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Effects of Genetic Heterogeneity Within Two Soybean Populations. II. Competitive Responses and Variance Due to Genetic Heterogeneity for Nine Agronomic and Chemical Characters¹

Effects of genotypic heterogeneity within relatively homozygous lines of soybeans evaluated in three environments were essentially additive for most traits when averaged over populations of lines within a cross. This was related to equivalence of competitive advantages and disadvantages expressed by genotypes within these populations of heterogeneous lines; that is, competitive effects were compensatory.

Such additivity apparently held, on the average, for individual heterogeneous lines in the simpler agronomic and chemical characters. However, effects of heterogeneity on character development were non-additive in individual lines for yield, height, and seed size. Differential responses for characters and crosses were related to differences in the degree of heterogeneity within the lines. In general, interactions among relatively diverse genotypes conditioned more extreme responses, and interactions among relatively similar genotypes conditioned less extreme responses than expected, on the basis of additivity of genotypic effects.

Substantial over- and under-compensatory competitive responses occurred for yield, and competitive response was positively correlated with heterogeneous line performance in both crosses. Some implications of these responses, and potential biases introduced into bulk population breeding procedures as a result of their action, are discussed.

Key Words: Glycine max • Quantitative genetics • Compensation • Breeding

² Graduate Assistant and C.S.I.R.O. Student, Iowa Agr. Exp. Sta. (Present address: Department of Agriculture, University of Queensland, Brisbane, Australia); and Agronomist, Crops Research Division, ARS, USDA, Beltsville, Md. 20705.

Received for publication February 1, 1969.