Allozyme frequencies, heterozygosity and genetic distances following S-1 recurrent selection in two synthetic maize populations

Three cycles of S-1 recurrent selection for yield were carried out in two synthetic maize populations, EPS6 from humid Spain and EPS7 from arid Spain. One-hundred S-1 lines were evaluated from each cycle of selection and the ten highest-yielding S-1 lines were recombined to produce the next cycle. Changes in variability and genetic distances in two synthetic maize populations, following three cycles of recurrent selection, recombining ten S-1 lines in each cycle, were determined. Isozyme analysis was performed on 125 seedlings per cycle of selection (four cycles in each of two populations). Regressions of each allozyme frequency on cycles of selection were performed, genetic distances between populations were determined, and simple correlations between genetic distances and heterosis were calculated. The average heterozygosity per locus was also calculated for each population. Regression analysis did not reveal any common trend between EPS6 and EPS7 for changes in allele frequencies presumably due to selection. The number of polymorphic loci, the mean alleles per locus, and the mean heterozygosity did not show any reduction in variability. Finally, selection did not affect genetic distances among cycles of selection. The agronomic evaluation of the selection program, after three cycles of selection, revealed that the genetic variance was not significantly reduced for most traits, and that the heterosis among cycles of selection of both populations had not changed significantly. The conclusions based on isozyme data supported the deductions made from agronomic data. Three cycles of selection neither caused relevant changes on variability nor on genetic distance among cycles of selection of both maize synthetic populations. These data did not indicate any basis for increasing the number of S-1 lines recombined for recurrent selection.