the Sainsbury Laboratory, John Innes Institute, Colney Lane, Norwich, NR4 7UH .U.K.
Classical and RFLP mapping of the Cf-2, Cf-4, Cf-5 and Cf-9 genes in tomato for resistance to Cladosporium fulvum has continued. Cf-2 has been relocated on the classical map of chromosome 6 from the published position distal to yv (yellow virescent) (Tanksley and Mutschler, 1990) to a position 1.0+\-0.7% proximal to yv in an LA1178 (yv-coa-c) x Cf-2 F2 segregation (Fig. 1). Similar results have been obtained independently for the same cross by M. Koornneef (pers. comm.). With the greater resolution provided by additional markers, Cf-2 has been relocated on the RFLP map from the presumed position distal to TG232 given in last years report (Jones et al., 1991) to a position 9.3+\-3.9% proximal to Aps-1(Acid phosphatase-1) and 11.1+\-4.3% proximal to TG232 and cosegregating with GP79, in the susceptibles from a Cf-2 x LA716 F2 segregation (Fig. 1). From wide ranging classical mapping experiments using chromosome tester x Cf-5 testcross or F2 progeny, Cf-5 was finally located on chromosome 6 near to yv (Fig 1) in an LA1190 (yv pds) x Cf-5 F2 segregation. A three point classical mapping experiment, exactly as described above for Cf-2, showed Cf-5 to be 2.3+\-0.9% proximal to yv , in the vicinity of Cf-2 (Fig. 1), raising the possibility of allelism. RFLP mapping experiments locating Cf-5 between Aps-1 and GP79, 4.6+\-3.1% proximal to both Aps-1 and TG232 and 2.2+\-2.2% distal to GP79, in the susceptibles from a Cf-5 x LA716 F2 segregation, also place Cf-5 in the vicinity of Cf-2 (Fig. 1). An F1 between plants homozygous for Cf-2 and Cf-5 was test-crossed to Cf0 (a plant lacking a detectable Cf gene) and the progeny inoculated with race 0 of Cladosporium fulvum carrying both Avr-2 and Avr-5. No susceptible progeny were recovered in 223 progeny consistent with Cf-2 and Cf-5 being allelic or very closely linked (with an upper limit of 1.3% recombination at p = 0.05). These data are also consistent with Cf-2 and Cf-5 being in the vicinity of the Mi gene for resistance to the root knot nematode Meloidogyne incognita , which has been located between Aps-1 and GP79 (Klein-Lankhorst et al., 1991; Messeguer et al., 1991). However, Mi and Cf-2 are probably not allelic because a significant amount of recombination has been reported between the two (Kerr et al., 1980).
Cf-4 has been relocated on chromosome 1 from the published position distal to scf (scurfy) on the long arm (Tanksley and Mutschler, 1990), to a position 18.7+\-3.4% proximal to au (aurea) on the short arm and 39.1+\-5.1% proximal to scf on the long arm, in an LA1186 (au scf inv dgt) x Cf-4 F2 segregation (Fig 1). In our previously reported RFLP mapping experiments (Jones et al., 1991), we were unable to detect linkage between Cf-4 and RFLP markers on chromosome 1. We believe this to have been due to mis-scoring of resistance in the Cf0 x (Cf-4 x LA716) segregation, owing to a combination of weak resistance conferred by Cf-4 exacerbated by the genetic background of LA716 and a lack of aggressiveness in the pathogenicity of Cladosporium fulvum race 0 on susceptible plants. We have since obtained clear indications for linkage of Cf-4 to TG24 and TG301 on the short arm of chromosome 1 in the susceptibles from a Cf-4 x LA716 F2 segregation. A combination of two point classical mapping experiments show Cf-9 to be 20.6+\-4.6% proximal to au in a 2-655A(au) x Cf-9 F2 segregation and 28.6+\-5.6% proximal to ses in an LA826(ses) x Cf-9 F2 segregation, placing Cf-9 in the vicinity of Cf-4 (Fig. 1), again raising the possibility of allelism. RFLP mapping experiments placing Cf-9 between TG24 and TG301 on the short arm of chromosome 1, 5.3+\-3.6% distal to TG24 and 2.6+\-2.6% proximal to TG301 in the susceptibles from a Cf-9 x LA716 F2 segregation (Fig. 1), also raise the possibility of allelism with Cf-4. This RFLP location is consistent with the data of van der Beek et al. (1991, 1992) who have independently located Cf-9 near to TG301. An F1 between plants homozygous for Cf-4 and Cf-9 was test-crossed to Cf0 and the progeny inoculated with Cladosporium fulvum race 0 carrying both Avr-4 and Avr-9. One possible susceptible plant was recovered in 214 progeny. This plant is being tested further to check its authenticity. Nevertheless, the data are consistent with Cf-4 and Cf-9 also being allelic or very closely linked. Cf-4 has previously been shown to be allelic or very closely linked to Cf-1 (Kerr and Bailey, 1964).
Fig. 1. Simplified classical and RFLP maps of chromosomes 1 and 6 showing the old locations of Cf-1, Cf-2 and Cf-4 indicated by asterisks and the new locations of Cf-1, Cf-2, Cf-4, Cf-5 and Cf-9 indicated by boxes.
There now appear to be at least two complex resistance loci in tomato, one on chromosome 6, of which Cf-2, Cf-5 and possibly Mi are members, and another on chromosome 1, of which Cf-4, Cf-9 and presumably Cf-1 are members (Fig. 1). This reveals the genetic control of resistance to leaf mould in tomato to be similar to the genetic control of resistance in other plants which often also possess complex resistance loci e.g. resistance to rust in maize, powdery mildew in barley, rust in flax and downy mildew in lettuce.
This report shows that, to various degrees, the Cf-2, Cf-4, Cf-5 and Cf-9 genes have been poorly placed on the classical genetic map of tomato. Cf-2 was located previously to chromosome 6 (Langford, 1937; Kerr et al., 1980a), but was placed on the map distal to yv (Fig. 1), although no three point mapping data involving Cf-2 and yv existed to support this location. We have three point data to show that Cf-2 is in fact proximal to yv. Similarly, Cf-4 was located previously to chromosome 1 (Kerr and Bailey, 1964: Kanwar et al., 1980a), but was placed on the map distal to scf (Fig. 1), although again no relevant three point data existed to support this location. We have three point data to show that Cf-4 is in fact proximal to scf. This highlights the fact that many locations on the classical genetic map of tomato are compromises designed to accommodate data from different sources and therefore cannot be regarded as definitive. Cf-5 and Cf-9 were placed on chromosomes 4 and 10, respectively, by Kanwar et al. (1980a). We have shown that Cf-5 and Cf-9 are in fact on chromosomes 6 and 1, respectively. Others (Kerr, 1982; Gerlagh et al., 1989; Laterrot and Moretti, 1989, 1991) have also produced data contradicting some of the Cf gene locations reported by Kanwar et al. (1980a, 1980b).
We are grateful to C. Rick for the many chromosome tester lines used in the classical mapping experiments, and to S. Tanksley for the many TG clones, V. Williamson for the Aps-1 clone and C. Gebhardt for the genomic potato clone GP79 used as probes in the RFLP mapping experiments.
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Laterrot, H. and A. Moretti 1989. Allelism of Cf-16 and Cf-21. TGC Report 39: 22-23.
Laterrot, H. and A. Moretti 1991. Independence of Cf-10 and Cf-12. TGC Report 41: 26-27.
Messeguer, R., M. Ganal, M.C. de Vicente, N.D. Young, H. Bolkan and S.D. Tanksley 1991. High resolution RFLP map around the root knot nematode resistance gene (Mi) in tomato.
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van der Beek, H., R. Verkerk, P. Zabel and P.Lindhout 1992. Mapping strategy for resistance genes in tomato based on RFLPs between cultivars:Cf-9 (resistance to Cladosporium fulvum) on chromosome 1. Theor. Appl. Genet. In press.