MAINTENANCE OF TROPICAL ISOZYME ALLELES DURING RANDOM MATING IN A SEMIEXOTIC MAIZE COMPOSITE

Information on the maintenance of tropical germplasm in temperate sweet corn x tropical maize composites would influence decisions about the number and geographic location of random mating cycles to facilitate gene flow before recurrent selection for temperate-zone adaptation. This study was conducted to determine if tropical germplasm is lost during random mating as a result of natural selection in a temperate environment. Frequencies of unique tropical isozyme alleles were estimated for the four initial cycles of random mating following the intercrossing of 11 temperate sweet corn inbred parents with 17 tropical maize parents. Variations in the frequencies of two of nine alleles of tropical origin were significantly (P < .05) greater than could be attributable to genetic drift alone. However, only allele Glu 1-9, contributed by the Mexican parent Elotes Conicos (Comp. Gto. Gpo. 13), appeared to have a significant (P < .05) linear decrease in frequency over cycles. Directional natural selection did not have a large impact on the five genomic regions marked by segregating isozyme loci, reflecting conservation of tropical germplasm during the early years of adaptation. Tropical germplasm in temperate x tropical maize composites can be maintained even when the composite is random mated in a temperate environment.