The genetic, developmental, and molecular bases of fruit size and shape variation in tomato.

Fruit, corresponding to the plant’s ovary (or ovaries), protect seed development and serve as the vehicle for seed dispersal to different habitats for species propagation. Fruit also provide humans with a source of nutrition, culinary diversity, and often great pleasure. Humans consume fruit from a wide range of plants, including members of the Rutaceae (citrus), Rosaceae (stone fruits, apples, pears), Solanaceae (eggplant, pepper, tomato, husk tomato), Cucurbitaceae (melons, squash, cucumbers), Ribaceae (berries), Vitaceae (grapes), and a multitude of additional species. Although fruit-bearing crop species are taxonomically diverse, they share a common feature. Fruit from domesticated species often have been tremendously enlarged over that normally found in the progenitor wild species. For example, the putative wild ancestor of the cultivated tomato, Lycopersicon esculentum cv Cerasiforme, bears fruit (composed of two locules) weighing only a few grams. By contrast, a single fruit from a modern tomato variety may contain many locules and weigh up to 1 kg, a nearly 1000-fold increase in weight (Figure 1A). A similar situation exists for most other domesticated fruit species (Smartt and Simmonds, 1995). In addition to increases in fruit size, the domestication of fruit-bearing species often has resulted in tremendous shape variation: wild and semiwild forms of tomato bear fruit that are almost invariably round, whereas cultivated tomatoes come in a wide variety of shapes: round, oblate, pearshaped, torpedo-shaped, and bell pepper–shaped (Figure 1). The topic of fruit growth and development can be approached with several questions in mind. From a global perspective, one can ask, What are the genes, proteins, and processes that specify or affect the formation of fruit? Of course, the answer to this question is that many if not most plant genes/proteins/ processes influence fruit development. Plants compromised in photosynthesis, phloem transport, floral initiation/development, or male or female fertility either cannot produce fruit or are abnormal in their fruit production (e.g., parthenocarpic fruit, reduced fruit size, or reduced fruit load). As a result, many if not most tomato mutants have some alteration in their fruit phenotypes (Balbi and Lomax, 2003). Another question regarding fruit growth and development is, What are the underlying genetic, molecular, and developmental changes that permitted wild progenitors to produce the large, highly variable edible fruit associated with modern agriculture? Because this question is more narrowly defined, it is more likely to yield specific answers concerning processes unique to fruit development and relevant to agriculture. Thus, this review is focused on the question of how tomato fruit were changed by domestication. In approaching this topic, I try to summarize what is known about the alteration of fruit development during tomato domestication and to set forth a series of questions that need to be answered before we can claim any general understanding of this topic.