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.
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.
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).
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.
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.