A testcross procedure for selecting exotic strains to improve pure-line cultivars in predominantly self-fertilizing species.

Methods for identifying germplasm carrying alleles with the potential to improve a particular single-cross hybrid have been proposed and discussed in recent years. There is a need for similar methods to be used in breeding crops for which pure-line cultivars, rather than hybrids, are the goal. The objective of this research was to develop a method to identify germplasm lines with the potential to contribute favorable alleles not present in a specified pure line or set of pure lines. Given a set of adapted pure lines (A 1, A 2 ..., A m) to be improved and a set of germplasm lines (P 1 P 2 ..., P f), the procedure consists of producing all f x m possible hybrids and evaluating them along with the parents. The testcross statistic T ij is defined by T ij=γ(F ij-A j)+(1-γ) (F ij-P i), where A j, P i, and F ij represent the performance of thej (th) adapted line, the i (th) germplasm line, and their hybrid, respectively. The statistic [Formula: see text] is the mean value of T ij over all adapted parents A j. If γ=(1/2)(1+d'), where d' = the mean degree of dominance, then T ij measures the potential for alleles from P i to improve A j and [Formula: see text] measures the potential for alleles from P i to improve the set A 1, A 2 ..., A m. Use of data on soybean and peanut hybrids published by other researchers suggests that the value assumed for d' has little effect on the P i chosen. The ability of the T ij and [Formula: see text] statistics to identify germplasm strains carrying rare favorable alleles should be assessed in empirical studies.