We intend using the maize transposable element Ac to tag the Cf-2, Cf-4, Cf-5 or Cf-9 genes in tomato for resistance to Cladosporium fulvum, the causal agent of tomato leaf mould. Our preferred strategy is designed to take advantage of the fact that Ac transposes preferentially to genetically linked sites, so the genetic locations of the target Cf genes need to be well defined. The Cf-2, Cf-4, Cf-5 and Cf-9 genes have been mapped previously; Cf-2 by Langford (1937) and Kerr et al. (1980); Cf-4 by Kerr and Bailey (1964); andCf-5 and Cf-9 by Kanwar et al. (1980); but the locations of some of these genes, particularly Cf-5 and Cf-9, are suspect (Kerr, 1982). We set out to check these locations using a combination of classical genetic and RFLP mapping. We were able to confirm the location of Cf-2 on chromosome 6 by both classical and RFLP mapping. In progeny from the cross LA1190 x (LA1190 x Cf-2), Cf-2 was found to be linked with 1% +/- 1% recombination to the chromosome 6 marker yv, and in progeny from the cross Cf-0 x (Cf-2 x LA716), Cf-2 was located between the RFLP markers TG118 and TG232, linked with 8.6% +/- 4.7 recombination to TG118 and 14.3% +/- 5.9 recombination to TG232. We were not able to confirm the location of Cf-4 on chromosome 1. In progeny from the cross Cf-0 x (Cf-4 x LA716), Cf-4 was unlinked to the RFLP markers TG24, TG71, TG83 and TG53, which together cover the majority of chromosome 1. Similarly, we were not able to confirm the location of Cf-5 on chromosome 4. In progeny from the cross Cf-0 x (Cf-5 x LA716), Cf-5 was unlinked to the RFLP markers TG123, TG62 and TG22, which together cover all of chromosome 4. Cf-5 has also been reported to be on chromosome 1 (Lenhardt and Kerr, 1972). We were not able to confirm this observation either, since Cf-5 was unlinked to the RFLP markers TG24, TG71, TG83 and TG53, which together cover the majority of chromosome 1. Efforts to locate Cf-4 and Cf-5 are continuing. Similarly, we were not able to confirm the location of Cf-9 on chromosome 10. In progeny from the cross LA1002 x (LA1002 x Cf-9), Cf-9 was unlinked to the chromosome 10 markers h, l-2 or ag, and in progeny from the cross Cf-0 x (Cf-9 x LA716), Cf-9 was unlinked to the RFLP markers TG122, TG43 and TG63, which together cover the majority of chromosome 10. By classical mapping, using chromosome tester x Cf-9 F1 testcross or F2 progeny, we were able to exclude Cf-9 from about 3/4 of the classical map. By RFLP mapping in the remaining regions, using Cf-0 x (Cf-9 x LA716) progeny, Cf-9 was found to be on chromosome 1, completely linked to TG24 with an upper limit of 12.2% recombination (p = 0.05).
Acknowledgements:
We are grateful to Prof. C. Rick for the provision of the many chromosome tester lines used in the classical mapping experiments and to Prof. S. Tanksley for the provision of the many TG probes used in the RFLP mapping experiments.
Literature cited:
Kanwar, J.S., Kerr, E.A. and Harney, P.M. (1980) Linkage of Cf-1 to Cf-11 genes for resistance to tomato leaf mold, Cladosporium fulvum Cke. TGC Report 30: 20-21.
Kerr, E.A. (1982) Further studies of genes for resistance to Cladosporium fulvum . TGC Report 32: 19.
Kerr, E.A. and Bailey, D.L. (1964) Resistance to Cladosporium fulvum Cke. obtained from wild species of tomato. Can. J. Bot. 42:1541-1554.
Kerr, E.A. , Kerr, E. Patrick, Z.A. and Potter, J.W. (1980) Linkage relation of resistance to Cladosporium leaf mold (Cf-2) and root-knot nematodes (Mi) in tomato and a new gene for leaf mold resistance (Cf-11). Can. J. Genet. Cytol. 22:183-186.
Langford, A.N. (1937) The parasitism of Cladosporium fulvum Cooke and the genetics of resistance to it. Can. J. Res. C. 15:108-128
Lenhardt, L.P. and Kerr, E.A. (1972) New genes for resistance to tomato leaf mold, Cladosporium fulvum. TGC Report 22:15-16.