QTL analysis for capsaicinoid content in Capsicum.

Pungency or "heat" found in Capsicum fruit results from the biosynthesis and accumulation of alkaloid compounds known as capsaicinoids in the dissepiment, placental tissue adjacent to the seeds. Pepper cultivars differ with respect to their level of pungency because of quantitative and qualitative variation in capsaicinoid content. We analyzed the segregation of three capsaicinoids: capsaicin, dihydrocapsaicin and nordihydrocapsaicin in an inter-specific cross between a mildly pungent Capsicum annuum 'NuMex RNaky' and the wild, highly pungent C. frutescens accession BG2814-6. F(3) families were analyzed in three trials in California and in Israel and a dense molecular map was constructed comprised mostly of loci defined by simple sequence repeat (SSR) markers. Six QTL controlling capsaicinoid content were detected on three chromosomes. One gene from the capsaicinoid biosynthetic pathway, BCAT, and one random fruit EST, 3A2, co-localized with QTL detected in this study on chromosomes 3 and 4. Because one confounding factor in quantitative determination of capsaicinoid is fruit size, fruit weight measurements were taken in two trials. Two QTL controlling fruit weight were detected, however, they did not co-localize with QTL detected for capsaicinoid content. The major contribution to the phenotypic variation of capsaicinoid content (24-42% of the total variation) was attributed to a digenic interaction between a main-effect QTL, cap7.1, and a marker located on chromosome 2 that did not have a main effect on the trait. A second QTL, cap7.2 is likely to correspond to the QTL, cap, identified in a previous study as having pronounced influence on capsaicinoid content.

fs3.1: a major fruit shape QTL conserved in Capsicum.

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.