Effect of recombination in the parent populations on the means and combining ability variances in hybrid populations of maize (Zea mays L.)

Recombination of selected genotypes plays a key role in plant breeding for generating new base populations. We investigated the influence of recombination in two parent populations on the means and combining ability variances of their hybrid population by (1) quantitative genetic theory and (2) experiments with maize. The two parent populations were founded by four early flint and four early dent inbred lines, respectively. Each population was studied in three generations: Syn-0, the four inbred lines themselves; Syn*-1, the six intrapool single crosses (SC); and Syn*-2, the three intrapool double crosses (DC). Four interpool hybrid populations were created: (1) all 16 SC and (2) all 36 DC were produced from generations Syn-0 and Syn*-1, respectively, (3) 168 biparental progenies (BIP) of type flint x dent (female x male), and (4) 168 BIP of type dent x flint were produced according to NC-design I with randomly sampled plants of generation Syn*-2. The half-sib and full-sib families obtained in this manner were evaluated for grain yield, dry matter concentration and plant height. According to theoretical results, differences in the population means of these hybrid populations indicate the presence of various types of epistasis. Changes in combining ability variances from SC to DC reflect different levels of parental inbreeding (F = 1 vs F = 0), whereas changes from DC to BIP only reflect the effects of recombination and are attributable to covariances between additive and dominance effects caused by linkage disequilibrium in the Syn-0 generations. The experimental results showed a significant decline in yield from DC to BIP due to a loss of gene combinations with favourable epistatic effects. Estimates of sigma(2)(GCA) attributable to flint or dent lines decreased or remained unchanged from SC to DC, but generally increased in the BIP populations. The consequences of these trends for developing improved interpool hybrids are discussed.