6.3 Results
6.3.8 Allelism test for Fusarium root rot resistance genes from several potential sources of
The chi-square test (X2) results for the goodness of fit of the phenotypic classes of F2
segregants is presented in Table 6.18. Four out of the 11 ratios were fitted. The test indicated the presence of one dominant and two recessive genes in the cross MLB-49-89A x Vuninkingi, and two complementary dominant genes in the cross MLB-49-89A x G4795.
Three complementary dominant genes were suggested by the chi square test in the crosses RWR719 x Vuninkingi, RWR719 x Vuninkingi, MLB-48-89A x Umubano, and MLB-48-89A x G4795. All the other crosses had more than three genes involved and did not fit into any of the ratios tested. The involvement of more than three genes is explained by the continuous distribution exhibited by their progeny, suggesting the importance of polygenic inheritance in these crosses.
Table 6.18. Chi square testing for goodness of fit of phenotypic classes in F2.
Cross Hypothesis X2
Value Df P
value Implication
MLB-49-89A x Vuninkingi 49:15 0.93 1 0.336 one dominant and two recessive genes MLB-49-89A x G4759 9:7 0.49 1 0.482 two complementary dominant genes MLB-49-89A x Umubano 57:7 0.01 1 0.931 one dominant and two complementary
genes MLB-49-89A x MLB-48-
89A
57:7 0.60 1 0.438 one dominant and two complementary genes
MLB-48-89A x G4759 27:37 0.16 1 0.693 three complementary dominant genes RWR719 x Vuninkingi 27:37 0.29 1 0.591 three complementary dominant genes
F2 populations that involved RWR719 (Figure 6.2a, b, c, d and e) had continuous distribution. This indicated the action of more than one or two additive resistance genes in these crosses. However, the cross RWR719 x MLB-49-89A (Figure 6.2a) was skewed to the susceptible side indicating the presence of major additive recessive susceptibility and resistance genes in both these varieties, probably located within the same quantitative trait loci (QTL) because they only expressed themselves in the F2. It also implied that the resistance genes were fewer than the susceptibility genes in this cross.
Most of the crosses with MLB-49-89A, that is, MLB-49-89A x MLB-48-89A (Figure 6.3a), MLB-49-89A x Vuninkingi (Figure 6.3b), MLB-49-89A x Umubano (Figure 6.3c), and MLB- 49-89A x G4795 (Figure 6.3d) tended towards resistance which indicated the presence of major additive resistance gene effects, probably located at the same locus in these bean varieties. However, the crosses MLB-49-89A x RWR719 (Figure 6.2a) tended towards susceptibility
indicating the possibility of recessive genes for resistance and susceptibility in MLB-49-89A and RWR719, probably located within the same quantitative trait loci (QTL). The distribution of MLB-49-89A x G4795 (Figure 6.3d) was discontinuous (R2=0.27) because there seemed to be two distinct classes of susceptible and resistant plants in this cross which may indicate the presence of recessive resistance gene(s) in either MLB-49-89A or G4795 or recessive susceptibility gene(s) in both parent and the presence of probably two loci governing resistance to FRR in these varieties.
a. RWR719 x MLB-49-89A.
R2 = 0.7635
0 50 100 150 200
2 3 4 5 7 9
Severity sco re
b. RWR719 x MLB-48-89A.
R2 = 0.7728
0 50 100 150 200 250
2 3 4 5 7 9
Severity score
c. RWR719 x Vuninkingi.
R2 = 0.9052
0 20 40 60 80 100 120 140 160
2 3 4 5 7 9
Severity score
d. RWR719 x Umubano.
R2 = 0.7103
0 50 100 150 200 250
2 3 4 5 7 9
Severity sco re
e. RWR719 x G4795.
R2 = 0.7865
0 20 40 60 80 100 120 140 160 180
2 3 4 5 7 9
Severity sco re
Fig. 6.2. F2phenotypic segregation for R x R crosses involving RWR719.
a. MLB-49-89A X MLB-48-89A.
R2 = 0.7989
0 10 20 30 40 50 60 70 80 90
2 3 4 5 7 9
Severity scores
b. MLB-49-89A X Vuninkingi
.
R2 = 0.5808
0 50 100 150 200 250
2 3 4 5 7 9
Severity score
c. MLB-49-89A x Umubano.
R2 = 0.5915
0 20 40 60 80 100 120
2 3 4 5 7 9
Severity score
d. MLB-49-89A x G4795.
R2 = 0.2767
0 20 40 60 80 100 120 140 160
2 3 4 5 7 9
Severity score
Fig. 6.3. F2phenotypic segregation for R x R crosses involving MLB-49-89A.
Crosses involving MLB-48-89A had a continuous distribution of severity scores for some of the crosses, such as MLB-49-89A x MLB-48-89A (Figure 6.3a), tending towards resistance, while the cross MLB-48-89A x Vuninkingi (Figure 6.4a) and MLB-48-89A x G4795 (Figure
6.4c), tended towards susceptibility. The crosses MLB-48-89A x Umubano (Figure 6.4b) had a discontinuous distribution (R2=0.14), with the appearance of two distinct separate classes of both resistant and susceptible plants at F2. It is probable that the resistance genes were on different loci in the two bean varieties.
a. MLB-48-89-A x Vuninkingi
R2 = 0.8553
0 20 40 60 80 100 120 140 160 180
2 3 4 5 7 9
Severity score
b. MLB-48-89A x Umubano R2 = 0.1409
0 10 20 30 40 50 60 70 80
2 3 4 5 7 9
Severity score
c. MLB-48-89A x G4759 R2 = 0.7586
0 20 40 60 80 100 120 140 160
2 3 4 5 7 9
Severity score
Fig. 6.4. F2phenotypic segregation for R x R crosses involving MLB-48-89A.
Crosses involving Vuninkingi (Figures 6.2c, 6.3b, 6.4a, 6.5a and 6.5b) had continuous distributions, indicating the presence of additive resistance genes in these varieties. Most of the populations with Vuninkingi as a parent tended towards susceptibility, with the exception of that with RWR719, which was >90% continuous, and MLB-49-89A, which tended towards resistance. This, therefore, indicated that MLB-49-89A had more additive resistance genes than all the other resistant parents, and that the genes for resistance to FRR are recessive in nature.
a. Vuninkingi x Umubano R2 = 0.8638
0 20 40 60 80 100 120 140 160
2 3 4 5 7 9
Severity score
b. Vuninkingi x G4795
R2 = 0.6898
0 20 40 60 80 100 120 140 160 180 200
2 3 4 5 7 9
Severity score
Fig. 6.5. F2 phenotypic segregation for R x R crosses involving Vuninkingi.
The crosses involving Umubano, that is, RWR719 x Umubano (Figure 6.2d), Vuninkingi x Umubano (Figure 6.5a), and Umubano x G4759 (Figure 6.6), also tended towards susceptibility, while the cross MLB-49-89A x Umubano (Figure 6.3c), tended towards resistance. This again showed that MLB-49-89A had a greater number of resistance genes than any of the other parents. The cross MLB-48-89A x Umubano (Figure 6.4b) had almost equal numbers of both resistant and susceptible plants in the F2 generation, which indicated the possibility of two loci governing resistance in this cross.
Crosses involving G4795 (Figures, 6.2e, 6.4c, 6.5b, and 6.6) were continuous in nature indicating the involvement of many additive genes, with the exception of MLB-49-89A x G4795, which was discontinuous (Figure 6.3d) as already discussed. Results indicate two distinct classes of susceptible and resistant plants which suggested the presence of a recessive resistance gene or recessive susceptibility gene(s) governing resistance to FRR in both MLB-49-89A and G4795, probably located at two loci in these varieties. The cross Umubano x G4795 (Figure 6.6) was skewed to the susceptible side, indicating that the resistance genes in these varieties were recessive in nature and possibly at the same locus.
Umubano x G4795
R2 = 0.8986
-20 0 20 40 60 80 100 120 140 160 180 200
2 3 4 5 7 9
Severity score
Fig. 6.6. F2phenotypic segregation for Umubano x G4795.
In summary, F2 populations that involved RWR719 had skewed continuous distribution (Table 6.19). This indicated the action of additive recessive resistance genes in these populations, with the possibility of more than one locus. Populations with MLB-49-89A tended towards resistance which indicated the presence of many additive resistance genes in this line. However, the distribution of the RWR719 x MLB-49-89A indicated that the number of susceptibility genes in this combination was greater than the number of resistance genes, hence the susceptible reaction in the F2. MLB-49-89A x G4795 showed two distinct classes of susceptible and resistant plants, probably indicating the presence of two loci governing resistance to FRR in these varieties. Crosses involving MLB-48-89A had continuous distribution, with some of crosses tending towards resistance while others tended towards susceptibility, thus indicating the presence of more than one locus. The cross MLB-48-89A x Umubano had a discontinuous distribution, with the appearance of two distinct separate classes of both resistant and susceptible plants in the F2 generation which indicated the presence of two loci (Table 6.19). All crosses involving Vuninkingi had continuous distributions, indicating the presence of several additive genes on different loci.
Crosses involving Umubano either tended towards resistance or towards susceptibility, depending on the parents indicating the presence of more than one locus. Crosses involving G4795 were continuous in nature, indicating the involvement of many additive genes; the exception was MLB-49-89A x G4795, the distribution of which discontinuous with two distinct classes of susceptible and resistant plants, indicating the presence of two loci governing resistance to FRR in these varieties (Table 6.19).
Table 6.19. Distribution of F2 populations used in testing allelism of resistance genes to Fusarium root rot.
Populations skewed Distribution Line
S R Continuous Discontinuous
RWR719 MLB-49-89A Vuninkingi Umubano G4795 MLB-49-89A RWR719 MLB-48-89A
Vuninkingi Umubano
G4795
MLB-48-89A MLB-49-89A RWR719 Vuninkingi G4795
Umubano
Vuninkingi Umubano MLB-49-89A RWR719 G4795
MLB-48-89A Umubano Vuninkingi
G4795 MLB-49-89A RWR719 MLB-48-89A
G4759 Umubano Vuninkingi
MLB-48-89A RWR719
MLB-49-89A