A study was made of the genetics of resistance to the soybean …

• Abstract
• Introduction
• Materials and Methods
• Results and Discussion
• References
• Table 1

The means, standard errors and variances for the number of cysts of H. glycines (race 3) found on parents and generations F₁ and F₂ for each generation of the cross between the soybean genotypes BR 90-4722 and FT-Cristalina were determined (Table I). The standard errors for all the means were low, suggesting reasonable precision for the estimates. The largest variance estimate, as expected, was found for the F₂ generation; on the other hand, the homogeneous generations had low estimates for their variances.

The resistant parent (BR 90-4722), as expected, had the lower mean number of cysts, while the FT-Cristalina (standard of susceptibility) had the largest and the F₁ generation had an intermediary mean number (Table I). In terms of genotypic values, estimated according to Falconer (1989), the susceptible parent had a value of 9.2 (a), the resistant parent -9.2 (-a) and the segregating population -2.8 (d). These results show that the F₁ generation, though it was classified as susceptible, had a degree of resistance only 30% inferior to the resistant parental.

Comparisons among observed and expected means for F₁ and F₂, between F₁ and F₂ and between the resistant parent and the F₁ were significant by the t-test at the level of 5% of probability. These results suggest that there was no additivity, complete dominance or complete recessivity in connection with the resistance of the studied population to SCN. Instead, these results reinforce the hypothesis that the reaction of resistance in this population is controlled by dominant and recessive genes, with a greater expression of the dominant gene.

The data (Table I) also provided an estimate for the number of genes related to resistance of the population to SCN. Only three genes were found to be related to resistance. The heritability of the character, according to Mahmud and Kramer (1951), was 0.96, which suggests a low magnitude for the environmental component and a probable qualitative nature for the character. These results also show that selection within this population for the development of resistant soybean cultivars is feasible and can easily be achieved. Caviness and Riggs (1976) also estimated the heritability for the resistance of soybean to SCN; however, they found lower values in their estimates.

The parental, F₁ and F₂ generations were classified as a function of the inoculations performed and the results were submitted to the chi-square test (Table II). All plants of the parents BR 90-4722 and FT-Cristalina were classified, respectively, as resistant and susceptible to SCN, showing their suitability for this study. As the F₁ generation is genetically homogeneous, only four individuals were analyzed and all four were classified as susceptible to the nematode, however with a degree of resistance superior to that observed for the susceptible parent (Table II). Only seven individuals of the F₂ generation were resistant to SCN, while 121 were classified as susceptible to the nematode.

Considering that the resistance of soybeans to SCN (race 3) is controlled by one dominant and two recessive genes, the expected ratio for individuals resistant:susceptible in the F₂ generation is 3:61. There were no significative differences, by the chi-square test at the level of 5% of probability, between the observed and expected ratios (Table II). This confirms the results obtained in the previous analysis and shows that the studied population has the same genetic control for resistance to SCN, race 3, as do the North American soybean genotypes, e.g., one dominant and two recessive genes (Myers and Anand, 1991; Rao-Arelli et al., 1992).

Based on these results, it can be concluded that the resistance to SCN, race 3, in the population derived from crosses between the Brazilian soybean genotypes BR 90-4722 and FT-Cristalina is controlled by three genes, one dominant and two recessive. The character has a high heritability (0.96) and a qualitative nature. The environmental component of the variation, under the conditions of the present study, was of low magnitude. Therefore, the population is suitable for the development of new soybean cultivars resistant to the soybean cyst nematode.

ACKNOWLEDGMENTS

The authors are thankful to FAPESP for supporting this research, to CAPES for the doctoral fellowship awarded to A.L. de Oliveira and to Dr. Neylson Eustáquio Arantes (Embrapa/CNPS) for valuable suggestions. Publication supported by FAPESP.