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Genetic Variances and Selection Responses in an Advanced Generation of a Hybrid of Widely Divergent Populations of Maize¹

A resonable approach to the utilization of exotic germplasm of maize (Zea mays L.) is through the development and improvement of composites of exotic and locally adapted maize populations. The effectiveness of conventional recurrent selection procedures relative to the large genetic variances anticipated has not been adequately investigated. A series of experiments was undertaken in a composite of two widely divergent populations, one of which is locally adapted. Experiments were designed to (1) carry out five cycles of full-sib family selection and S₁ progeny selection for yield, (2) evaluate the responses to the two selection methods, (3) characterize genetic variances for yield the composite population before and after selection, and (4) compare expected responses based on variance estimates with responses observed.

Full-sib family selection was conducted in duplicate, and the selection responses obtained were in close agreement. The rate of response over five selection cycles was slightly greater than those reported for locally adapted populations. Response to S₁ progeny selection was approximately 50% greater than responses to fullsib family selection.

Predictions of selection responses computed from estimates of genetic variances were too high when compared with realized responses. Selection resulted in a reduction of genetic variance by about one-third.

In view of the mean yield of the original composite and the rates of gain with selection, the implications with regard to utilization of semi-exotic composites in practical breeding programs are encouraging.

Key Words: Exotic germplasm • Repeatability of selection responses • Modified diallel analysis • Evaluation of selection response • Maize

² Professor of genetics, North Carolina State Univ., Raleigh, NC 27650, and research geneticist, USDA, SEA-AR, Iowa State Univ., Ames, IA 50011.

Received for publication September 2, 1980.