selfed and a homozygous resistant stock was selected from the progeny. This homozygous resistant NIL was then
crossed with E6203 to produce seed heterozygous for the I-2 gene.
E6203 susceptible plants and nearly isogenic heterozygous resistant plants were transplanted into field
plots of 20 plants each at standard densities in the summer of 1996. The sites were in Akko, Israel (IS); Badajoz,
Spain (SP), Stockton, California (CAH); Woodland, California (CAP); and Numata, Japan (JP). There were 3 reps
of the heterozygous resistant NIL and 9 reps of E6203 at each site. The only exception was JP where there were
2 reps of the resistant NIL and 6 reps of E6203. Details of field condition and cultural practices can be found in
Tanksley et al. (1 997a). All plots were harvested at the end of the season and evaluated for plant growth/cover,
maturity, stem scar size, fruit color (internal and external), fruit shape, fruit firmness, pericarp thickness, puffiness,
stem core depth, pH, soluble solid, yield (total, red and green), brix*yield, fruit weight and viscosity. The methods
used for these evaluations are described in Tanksley et al. (1997a). The only exception was for yield which was
not measured at CAH. Analysis of variance was used to compare the performance of E6203 and the
heterozygous resistant NIL with respect to each of the measured traits. Table 1 summarizes those traits for which
a significant difference (P<0.1) was observed.
Table 1. p values derived from ANOVAs comparing E6203 and heterozygous 1-2 resistant NIL.
S = susceptible allele, R = resistance allele. The genotype with the higher value for trait is listed in parenthesis
beside the p value.
E6203 and the heterozygous NIL differed with marginal significance (p = 0.05-0.1) for pericarp thickness,
puffiness, pH and soluble solids (Table 1). The heterozygous NIL tended to have thinner pericarp, puffier fruit,
lower pH and lower soluble solids (average brix 4.75 vs 4.90 for E6203). However the most significant differences
were for yield (both total and red yield). The heterozygous resistant NIL averaged approximately 30% greater yield
than E6203. The yield increase was evident in all locations tested (fig 1).
This is the third instance in which disease resistance genes, in the heterozygous state, have been shown to
increase yield in processing tomatoes. The other cases were for Mi (Tanksley et al, 1997b, this issue of TGC) and
Tm2a (Tanksley et al. 1997a). In both of these
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