Perspectives of molecular marker assisted breeding for earliness in tomato

Abstract

In a tomato (Lycopersicon esculentum) breeding programme aimed at earliness, a breeding line, designated ‘IVT-KT₁’, was developed by several rounds of crossing and selection. Among its ancestors were the two wild relatives L. pimpinellifolium and L. parviflorum. The breeding line flowered and set fruit one to four weeks earlier than old and modern cultivars. To identify QTLs for earliness, an F₂ population was obtained by crossing ‘IVT-KT₁’ with the ‘late’ true breeding cultivar ‘Premier’. In winter and early spring, 690 plants of this F₂ population were evaluated for earliness of which 292 were selected for RFLP analysis. Only limited parts of the genomes of ‘IVT-KT₁’ and ‘Premier’ were polymorphic and these polymorphisms were likely due to introgression from the ancestor wild species. By using the interval mapping method on the combined data of the greenhouse evaluations and RFLP analyses, three loci were identified that were associated with earliness. One was mainly associated with flowering time, another with fruitset time and a third one with ripening time. Two of these loci were also associated with fruit weight. F₃ progenies selected for earliness on the basis of the QTL-data did confirm the F₂ results. Two major earliness genes explained a difference in earliness of more than three weeks. The association with reduced fruit weight seemed to be due to pleiotropic effects rather than to linked genes. Consequently, the gain in earliness will cause a reduction in fruit weight that may not be acceptable for breeding applications.