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S₁-line Recurrent Selection for Cold Tolerance in Two Maize Populations¹

S₂-line recurrent selection for improved cold tolerance was evaluated after five cycles in two maize (Zea mays L.) populations, BSI3(SCT) and BSSS2(SCT). Changes in population means for percentage emergence, rate of emergence, seedling dry weight, and seedling vigor score were expressed in terms of changes in allelic frequency, allelic effects, and drift due to restricted population size. Selection resulted in significant genetic gains (2p) for percentage emergence (2.1% cycle^–1), seedling dry weight (0.04 g cycle^–1), and seedling vigor score (0.3 units cycle^–1 when averaged over both populations). Estimates of 2p were not significant for rate of emergence in either population. Genetic drift (2p²d) due to restricted population size was significant for seedling dry weight (0.01 g cycle^–1) and seedling vigor score (0.1 units cycle^–1), emphasizing the need for effective population sizes larger than those used in this study. Estimates of dominance effects were important for percentage emergence and seedling dry weight. Heterosis estimates indicated that BS13(SCT)CO and BSSS2(SCT)CO differed in allelic frequency. Selection increased frequencies of alleles with dominance effects controlling percentage emergence and seedling dry weight. Heterosis for percentage emergence and seedling dry weight decreased with selection. The data indicated that selection for cold tolerance would be effective with use of a selection index including percentage emergence and seedling dry weight, based on data from more than one year. Rate of emergence was not a dependable selection trait.

Key Words: Corn • Population improvement • Genetic gain • Direct response • Zea mays L.

² Former graduate assistant, Iowa State Univ., Ames, IA 50011; director of research, Garst Seed Company, Ames, IA 50010 (former assistant professor of agronomy, Iowa State Univ., Ames, IA 50011).

Received for publication January 14, 1985.