Linkage relationship between genes Fw and Fnw

  Fusarium oxysporum Schlecht. emend. Snyd. and Hans. f. sp. pisi (van Hall) Snyd. and Hans. is a causal agent of wilting in peas. Four races, 1, 2, 5 and 6, are economically important. Races 1 and 2 of the pathogen have a worldwide distribution while races 5 and 6 are mostly confined to the western part of Washington State in the U.S.A. and to British Columbia in Canada (1). Resistance to race 1 of the pathogen is conferred by a single dominant gene Fw that has been reported to be located 30 map units from Le (4). Resistance to race 2 of the pathogen is also conferred by a single dominant gene Fnw (2, 5). Wells et al. (5) reported loose linkage between Fnw and Fw with 40% recombination; however, no linkage was found between Fnw and Le. The putative location of Fnw has been on chromosome 4 but not linked to Le. Since the data of Wells et al (5) for F₃ families did not support linkage between Fnw and Fw, it was our goal to determine where the two genes are located and whether or not they are linked.

Crosses between lines resistant or susceptible to races 1 and 2 of the fungus were made, and the F₃ families were evaluated for their response to the two races. Resistance or susceptibility to race 1 was evaluated in the field in a disease nursery, while resistance to race 2 of the pathogen was evaluated under greenhouse conditions following standard inoculation procedures (3). Crosses used in this study were: Mini 93 (Fw, Fnw) x M410 (fw, fnw), 74SN5 (Fw, Fnw) x WA788 (fw, fnw), and 74SN5 (Fw, Fnw) x M410 (fw, fnw).

We analysed the data for each cross separately and later combined the data (123 F₃ families). Fw and Fnw segregated as expected for single dominant genes (Table 1).

The joint segregation data for the two loci (Table 2) indicated a recombination fraction of around 46%, and the likelihood that the two genes assorted independently. The linkage relationship of 40% between the two loci reported by Wells et al. (5) was never confirmed in subsequent studies. Based on our data, it appears that the genes are transmitted independently; however, the possibility that the two genes are in the same linkage group has not been ruled out. We are conducting further studies on linkage of various genes with Fw and Fnw which may help to determine the linkage groups that contain these genes.

1.  Hagedorn, D.J. 1984. In Compendium of Pea Diseases. Ed D.J. Hagedorn, Am. Phytopath. Soc., St Paul, Minnesota, pp.22-25.

2. Hare, W.W., Walker, J.C. and Delwiche, E.J. 1949. J. Agric. Res. Washington D.C. 78:239-250.

3. Kraft, J.M. and Haglund, W.A. 1978. Phytopathology 68:273-275.

4. Wade, B.L. 1929. Wis. Agric. Exp. Sta. Res. Bull. 97:1-32.

5. Wells, D.G., Hare, W.W. and Walker, J.C. 1949. Phytopathology 59:907-912.

Table 1. Single locus goodness of fit to a 1:2:1 ratio for F₃ families from crosses segregating for Fw and Fnw.

Table 2. Joint segregation data for Fw (resistance to race 1) and Fnw (resistance to race 2) for 116 F₃ families from three crosses.