Domestication

Although the natural distribution of the wild species is restricted to the Andean region, the site(s) of domestication remain uncertain. Two alternative hypotheses have been proposed: southern Mexico (Jenkins 1948; Rick and Fobes 1975) versus Peru (De Candolle 1886). The Mexican domestication hypothesis states that the populations of wild cherry tomatoes (S. lycop-ersicum var. cerasiforme1) migrated from Peru into Meso-America, and became domesticated in Mexico (Jenkins 1948; Rick 1995). There is scant informa tion on when domestication occurred, however; by the time of the Spanish conquest of Mexico in 1521, large-fruited types - a sign of human selection - were already being grown for food (Hancock 2004). Domestication and use of tomato as food probably first occurred in Central America as evidenced by cultural, linguistic, and historical records as well as genetic findings (Rick 1995; Cox 2000). With respect to the cultural evidence, decoration of textiles and ceramics with depictions of crop plants was a common practice of Pre-Columbian cultures in Peru. Important food plants commonly represented in this way include maize, potato, pepino, and others, but, conspicuously, not tomato. Putative archaeological evidence of tomato is decorated functional ceramics ("spindle whorls") produced by the native Quimbaya culture (500-1000 AD) of Colombia (McMeekin 1992). However, these ceramics could also be interpreted as representations of other Solanum flowers (possibly potato). Furthermore, there have been no discoveries of preserved tomato fossils or other archaeological remains in Peru, suggesting it is unlikely that tomato had undergone domestication there (Rick 1995). However, soft plant tissues generally do not preserve well. Studies of microfossils (starch grains, phytoliths, etc.) have revealed evidence of the early history of pepper in South America. Linguistic records also support the theory of domestication in Central rather than South America. The modern name "tomato" is derived from "tomatl", the word for this plant in the native language of the Aztecs (Gould 1983). The Aztecs mixed tomatoes, chilies and ground squash seeds into a concoction that seems likely to be the original salsa recipe (Cutler 1998). However, no writings or records from ancient Peruvian tribes ever mentioned a tomato-like fruit as being an important part of the diet or even a word meaning tomato. Supporting evidence also comes from the relationship between the introductions of the tomato to Europe relative to the conquests of Mexico and Peru. In possibly the first Italian herbal to mention tomato, Matthiolus (1544) implies a considerably earlier introduction of the tomato by his statement that it was already "eaten in Italy with oil, salt, and pepper". The earliness of this date favors a Mexican origin considering the capture of Mex-

1 The correct name for the cherry tomato under the International Code of Botanical Nomenclature is S. lycopersicum var. leptophyllum. The more widely applied name, S. lycop-ersicum var. cerasiforme, is used in this chapter to avoid confusion.

ico City in 1521 vs. the conquest of Peru in 1531 (Rick 1995).

In addition to these cultural considerations, genetic evidence also supports Central American domestication. Data from allozymes demonstrated that older European cultivars, i.e., descendents of the tomatoes introduced by the Spanish explorers, are extremely homogeneous at most loci and are closely related to cultivars and wild cherry tomatoes from southern Mexico and Central America, but differ from those of South America (Rick and Holle 1990). In addition, genetic variability in wild cherry tomato from the Andean region is greater than in accessions collected outside of this region (Rick and Holle 1990). This is consistent with the model of migration beyond the center of origin, which would entail founder effects, selection, and the consequent loss of variation associated with those processes. In many crops, a reduction in genetic diversity has been one of the major features accompanying domestication, and this seems to be true for tomato.

Arguments supporting the Peruvian domestication hypothesis were presented by Moore (1935), Müller (1940a, b), and Luckwill (1943a, b). These arguments trace back to botanical (Bauhin 1623; Ruiz and Pavón 1797), historical (Hernández 1651), and linguistic (Roxburgh 1832) evidence summarized and interpreted by De Candolle (1886). De Candolle (1886) found that there were no definitive original records of tomato outside of the Americas before its European discovery there. Initial names for tomato cited by early 16th century botanists, "mala peruviana" and "pomi del Peru" (Bauhin 1623), suggested Peruvian origin and subsequent dispersal of tomato from Peru to Europe. De Candolle (1886) also argued that (i) its evolutionary origin was from the wild cherry tomato, that by the mid- to late 19th century was spread from coastal Peru, to Mexico, and to southwestern USA (California), and also reported in Asia, (ii) the crop had only recently been domesticated before the discovery of the Americas, and (iii) the distribution of cultivated tomato and its progenitor outside of Peru originated by garden escapes. Contrary to Jenkins' (1948) statements that there are no indigenous Peruvian names for tomato, Horkheimer (1973) documented a Quechua name for tomato (pirca), and Yakovleff and Herrera (1935) cited another Quechua name (pescco) possibly referring to the cherry tomato. Peralta and Spooner (2007) reviewed evidence for a Mexican versus Peruvian origin of domesticated tomato, as well as methods applied for inferring geographical location of crop origins. They concluded that the original site of tomato domestication remains uncertain, and may be unfeasible to resolve.

De Candolle's (1886) assertion that wild cherry tomato is the immediate progenitor of cultivated tomato also continues to be actively debated (Nes-bitt and Tanksley 2002) (Table 1). This hypothesis is supported by S. lycopersicum var. cerasiforme's exceedingly close genetic resemblance to the cultivated forms of tomato, and its common occurrence in Central America. In terms of morphology, the cherry and modern domesticated types differ primarily by the larger fruit size of the latter. Like cultivated tomato, the wild cherry tomato is naturally self-pollinated. In contrast, many of the other wild species are either obligate outcrossers (SI) or have a mixed mating system (see Sect. 1.2.3). Self-pollination in the cherry tomato is promoted by a stigma and style that protrudes only a short distance or not at all, beyond the end of the anther cone (Müller 1940a; Luckwill 1943b; Rick 1995; Cox 2000). Aside from increased fruit size, the gradual shortening of style length and other factors favoring self-pollination are believed to be a major feature of domestication. Besides the uniformity associated with inbreeding, a shorter style and recessed stigma position tend to favor fruit-setting ability, hence yield (Rick 1995; Scott and Angell 1998). Early North American cultivars tended to have slightly exserted stigmas. This resulted in diminished fruit setting, especially in the absence of appropriate pollinating insects or under conditions of excessive temperature (which tends to reduce pollen fertility). Selection for less exserted stigmas resulted in stigmas approximately flush with the mouth of the anther tube, as seen in many early European cultivars. Full enclosure of the pistil by the anthers was inadvertently selected during breeding of mechanically harvested processing tomatoes for California conditions, apparently because it ensured a reliable, concentrated fruit set. This improvement virtually guarantees self-pollination and has been bred into many modern cultivars (Rick 1995).

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