Exploitation of resistance genes found in wild tomato species to produce resistant cultivars; Pile up of Resistant Genes

Begomoviruses, whitefly-transmitted geminiviruses, are a major constraint for tomato production in many parts of the world. This is a highly profitable but costly crop, due to the labour and amount of chemical inputs usually required to protect tomato from the various pests and diseases that attack this crop. The well-documented pesticide abuse associated with tomato production has greatly contributed to the development of pesticide-resistant Bemisia tabaci populations (Horowitz et al., 2005). This whitefly specie is capable of vectoring over 20 different begomoviruses that attack tomato in tropical and subtropical regions of the world (Polston & Anderson, 1997; Zeidan et al., 1999).

Although most of the begomoviruses that attack tomato are found in the New World, very little breeding work has been done to minimize the severe damage that these viruses cause to tomato plantings in this region. Moreover, despite its tropical American origin, most of the tomato breeding work has been conducted in temperate countries. Hence, tomato growers in tropical and subtropical America have relied almost exclusively on pesticides to control B. tabaci and the geminiviruses this vector transmits. The situation in the Old World is similar, due to the severe damage caused by a group of geminiviruses transmitted by B. tabaci in tomato fields throughout the Mediterranean region, the Middle East, North Africa, central Africa and Southeast Asia (Czosnek & Laterrot, 1997). These related although distinct geminivirus species, are collectively referred to as Tomato yellow leaf curl virus (TYLCV). This geminivirus was accidentally introduced in the last decade into the Americas in the early 1990s (Nakhla et al., 1994), where it has already caused millions of dollars worth losses of industrial and fresh tomato.