Solanum sect. Lycopersicon

In this review, we examine the plant group Solanum sect. Lycopersicon - a clade of 13 species, including the domesticated tomato (Solanum lycopersicum L.) and its wild relatives - along with four allied species in the immediate outgroups Solanum sects. Juglandifolia and Lycopersicoides. We summarize the geographic distribution and morphological characters of these plant groups, describing their evolutionary relationships in the context of a new taxonomic revision at the species level of all these groups. We provide an overview of the role that wild tomato species have played in the development of cytogenetic stocks, in classical and molecular genetic studies as well as in crop improvement through traditional and advanced tools. We discuss how the very narrow genetic basis of cultivated tomato germplasm has forced tomato geneticists and breeders to rely on the wealth of genetic variation present in the wild relatives to address the many breeding challenges. The numerous molecular mapping studies conducted using interspecific crosses have clearly demonstrated that the breeding value of exotic (wild) tomato germplasm goes far beyond its phenotype. These studies also show that we are still far from being able to fully exploit the breeding potential of the thousands of accessions stored in seed banks around the world, in addition to those that may still be found in natural habitats. Over the past decades, tomato breeders have been at the forefront of establishing new principles for crop breeding based on the use of wild species to improve modern cultivars. In this respect, among all model systems, the wild and domesticated species of the tomato clade have pioneered development of novel populations such as "exotic libraries." These genetic resources, combined with the increasing knowledge deriving from the many "omics" tools, including the tomato genome sequence, are expected to further improve the efficiency with which wild tomato relatives will contribute to the improvement of this important crop.