Responses and allelic frequency changes associated with recurrent selection for plant regeneration from callus cultures in maize

Improving the regeneration capacity of the calli obtained from maize (Zea mays L.) immature embryos is an important breeding objective. Three cycles of recurrent selection for regeneration capacity were conducted in the source double cross (A188 x W64A) x (A634 x B79). Objectives of this study were to evaluate the selection responses for in vitro culture traits, the associated allelic frequency changes at RFLP loci, and the correlated responses for agronomic traits. Population C0 (obtained by intermating the source double cross) and selected populations (C1-C3) were tested together with parental inbreds. The response to selection was significant for in vitro culture traits and for regeneration capacity was equal to 41.6% cycle -1. Parental inbred A188 was the best genotype for regeneration capacity but upward transgressive segregation was observed within populations, particularly in C2 and C3. The RFLP analysis revealed that at six loci the allelic frequency changes followed a linear trend, suggesting an effect of directional selection. The favorable alleles at these loci had different origins. Correlated responses for agronomic traits were negative, especially for grain yield (-5.7% cycle -1) and plant height, while the within plot variance for plant height increased. The negative responses for agronomic traits were likely due to inbreeding depression, to possible somaclonal variation, and to the prevailing recovery of chromosomal regions of the parent (A188) with low agronomic performance.