Pisum Genetics 

Volume 24

1992

Research Reports

pages 52-53

Linkage relationship between genes Fw and Fnw

Grajal-Martin, M.J.

and

Muehlbauer, F.J.

 

Dept of Crop and Soil Sciences

WSU, Pullman, WA 99164, USA

USDA-ARS, Johnson Hall

WSU, Pullman, WA 99164, USA

  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.

Gene

Cross

F3 families

c2

df

P

+/+

+/-

-/-

Fw

Mini 93 x M410

8

22

12

0.85

2

0.65

 

74SN5 x WA788

6

15

8

0.31

2

0.86

 

74SN5 x M410

14

26

11

0.37

2

0.83

 

Combined

28

63

31

0.28

2

0.87

Fnw

Mini 93 x M410

12

18

6

2.00

2

0.37

 

74SN5 x WA788

7

14

8

0.10

2

0.95

 

74SN5 x M410

7

28

16

3.67

2

0.15

 

Combined

27

59

30

0.19

2

0.91

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

Race 2

Race 1

Joint seg.

Prob.

Recomb. fraction

SE

+/+

+/-

-/-

c2 (df = 4)

+/+

9

12

6

2.58

0.63

46.2

4.0

+/-

11

32

16

 

 

 

 

-/-

6

17

7