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Gene Effects Responsible for Inbreeding Depression in Autotetraploid Maize¹

Inbreeding depression was studied in autotetraploid maize (Zea mays L.). Experimental material included the original population, S₂ through S₄ generations derived by selling, and the sib 1 through sib 6 generations resulting from full-sib mating in an open-pollinated autotetraploid synthetic. The genetic model used was µ₁ = µ + 6Fß_H + (4P₀ + P₁)_iii + P_0iii + P_2iiii, where µ₁ is the mean of an inbred generation; µ is the mean of the noninbred generation; ß_ii, _iii, and _iiii are interaction effects resulting from the presence in a genotype of two, three, and four alleles, respectively, identical by descent; and _iiii is the interaction effect resulting from the presence of two pairs of alleles identical by descent. F is the inbreeding coefficient (probability that two alleles are identical by descent); P₀, P₁ and P₂ are the probabilities of obtaining genotypes with four alleles, three alleles, and two pairs of alleles identical by descent, respectively.

Least squares estimates of ß_ii, _{iii iiii}, and _iii,showed that for most characters, much of the depression could be accounted for by ß_ii and ß_iiii. These results were supported by multiple regression analyses ushig successively more complex models and measuring the sum of squares accounted for by each term in the model. It was concluded that the amount of inbreeding depression per unit of F in early generations of autotetraploids may differ for selfing and full-sib mating. The significance of the results in terms of expected inbreeding depression in autotetraploid species is discussed.

Key Words: Generation means • Full-sib mating • Gene interactions • Zea mays L.

² Assistant Professor, Department of Agronomy and Soils, Clemson University (formerly Graduate Assistant) and Professor of Plant Genetics in Agronomy, University of Illinois.

Received for publication August 2, 1973.

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