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Heterosis, Inbreeding, and Line Performance in Crosses of Adapted Spring Wheats¹

Eight adapted high-yielding spring wheat (Triticum aestivum L.) parents were crossed in a diallel to produce 28 hybrid populations. The F₁, F₂ through F₅ bulks, and a varying number of F₅ lines/cross were grown at two locations. The F₁ yields exhibited midparent heterosis from 5 to 58% ( = 20%) and highparent heterosis from _17 to 41% ( = 8%). Compared to the respective highest yielding line (HL) from each cross, F₁ yields ranged from 75 to 119% ( = 99.4%). The three highest yielding entries were F_l's even though the average HL and F₁ over all crosses were similar.

Significant general combining ability (GCA) mean squares for yield were noted in the combining ability analyses of the F₁, F₂ through F₅ bulks, HL, and the of F₅ lines from each cross (F₅L). Significant specific combining ability (SCA) mean squares were detected the analyses of the F, and F₅L. The significant SCA mean square for F₅L suggested the presence of additive x additive epistasis. The GCA effects generally were positively correlated across the various hybrid generations, which indicated consistent performance of the parents when measured by bulks or F₅ lines.

The regression analysis of F₁ and F₂ through F₅ bulk yields on heterozygosity detected a highly significant mean square (r² = 0.947). The mean yield of the 28 crosses decreased at a linear rate of 0.23% per 1% decrease in heterozygosity with a maximum rate of 0.4% in one cross. Even this maximum rate was less than the lowest reported in maize (Zea mays L.) suggesting lesser amounts of dominance genetic variance in these materials.

The significant F₁ heterosis and SCA for yield and the significant inbreeding depression indicated the possible desirability of F₁ hybrids to maximize yield in these materials. However, since no F₁ hybrid significantly outyielded the best F₅ line tested, it was unclear if increased yield would best be obtained via pureline or F₁ hybrid development.

Key Words: Natural selection • Triticum aestivum L. • Combining ability

² Former graduate research assistant (Present address: Cell Culture and Nitrogen Fixation Laboratory, Beltsville Agric. Res. Ctr.-West, USDA, Building 001, Beltsville, MD 20705) and profsesor, Agronomy Dep., North Dakota State Univ., Fargo, ND 58102.

Received for publication March 24, 1977.