Analysis of Quantitative Traits in Pepper Using Molecular Markers

Original objectives: The overall goal of the proposal was to determine the genetic and molecular control of pathways leading to the production of secondary metabolites determining major fruit quality traits in pepper. The specific objectives were to: (1) Generate a molecular map of pepper based on simple sequence repeat (SSR) markers. (2) Map QTL for capsaicinoids content (3) Determine possible association between capsaicinoids and carotenoid content and structural genes for capsaicinoid and carotenoid biosynthesis. (4) Map QTL for quantitative traits controlling additional fruit traits. (5) Map fruit-specific ESTs and determine possible association with fruit QTL (6) Map the C locus that determines the presence and absence of capsaicinoids in pepper fruit and identify candidate genes for C. Background: Pungency, color, fruit shape and fruit size are among the most important fruit quality characteristics of pepper. Despite the importance of the pepper crop both in the USA and Israel, the genetic basis of these traits was only little known prior to the studies conducted in the present proposal. In addition, molecular tools for use in pepper improvement were lacking. Major conclusions and achievements: Our studies enabled the development of a saturated genetic map of pepper that includes numerous simple sequence repeat (SSR) markers and the integration of several independent maps into a single resource map that consists of over 2000 markers. Unlike previous maps that consisted mostly of tomato-originated RFLP markers, the SSR-based map consists of largely pepper markers. Therefore, the SSR and integrated maps provide ample of tools for use in marker-assisted selection for diverse targets throughout the Capsicum genome. We determined the genetic and molecular bases of qualitative and quantitative variation of pungency, the most unique characteristics of pepper fruit. We mapped and subsequently cloned the Pun1 gene that serves as a master key for capsaicinoids accumulation and showed that it is an acyltransferase. By sequencing the Pun1 gene in pungent and non-pungent cultivars we identified a deletion that abolishes the expression of the gene in the latter cultivars. We also identified QTLs that control capsaicinoids content and therefore pungency level. These genes will allow pepper breeders to manipulate the level of pungency for specific agricultural and industrial purposes. In addition to pungency we identified genes and QTLs that control other key developmental processes of fruit development such as color, texture and fruit shape. The A gene controlling anthocyanin accumulation in the immature fruit was found as the ortholog of the petunia transcription factor Anthocyanin2. The S gene required for the soft flesh and deciduous fruit nature typical of wild peppers was identified as the ortholog of tomato polygalacturonase. We identified two major QTLs controlling fruit shape, fs3.1 and fs10.1, that differentiate between elongated and blocky and round fruit shapes, respectively. Scientific and agricultural implications: Our studies allowed significant advancement of our understanding at the genetic and molecular levels of important processes of pepper fruit development. Concomitantly to gaining biological knowledge, we were able to develop molecular tools that can be implemented for pepper improvement.