Fine mapping of a quantitative trait locus (QTL) from Lycopersicon hirsutum chromosome 1 affecting fruit characteristics and agronomic traits: breaking linkage among QTLs affecting different traits and dissection of heterosis for yield

The near-isogenic Line TA523, containing a 40-cM introgression at the botto m of chromosome 1 from Lycopersicon hirsutum acc. LA1777, affects several a gronomically important traits. A set of recombinant lines (subNILs) derived from the original NIL TA523 were developed in order to fine-map, by substi tution mapping, the genetic factors included within the original introgress ion. In the current experiment, TA523 showed redder, rounded, less pigmente d shoulder, lower-weighted fruits and higher brix, whereas higher yield and brix*yield was observed only in the hybrid TA253xTA209 suggesting heterosi s for these traits. By substitution mapping we mapped independent genetic l oci affecting brix, yield and fruit shape, whereas fruit weight, shoulder p igmentation and external color mapped to a position coincident with the bri x locus. Analysis of the subNILs revealed that the gene action of most of t he QTLs was additive or nearly additive. The exception was for the yield QT L which was dominant (d/a=0.7), eliminating the possibility that yield incr ease is due to true overdominance at a single gene locus. However, no negat ive yield effects were detected in other regions of the introgressed segmen t, as would be predicted by a dominance complementation model. There fore, epistatic interactions among genetic factors along the introgressed segment are suggested as the cause of yield heterosis. Results from this study, co mbined with previous experiments involving different tomato wild species, d emonstrate that the base of chromosome 1 of tomato contains multiple QTLs a ffecting various agronomic and fruit traits and that these effects can not be attributed to the pleiotropic effects of a single locus.