**Genetic Diversity in Tomato (***Solanum lycopersicum***) and Its Wild Relatives**

Guillaume Bauchet and Mathilde Causse

*Institut National de la Recherche Agronomique (INRA), Unité de Génétique et Amélioration des Fruits et Légumes (GAFL), France* 

#### **1. Introduction**

Tomato, ranking 1st in the world for vegetables, accounts for 14% of world vegetable production (over 100 million metric tons/year \$ 1.6 billion market; (Food and Agriculture Organisation [FAO] 2010). Tomato is a rich source of micronutrients for human diet. It is also an acknowledged model species for research on fruit development and metabolite accumulation. The major goals of tomato breeders (higher productivity, better tolerance to biotic and abiotic stresses and increased sensory and health value of the fruit) require a good comprehension and management of tomato genetic resources diversity.

Due to its Latin American origin and related domestication history, cultivated tomato has faced several bottlenecks over ages. This led to a drastic reduction of its genetic diversity. Explorations of tomato center of origin permitted major advances in the characterization of its diversity. In parallel, *ex situ* plant conservation initiatives bloomed, ensuring the collection and conservation of landraces and wild species through development of seed banks. Thus, unraveling the genetic potential of tomato's wild relatives for breeding purpose emerged. In parallel, the ecological and taxonomic diversity of tomato turned it into a model species for evolutionary studies. Since the mid-20th century, new methods such as controlled hybridization allowed crossing between wild and cultivated tomato. Modern genetics and breeding methods contributed to understand the genetic control of agronomical traits but also accentuated the progress. If successful, the accuracy to introgress agronomic traits of interest from wild relatives into cultivated tomato was not always straightforward. This was notably due to inherent linkage between "favorable" and "unfavorable" effects of introgressed fragments.

The advent of molecular biology in the 80's raised great hopes in terms of characterization of the genetic diversity present in both wild and cultivated compartments. Also, great expectations emerged since the development of molecular techniques to "pinpoint" genomic regions involved in targeted traits. Dissection of the genetic control of complex traits, using ad hoc techniques from quantitative genetics, was possible, leading to the identification of key alleles involved in diverse agronomic traits, originating from several wild relatives.

Today the tomato genome is fully sequenced. A new step in the knowledge on tomato diversity with the so called "-omics" and next generation sequencing techniques is coming. These technologies and related data analysis allow a complete and combined reading of genomes and related levels of expression (transcriptome, proteome, metabolome) in a high throughput way. Among the new approaches, QTL mapping techniques in natural populations or genome wide association studies will facilitate the genetic characterization of complex traits and germplasm management of both wild and cultivated tomatoes.

In this chapter we will first show how tomato diversity evolved from its early domestication until today. We will discuss how valuable tomato genetic resources are, and that investigating natural variation not only highlights existing diversity -which is of critical use for cultivated tomato improvement- but can also provide insights into the evolution and genetic bases of complex traits. In the last part, we will present how molecular markers have completed our view.
