An alternative statistic for identifying lines useful for improving parents of an elite single cross

Theory and methods for identifying inbred lines (I_w) with favorable dominant alleles not present in an elite single cross (I₁×I₂) have been developed recently. Selected I_w lines can be crossed to I₁ or to I₂ to transfer new favorable alleles to the single cross. However, favorable alleles already present in the single cross may be lost during selection. It is important to consider both potential gain of favorable alleles from I_w and loss of favorable alleles already present in I₁×I₂. The "net improvement" statistic (NI)=maximum [(I₁×I_w-I₁x I₂)/2, (I₂×I_w-I₁×I₂)/2] estimates the number of loci where favorable alleles can be gained minus the number of loci where favorable alleles can be lost in the single cross. Because I₁×I₂ is constant in an experiment, the method reduces to choosing I_w lines with the best mean performance in combination with either I₁ or I₁. NI was compared to estimators previously proposed for identifying lines, namely: (1) minimally biased estimates (μG′) of favorable dominant alleles present in I_w but not in I₁ and I₂; (2) minimum estimate of an upper bound (UBND) on μG; and (3) predicted three-way cross (PTC) performance. Based on a set of maize (Zea mays L.) grain yield data, correlations among the four estimators were relatively high, but indicated that rankings of I_w lines vary with the particular estimator used. Rankings of three I_w lines based on the frequency of F₂ test crosses superior to B73×Mo17 were identical to rankings based on NI, but differed from rankings based on μG′, PTC, and UBND. NI also was the best predictor of the mean of the upper 10% ({ov x}_0.1) of (I₂×I_w) F₂×I₁ or (I₁×I_w) F₂×I₂ test crosses based on simulated data. Being a simple statistic highly correlated to {ov x}_0.1, NI may be useful in applied breeding programs.