ORDER: Scrophulariales
Suborder: Solanineae
FAMILY:Solanaceae
TRIBE:Solaneae
GENUS:Lycopersicon
Subgenus: Eulycopersicon
Species:Lycopersicon esculentum
The tomato and its close relatives are believed to have originated in the mountainous regions of the Andes (Peru, Ecuador and Chile). Other distinct relatives are found among the flora of the Galapagos Islands. These relatives can be found in many environments, from high, moist elevations in the mountains to dry coastal regions and variations inbetween. Domestication of the tomato is believed to have occurred with the early civilizations of Mexico. The name tomato may have been derived from the Aztec word tomatl (which really means tomatillo). European explorers brought the tomato from the Americas and disseminated it throughout Europe and Asia sometime during the 16th century. The tomato is now cultivated world-wide in a variety of environments.
The common tomato, Lycopersicon esculentum Mill., belongs to an extremely diverse and large family the Solanaceae. Often refered to as the Nightshade family, this family contains many of our commonly cultivated plants (potato, peppers, eggplant, tobacco, petunias, tomatillo) and various weeds (nightshades, jimson weed). It may have been this association with the nightshades which originally led people to believe the tomato to be poisionous. It was therefore fortunate that Miller changed the species name to esculentum which roughly means "edible".
The common cultivated tomato belongs to the genus Lycopersicon. This genus exhibits a vast reservoir of genetic variability which remains largely underexploited. Some disease resistance, which is now a part of most commercially available cultivars, has been derived from related species of Lycopersicon. However, these other species may provide other potential benefits such as heat or cold tolerance, salt tolerance, higher nutrient amounts and possibly other yet undiscovered potentials. Some of these species cross readily to Lycopersicon esculentum, but a few others have compatability barriers which make gene transfer by sexual means more difficult.
The genus Lycopersicon has one closely related genus Solanum (of which Irish potatoes belong) which may one day provide other useful traits provided the technology of gene transfer progresses. It was at one time considered that the common tomato belonged to the genus Solanum. In fact Linnaeus originally classified the tomato within this genus as Solanum lycopersicon but each are now considered to belong to a seperate genus based on differences in leaf type and pollen dispersal from the anthers. Lycopersicon has anthers which dehisce (release pollen) laterally (along the length of the anthers) and leaves are almost always pinnate or pinnatifid. Solanum has anthers which dehisce pollen from the terminal ends of the anthers and the leaves tend to be simple.
Although the debate concerning Lycopersicon is still on, it is generally considered to consist of eight species. Dr. C.M. Rick, whom has spent a lifetime collecting and researching tomato genetics, has divided the species into two complexes (the L.esculentum complex and the L. peruvianum complex) based on their ability to cross with the common tomato Lycopersicon esculentum. The species can be classified differently when one considers geographic distribution, morphological features and compatability relations (see Taylor, 1986). We will consider the classification by Rick to examine the differing species.
The L.esculentum complex consists of six other species which are relatively easy to cross with the common tomato. All species in this group are diploid (2n = 2x = 24). Most (except L. hirsutum f. typicum and some L. pennellii) are self-compatible. They have yellow flowers and the stamens are joined to produce an anther cone. Fruit color varies depending on the species. Several members of this complex have provided sources of pest resistance in the cultivated tomato.
The L. peruvianum complex consists of two extremely diverse species which only hybridize with great difficulty. Both are diploid (2n = 2x = 24) and both occupy unique environments. These two species represent a wealth of characteristics which are potentially valuable to the cultivated forms. Their fruits are green and they are mostly self-incompatible. These species have been limited in their usefulness to cultivated forms due to various barriers present in sexual transfer (convential breeding). Often expensive techniques like embryo rescue are required to obtain plants from these crosses. However there are a few documented examples of crosses which have been utilized, for instance tobacco mosaic virus resistance and nematode resistance can be traced to sucessful sexual crosses between L. esculentum and L chilense. Other documented examples include pest resistance from the L. peruvianum complex are citied by M.A.Stevens or Rick (search the Tomato Genetics Cooperative for further info).
There are quite possibly >10,000 varieties in existence today. Searching just one of the USDA holdings will give one the idea of what is available (~5,000). The C.M.Rick Tomato Genetics Resource Center in Davis, Ca. has approximately 3,000 varieties most of which are genetic mutants and wild-type accessions available for researchers. Seed Savers Exchange in Decorah, Iowa lists approximately 2,000. This does not take into account other collections in existence in Europe, Asia or Africa. To what extent these varieties are duplicated within these seedbanks is not know. It would be difficult (and expensive) to determine the "relatedness" of each variety if not impossible.
