogeneous genetic background of L. esculentum (cv. M82). Ten different homozygous introgression lines, each containing from 4 t o 58 cM of introgressed DNA, were crossed in a half diallele scheme. T he 45 derived double heterozygotes were evaluated in the field for fou r yield-associated traits, along with the 10 single heterozygotes and M82. Of 180 (45 x 4) tested interactions, 28% were epistatic (P < 0.05 ) on both linear and geometric scales. The detected epistasis was pred ominately less-than-additive, i.e., the effect of the double heterozyg otes was smaller than the sum of the effects of the corresponding sing le heterozygotes. Epistasis was also found for homozygous linked QTL a ffecting fruit mass and total soluble solids. Although the frequency o f epistasis was high, additivity was the major component in the intera ction of pairs of QTL. We propose that the diminishing additivity of Q TL effects is amplified when more loci are involved; this mode of epis tasis may be an important factor in phenotype canalization and in bree ding.