An advanced backcross QTL study was performed in pepper using a cross between the cultivated species Capsicum annuum cv. Maor and the wild C. frutescens BG 2816 accession. A genetic map from this cross was constructed, based on 248 BC(2) plants and 92 restriction fragment length polymorphism (RFLP) markers distributed throughout the genome. Ten yield-related traits were analyzed in the BC(2) and BC(2)S(1) generations, and a total of 58 quantitative trait loci (QTLs) were detected; the number of QTLs per trait ranged from two to ten. Most of the QTLs were found in 11 clusters, in which similar QTL positions were identified for multiple traits. Unlike the high percentage of favorable QTL alleles discovered in wild species of tomato and rice, only a few such QTL alleles were detected in BG 2816. For six QTLs (10%), alleles with effects opposite to those expected from the phenotype were detected in the wild species. The use of common RFLP markers in the pepper and tomato maps enabled possible orthologous QTLs in the two species to be determined. The degree of putative QTL orthology for the two main fruit morphology traits-weight and shape-varied considerably. While all eight QTLs identified for fruit weight in this study could be orthologous to tomato fruit weight QTLs, only one out of six fruit shape QTLs found in this study could be orthologous to tomato fruit shape QTLs.

fs3.1 is a major fruit shape (defined as the ratio of fruit length to fruit width) quantitative trait locus (QTL) originally detected in an intraspecific cross of Capsicum annuum between the blocky and elongated-fruited inbreds 'Maor' and 'Perennial', respectively. In addition to increasing fruit shape index, the 'Perennial' allele at fs3.1 increased fruit elongation and decreased fruit width and pericarp thickness. We verified the effect of fs3.1 in backcross inbred lines (BILs) derived from crossing 'Perennial' with 'Maor' and with a second blocky-type inbred line of C. annuum. To determine the effect of the fs3.1 region in additional Capsicum species, we constructed an advanced backcross population from the cross of 'Maor' and the oval-fruited Capsicum frutescens BG 2816 and an F2 of the introgression line IL 152 that contains an introgression of the fs3.1 region from Capsicum chinense PI 152225. QTLs for fruit shape, fruit width, and pericarp thickness, but not for fruit length, were detected in both crosses, indicating the conservation of the fs3.1 region as a QTL affecting fruit shape in pepper. We also tested tomato (Lycopersicon spp.) introgression lines containing the corresponding fs3.1 region from L. pennellii and L. hirsutum, but we did not detect a significant fruit shape QTL in these lines. The effect of fs3.1 on the growth of fruit dimensions varied with the genetic background. By measuring the length and width of ovaries and fruits of near-isogenic C. annuum lines that differ in fs3.1 during fruit development, we determined that fs3.1 controls shape predominantly by increasing the growth rate of the longitudinal axis in the first 2 weeks after pollination. However, in the crosses of C. annuum with C. frutescens and C. chinense, fs3.1 predominantly exerted its effect on the width dimension.

Fruit size and shape are among the most important traits that were under selection during domestication of the fruit-bearing Solanaceae crops, tomato and pepper. To determine the level of conservation of the locations of quantitative trait locus (QTL) controlling these traits in the two species, we conducted whole genome comparative QTL analysis for fruit weight, pericarp thickness, and fruit shape. The pepper QTLs data were obtained from previous studies using two mapping populations from crosses of the same blocky-type parent, cv. Maor (Capsicum annuum), with small-fruited accessions of C. annuum and C. frutescens. The tomato QTLs data were derived from the analysis of two populations containing introgressions from Solanum pennellii and S. habrochaites in the background of the cultivated tomato S. lycopersicum. A total of 95 QTLs in 34 genomic regions were detected in the four populations. Most of the QTLs (52%, 64%, and 89% for fruit weight, pericarp thickness, and fruit shape, respectively) were population-specific. The degree of conservation of the QTL locations in pepper and tomato varied among the traits; eight QTLs for fruit weight, five QTLs for pericarp thickness, and only one QTL for fruit shape were identified in common genomic regions in the two genera. Only two QTLs for fruit weight were found to be common in all four populations. The difference in the degree of QTL conservation in pepper and tomato between fruit weight and fruit shape indicates that while the genetic control of fruit weight is in part similar in pepper and tomato and convergent domestication in these crops resulted from mutations and selection at common loci, selection for fruit shape occurred for the most part at independent loci in these two genera.

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