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

Abstract

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 $\bar T_i = \left( {{1 \mathord{\left/ {\vphantom {1 m}} \right. \kern-\nulldelimiterspace} m}} \right)\sum {\left( {T_{ij} } \right)}$ 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 $\bar T_i$ 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 $\bar T_i$ statistics to identify germplasm strains carrying rare favorable alleles should be assessed in empirical studies.