Temporal changes in allele frequencies in two reciprocally selected maize populations

The effects of breeding on allele frequency changes at 82 restriction fragment length polymorphism (RFLP) loci were examined in two maize (Zea mays L.) populations undergoing reciprocal recurrent selection, Iowa Stiff Stalk Synthetic and Iowa Corn Borer Synthetic #1. After 12 cycles of selection, approximately 30% of the alleles were extinct and 10% near fixation in each population. A test of selective neutrality identified several loci in each population whose allele frequency changes cannot be explained by genetic drift; interpopulation mean expected heterozygosity increased for that subset of 28 loci but not for the remaining 54 loci. Mean expected heterozygosity within the two subpopulations decreased 39%, while the between-population component of genetic variation increased from 0.5% to 33.4% of the total. Effective population size is a key parameter for discerning allele frequency changes due to genetic drift versus those resulting from selection and genetic hitchhiking. Empirical estimates of effective population size for each population were within the range predicted by the breeding method

Hardy-Weinberg and linkage equilibrium estimates in the BSSS and BSCB1 random mated populations

Because maize (Zea mays) is an annual species those working with it must frequently make crosses to preserve and periodically maintain populations. Random mating is performed either using hand-pollination techniques or in wind-pollinated isolated blocks. Eighty-two restriction fragment length polymorphism (RFLP) markers were used to examine samples of random mated, hand-pollinated BSSS(R) and BSCB1(R) maize populations to find out whether their genotypic proportions conformed to predicted outcomes of random mating. The majority of loci conformed to expectations for Hardy-Weinberg equilibrium (HWE). Excess homozygosity was observed at 87% of the loci where the null hypothesis of HWE was rejected. For pairs of polymorphic loci, linkage equilibrium was observed in the BSSS(R) and BSCB1(R) progenitor populations (fewer than 5% of all tests rejected the null hypothesis of equilibrium at the P≤0.05 significance level). The BSSS(R)CO, BSCB1(R)CO and BSCB1(R)C12 populations showed slight increases in the proportion of pairs of loci in linkage disequilibrium compared to the progenitors (approximately 8.4% of all pairs of loci rejected the null hypothesis at the P≤0.05 significance level). BSSS(R)C12 was an extreme outlier with 25.0% of all pairs of polymorphic loci displaying significant (P≤0.05) linkage disequilibrium. This result was likely caused by the artificial grouping of three BSSS(R)CO plants with 97 BSSS(R)C12 plants during sampling. Results from principal components analysis of all individuals based on RFLP alleles supported this interpretation. Overall, most of the observed deviations from equilibrium were likely to have been caused by positive assortative mating in the case of HWE, and natural selection for epistatic effects between unlinked loci in the case of linkage disequilibrium

Population Genetics of Increased Hybrid Performance between Two Maize Populations under Reciprocal Recurrent Selection

Heterosis, the superiority in one or more characteristics of crossbred organisms relative to their inbred parents, is the basis of the modern cultivars utilized in maize (Zed mays L.). Heterosis is of interest in nondomesticated species due to its relevance to the question how much polymorphism is maintained in natural populations due to selection? (Berger, 1976). For maize and certain other domesticated species that employ inbred lines to produce commercial hybrids, knowledge of the mechanisms of gene action producing heterosis could contribute to advances in breeding techniques

Response of Continuous Maize with Stover Removal to Living Mulches

Constraints to maize (Zea mays L.) stover biomass harvest may be mitigated by using a living mulch (LM) to offset C exports and control soil erosion. Living mulches can compete with the main crop for resources. The objective of this research was to quantify competitive effects of LM management systems grown in continuous maize with stover removal. Maize was planted into creeping red fescue (CF) (Festuca rubra L.), Kentucky bluegrass (KB) (Poa pratensis L.), and a mixture of CF and white clover (Trifolium repens L.) (MX) LMs in 2008, 2009, and 2010 near Ames, IA. Management treatments were fall strip-tillage (ST) and no-tillage (NT), with either a pre-planting paraquat burn-down followed by two glyphosate bands (PQ) or glyphosate bands only (GLY). Kentucky bluegrass PQ ST produced similar grain yields (11,230 kg ha−1) all 3 yr as the no LM control (11,810 kg ha−1) with a harvest index (HI) of 0.55 compared to 0.52 in the control, averaged across years. The control produced greater stover dry matter (SDM) (10,110 kg ha−1) 2 of the 3 yr compared to KB PQ ST (8600 kg ha−1). Total groundcover averaged 80% in KB PQ ST compared to only 45% in the no LM control. These results indicate that a combination of herbicide suppression and ST suppresses LMs adequately to maintain competitive maize grain yields. Additional research under varying climatic conditions will further quantify the risk of LM management systems to increase the sustainable stover harvest of maize biomass feedstocks

DNA methylation in diploid inbred lines of potatoes and its possible role in the regulation of heterosis

Self-incompatible diploid potatoes were altered to self-compatible ones by a function of S-locus inhibitor gene and continued selfing generated highly homozygous inbreds. In this study, this process was investigated for the status of DNA methylation by a simple method using genomic DNA digested by methylation-sensitive restriction enzymes prior to RAPD analysis. We detected 31 methylation-sensitive RAPD bands, of which 11 were newly appeared in the selfed progenies, and 6 of them stably inherited to subsequent generations. Aberrant segregations and paternal- or atavism-like transmission were also found. Segregating methylation-sensitive bands in initial populations became fixed in the advanced selfed progenies by 75.0–93.8%, of which 41.7% were fixed to all present and 58.3% to all absent. Because DNA methylation is generally recognized to suppress gene expression as regulatory factors, homozygosity/heterozygosity of methylated DNA may be involved in inbreeding depression/heterosis

Quantitative Genetics of Heterosis

Nearly 50 years have elapsed since the seminal heterosis conference was held at Iowa State College (Gowen, 1952). That conference undoubtedly grew out of the obvious importance of maize (Zea mays L.) hybrids in the agricultural economy of Iowa and the USA as well as the lack of understanding of the phenomenon of heterosis. Farmers in Iowa rapidly adopted maize hybrids. In just 15 years, Iowa went from 0 to 100% of the maize acreage being planted to hybrids. Gowen (1952) stated the following about hybrid maize "It seems likely that in no other period of like years has there been such an increase in food produced over so many acres of land. The return from hybrid corn has been phenomenal, but it is now evidently approaching an asymptotic value." If only Gowen could have looked ahead 50 years, because the best was yet to come (Fig. 4-1).This proceeding was published as Lamkey, K. R. and J. W. Edwards. 1999. Quantitative genetics of heterosis. p. 31-48. In: J.G. Coors and S. Pandey (ed.) Proceedings of the International Symposium on the Genetics and Exploitation of Heterosis in Crops, CIMMYT, Mexico City, Mexico, 17-22 Aug. 1997.</p

Evaluation of Populations for Their Potential to Improve Three Maize Hybrids

Identification of sources of favorable alleles to improve existing hybrids is one of the most important problems facing a maize (Zea mays L.) breeder. Previous work has demonstrated the effectiveness of a procedure developed by Dudley for identifying populations containing favorable alleles not present in an elite hybrid. However, previously reported work involved at most two elite hybrids. The objective of this study was to evaluate the potential of 20 improved populations to improve the three hybrids made from three inbreds in commercial use. Each of the populations was crossed to LH195, LH212, and LH216. The population × inbred crosses, the three hybrids among the inbreds, and the three inbreds were evaluated in seven U.S. midwestern environments in 1993 and four in 1994. Traits measured were grain yield, grain moisture, plant height, ear height, and concentration of protein, oil, and starch in the grain. For grain yield, 15 of the 20 populations had significant estimates of dominant favorable alleles not present in the highest yielding target hybrid (LH195 × LH212). None of the populations showed potential for reducing ear height. However, seven populations had more favorable recessive alleles than unfavorable dominants for plant height when LH195 × LH212 was the target hybrid. None of the populations tested appeared to have potential for increasing starch concentration in any of the target hybrids. Eight populations showed potential for increasing protein concentration in all three target hybrids. Assumptions required to identify parents were not met for grain moisture, oil concentration, and stalk and root lodging.This article is published as Dudley, J. W., K. R. Lamkey, and J. L. Geadelmann. "Evaluation of populations for their potential to improve three maize hybrids." Crop science 36, no. 6 (1996): 1553-1559. doi: 10.2135/cropsci1996.0011183X003600060025x. Posted with permission.</p