Abstract
Natural selection is used by plant breeders to increase the frequency of desirable genotypes in a population. When soybean [Glycine max (L.) Merr.] populations segregating for resistance to soybean cyst nematode (SCN) (Heterodera glycines Ichinohe) are grown on SCN‐infested soil, resistant genotypes produce more seed than susceptible genotypes. In a bulk‐harvested population this differential reproduction results in an increased frequency of resistant genotypes in the population. This research was conducted to determine the effects of tolerance to SCN and the frequency of resistant genotypes in the population on the effectiveness of natural selection for resistance to SCN. A second objective was to relate initial SCN population densities and parental pure‐stand yield ratios to the effectiveness of natural selection. Four 1:1 viable seed mixtures of resistant (R) and susceptible (S) genotypes were evaluated in four SCN‐infested fields for 2 yr. Two of the S genotypes were moderately tolerant and two were intolerant to SCN. The R genotypes in mixtures with moderately tolerant, S genotypes had an average reproductive advantage (ratio of R/S seed number) of 2.2 compared to 4.2 for R genotypes in mixtures with intolerant, S genotypes. In a second experiment, five ratios, 1:9, 3:7, 1:1, 7:3, and 9:1, of R to S genotypes were compared for two of the genotypic mixtures (‘Gordon’/‘ Bragg’ and Gordon/‘Wright’). The effectiveness of natural selection was not related to the frequency of the R genotype in the mixture. On an individual‐plot basis across the four mixtures the linear regression of the pure‐stand R/S yield ratios predicted 47% of the variation in reproductive advantage of R genotypes compared to 22% for log10 of initial density of SCN eggs. Effectiveness of natural selection for SCN resistance was dependent on the initial population density of SCN, the specific environmental conditions, and the tolerance level of the genotypes in the soybean population.
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