Insect Resistance

Tomatoes (S. lycopersicum) are susceptible to a wide array of arthropod pests (Lange and Bronson 1981; Berlinger 1986; Kennedy 2003). Currently available cultivars do not have sufficiently high levels of pest resistance to allow for significant reductions in the amount of pesticides used in the crop. Consequently, developing cultivars with increased levels of pest resistance is a major focus of some breeding programs, along with their adoption into integrated pest management programs aimed at reducing pesticide sprays and environmental impact.

The wild tomato species S. habrochaites, S. peru-vianum, and S. pennellii have been reported to be sources of resistance to many tomato arthropod pests (Gentile and Stoner 1968; Gentile et al. 1968, 1969; Rick 1973; Kennedy and Yamamoto 1979; Williams et al. 1980; Ecole et al. 1999; Kennedy 2007). However, introgression of these resistances into the cultivated tomato has been limited by the difficulties in maintaining the necessary uniform infestations to select for resistance (Stevens and Rick 1986). Indirect selection techniques based on correlated traits with high her-itability can be used to expedite introgression compared to expensive and slow direct selection methods (Juvik et al. 1982; Mutschler 2006).

Several allelochemicals are present in the wild relatives of the cultivated tomato and are associated with pest resistance: methyl-ketones such as 2-tridecanone in S. habrochaites (Williams et al. 1980; Fery and Kennedy 1987; Weston et al. 1989; Eigenbrode and Trumble 1993a, b; Maluf et al. 1997; Gon^alves et al. 1998), sesquiterpenes in S. habrochaites (Snyder et al. 1987; Eigenbrode et al. 1994), and acylsugarsin S. pennellii (Goffreda et al. 1989). These compounds are found in glandular trichomes present in the leaf surface (Williams et al. 1980; Carter and Snyder 1985, 1986; Snyder and Carter 1985; Carter etal. 1989), and are often associated with moderately high or high heritability values (Maluf et al. 1997). Efficient demonstrations of indirect selection methods for high allelochemical content have been reported for 2-tridecanone or sesquiterpene (zingiberene) content and resistance to the South American tomato pin-worm (Maluf et al. 1997), and spider mite repellence (Gon^alves et al. 1998). Quick and inexpensive techniques to measure these allelochemicals (Weston and Snyder 1990) for spider mite (Tetranychus spp.) repellence could also be indicative of their level of resistance to other pests.

S. pennellii accessions (particularly LA716) have shown a very high level of resistance to the whitefly Bemisia tabaci/Bemisia argentifolii complex, as well as to aphids (Macrosiphum euphorbiae, Myzus persi-cae), mites and Lepidopteran pests (Gentile et al. 1968, 1969; Juvik et al. 1982; Goffreda et al. 1989), including the South American tomato pinworm Tuta (= Scrobi-palpuloides) absoluta (Franca et al. 1989). The multiple pest resistance of S. pennellii is due to the presence of type IV glandular trichomes and the glucose and sucrose esters of fatty acids (acylsugars) that they secrete (Gentile et al. 1968; Goffreda et al. 1989; Shapiro et al. 1994). A reduction of insects on plants has been shown using purified acylsugars, where feeding is reduced by Macrosiphum euphorbiae and Myzus persi-cae, a reduction in feeding, larval development and survival of Helicoverpa zea and Spodoptera exigua, and by reducing oviposition and feeding of the leaf miner Liriomyza trifolii and of the silver leaf white-fly Bemisia argentifolii (Goffreda et al. 1988, 1989; Hawthorne et al. 1992; Rodriguez et al. 1993; Juvik etal. 1994; Liedl etal. 1995). The presence of type IV glandular trichomes is controlled by at most two unlinked genes in crosses of S. pennellii with S. lycopersicum (Lenke and Mutschler 1984). A study on the inheritance of type IV trichome density, acylsugar accumulation levels, percentage of acylsugars and leaf area in interspecific populations between S. pennel-lii LA716 and LA1912 identified three major loci and ten loci with modest associations (Blauth et al. 1998). A MAS breeding program was also attempted to transfer the acylsugar trait to cultivated tomato, however the accumulation in plants with the five target regions were lower than that of the interspecific Fi, suggesting another unidentified region is necessary for high accumulation levels (Lawson et al. 1997).

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