The I-3 gene from L. pennellii confers resistance to Fusarium oxysporum f. sp. lycopersici race 3 and is tightly linked to the isozyme locus Got-2 on chromosome 7 (Scott and Jones 1989, Bournival et al 1989, 1990). The same gene either confers resistance to or is linked to other genes that confer resistance to races 1 and 2 of the same pathogen (Bournival et al. 1990, Sarfatti et al. 1991). Currently a number of seed companies and tomato breeders are incorporating the I-3 gene into commerical tomato varieties via backcrossing.
We were interested in monitoring the progress of I-3 introgression by measuring the size of L. pennellii chromosome 7 segments carrying the I-3 gene. To do so, we obtained several different backcrossed-derived, I-3 containing breeding lines and probed them with RFLP markers from chromosome 7 (Table 1). The results of this probing are shown in figure 1.
The 3 breeding lines tested fell into two classes. Class A, including 89-508-1 and X4801-WC15, contain at least 19 cM of pennellii DNA. These two lines were independently derived and it is interesting to note that at least one of the crossovers in both lines occurred in the same interval on the map between TG216 and TG170. I3R1 has a much smaller piece of pennellii chromosome (ca. 2.8-14 cM) than the other two breeding lines. I3R1 and X4801-WC1 share the same donor line for I-3 (E427). E427 was not tested with RFLPs but must contain at least the same 19 cM of pennellii DNA as X4801-WC15. A crossover between TG217 and TG183 must have occurred during the subsequent backcrosses to produce I3R1 since it has a much smaller segment of foreign DNA. This is consistent with field observations that I3R1 has fewer associated negative horticultural characteristics than its progenitor line E427 (J. Scott, personal comm).
Our results indicate that I-3 resides somewhere between TG216 and TG183. Resistance to race 1 from L. pennellii (referred to as the I1 gene) has recently been mapped to chromosome 7, 11 cM below TG20 (Sarfatti et al. 1991) which places it very close to I-3 (see fig 1) and raises the possibility of a single gene conferring resistance to both races or the existence of a tightly linked cluster of resistance genes.
Literature Cited:
Bournival BL, Vallejos CE, Scott JW (1990) Genetic analysis of resistances to races 1 and 2 of Fusarium oxysporum f. sp. lycopersici from the wild tomato Lycopersicon pennellii. Theor Appl Genet 79:641-645
Bournival BL, Vallejos CE, Scott JW (1989) An isozyme marker for resistance to race 3 of Fusarium oxysporum f. sp. lycopersici in tomato. Theor Appl Genet 78:489-494
Sarfatti M, Abu-Abied M, Katan J, Zamir D (1991) RFLP mapping of I1 - a new locus in tomato conferring resistance against Fusarium oxysporum f. sp. lycopersici race 1. Theor Appl Genet (in press)
Scott JW, Jones JP (1989) Monogenic resistance in tomato to Fusarium oxysporum f. sp. lycopersici race 3. Euphytica 40:49-53
Table 1. I-3 breeding lines tested
_________________________________________________________ Line Source Generation RFLP type cM % chrm 7 _________________________________________________________ 89-508-1 Cornell U. BC5F2 A >19 >17% X4801-WC15 CIRT BC4F3 A >19 >17% I3R1 U. Flor. BC4F? B 2.8-14 > 2% _________________________________________________________CIRT (Campbell Institute for Research and Technology)
Cornell U. (from M.A. Mutschler)
U. Florida (from J. Scott)
Figure 1. RFLP map of chromosome 7. Bars indicate estimated size of introgressed L. pennellii chromosomal segment containing I-3 gene(see Table 1 for list of lines). If bar crosses lines, L. pennellii allele was detected at the corresponding marker. Arrows indicate that markers beyond this point have not been tested in these lines and thus the pennellii segment may be larger.