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Hybrid breeding facilitates a maximum exploitation of heterosis. The objectives of our research were to (i) examine the magnitude of heterosis over the mid- and better parent for yield, yield components, and quality traits in Central European spring durum wheat and (ii) investigate the efficiency of parental selection based on mid-parent value or general combining ability effects. Sixteen inbred lines and 40 incomplete factorial crosses were field-evaluated for eight agronomic traits at 10 environments in Germany. For grain yield, the hybrids yielded on average 10% higher than the mid-parent performance, and the maximum superiority was 22%. The significantly positive contrast between inbreeding depression and mid-parent heterosis for grain yield indicated the presence of positive additive x additive epistatic interactions. Furthermore, the best hybrid outperformed the best line variety by 1.01 Mg [ha.sup.-1]. We conclude that there is a potential for hybrid breeding in durum wheat. doi: 10.2135/cropsci2009.10.0637

Heterotic groups are an essential prerequisite for efficient hybrid breeding. The present study in winter triticale (xTriticosecale Wittmack) was performed to (i) evaluate the magnitude of heterosis, (ii) investigate the efficiency of the prediction of hybrid performance based on midparent value or general combining ability (GCA)effects, and(iii)identify two heterotic groups by employing principle coordinate analysis and an enumeration algorithm for maximizing [F.sub.1] performance, midparent heterosis, and [[sigma].sup.2.sub.GCA]/ [[sigma].sup.2.sub.SCA] ratio for grain yield. Twenty-one inbred lines and their 210 diallel crosses were field-evaluated for grain yield at five agroecologically diverse locations in Germany. On average, the hybrids yielded 8.6% higher than the midparent performance, and the maximum superiority was 12.4%. Hybrid performance was predicted more reliably with GCA effects rather than with midparent performance. An enumeration algorithm based on optimum allocation of parents to heterotic groups, improved all three criteria. [F.sub.1] performance, midparent heterosis, and [[sigma].sup.2.sub.GCA]/[[sigma].sup.2.sub.SCA] ratio increased by about 3, 24, and 70%, respectively, as compared with the base situation of having no heterotic groups. These improvements substantiate the potential advantages, when heterotic groups are developed in hybrid breeding. doi: 10.2135/cropsci2009.04.0225

The objective of our study was to examine the influence of genetic divergence on the ratio of the components of variance for specific ([[sigma].sup.2.sub.SCA]) and general [[sigma].sup.2.sub.GCA]) combining ability using experimental data in triticale (xTriticosecale Wittm.). In total, 21 lines and their 210 crosses were evaluated for grain yield in field trials. Published molecular data were reanalyzed, indicating an optimum of two subgroups. The estimates of [[sigma].sup.2.sub.SCA] and [[sigma].sup.2.sub.GCA] were determined for the total diallel and between the two subgroups. The ratio of [[sigma].sup.2.sub.SCA] VS. [[sigma].sup.2.sub.GCA] tended to be lower for crosses between than within genetically distinct groups. Our experimental findings can be interpreted as an indicator of a more favorable ratio of [[sigma].sup.2.sub.SCA] vs. [[sigma].sup.2.sub.GCA] in situations with two genetically distinct populations than in situations with populations that are not genetically distinct. doi: 10.2135/cropsci2008.12.0689

CIMMYT is the source of improved maize (Zea mays L.) breeding material for a significant portion of the nontemperate maize growing world. Landraces which did not serve as sources for improved maize germplasm may contain untapped allelic variation useful for future breeding progress. Information regarding levels of diversity in different germplasm would help to identify sources for broadening improved breeding pools and in seeking genes and alleles that have not been tapped in modern maize breeding. The objectives of this study were to examine the diversity in maize landraces, modern open pollinated varieties (OPVs), and inbred lines adapted to nontemperate growing areas to find unique sources of allelic diversity that may be used in maize improvement. Twenty-five simple sequence repeat markers were used to characterize 497 individuals from 24 landraces of maize from Mexico, 672 individuals from 23 CIMMYT improved breeding populations, and 261 CIMMYT inbred lines. Number of alleles, gene diversity per locus, unique alleles per locus, and population structure all differ between germplasm groups. The unique alleles found in each germplasm group represent a great reservoir of untapped genetic resources for maize improvement, and implications for hybrid breeding are discussed.

Characterization of genetic diversity of maize (Zea mays L.) germplasm is of great importance in hybrid maize breeding. The objectives of this study were to (i) investigate genetic diversity in CIMMYT subtropical, tropical midaltitude and highland inbreds with simple sequence repeat (SSR) markers, (ii) identify appropriate testers for the development of new inbred lines, (iii) compare this sample to U.S. and European elite maize lines and CIMMYT tropical lowland inbreds, and (iv) use the marker and pedigree information as a guide to understanding the heterotic relationship among the CIMMYT maize lines (CMLs) and their potential practical use in maize breeding programs worldwide. Inbreds included in the study were assayed with 79 SSR markers. The CIMMYT inbred lines originated from 35 mostly broad-based populations and pools with mixed origins. A total of 566 alleles were scored, (averaging 7.2 and ranging from 2 to 16 alleles per locus). The modified Roger's distance (MRD) between pairs of inbreds averaged 0.78, with a range of 0.45 to 0.93. Unweighted paired group method using arithmetic averages (UPGMA) cluster analysis revealed no clear clustering. This reflects a mixed composition of CIMMYT subtropical, tropical midaltitude and highland maize populations and pools, and indicates that large amounts of variation have been incorporated into CIMMYT germplasm. Temperate heterotic groups were separated based on the markers, and nontemperate CIMMYT maize was genetically distinct from temperate lines. Discrete clusters were difficult to identify within and often between megaenvironments. Specific recommendations for nontemperate hybrid maize breeding are made., THE DEFINITION OF HETEROTIC GROUPS and heterotic patterns is an empirical task in hybrid maize breeding that has, in temperate maize germplasm, contributed to large increases in yield. Reciprocal recurrent [...]

A proper choice of a dissimilarity measure is important in surveys investigating genetic relationships among germplasm with molecular marker data. The objective of our study was to examine 10 dissimilarity coefficients widely used in germplasm surveys, with special focus on applications in plant breeding and seed banks. In particular, we (i) investigated the genetical and mathematical properties of these coefficients, (it) examined consequences of these properties for different areas of application in plant breeding and seed banks, and (iii) determined relationships between these 10 coefficients. The genetical and mathematical concepts of the coefficients were described in detail. A Procrustes analysis of a published data set consisting of seven CIMMYT maize populations demonstrated close affinity between Euclidean, Rogers', modified Rogers', and Cavalli-Sforza and Edwards' distance on one hand, and Net's standard and Reynolds dissimilarity on the other hand. Our investigations show that genetical and mathematical properties of dissimilarity measures are of crucial importance when choosing a genetic dissimilarity coefficient for analyzing molecular marker data. The presented results assist experimenters to extract the maximum amount of information from genetic data and, thus, facilitate the interpretation of findings from molecular marker studies on a theoretically sound basis., QUANTIFYING THE DEGREE of dissimilarity among genera, species, subspecies, populations, and elite breeding materials is of primary concern in population genetics and plant breeding. Before 1970, measures of genetic dissimilarity [...]

Detailed knowledge about the genetic diversity among germplasm is important for hybrid maize (Zea mays L.) breeding. The objectives of our study were to (i) investigate the genetic diversity among tropical lowland inbred lines and (ii) delimit heterotic groups in this germplasm. Simple sequence repeat (SSR) analysis of 79 markers distributed uniformly throughout the maize genome was performed for 155 tropical lowland inbred lines from the International Maize and Wheat Improvement Center (CIMMYT). Inbred lines were extracted from 60 broad-based populations and pools with mixed origin. We observed an average of 7.4 alleles per marker with a range from 2 to 18. The polymorphic information content (PIC) of the 79 SSRs ranged from 0.13 to 0.87, with an average of 0.60. Cluster analysis of the tropical yellow and white lines revealed a lack of structure within this germplasm, which can be explained by the mixed origin of the populations used to extract these lines and the specific choice of testers for reciprocal recurrent selection (RRS). An SSR analysis can support the choice of representative testers for evaluating inbred lines in such broad-based populations. Thus, a more complete exploitation of the outstanding flint-dent heterotic pattern is possible., FEW AGRONOMIC IMPROVEMENTS during the 20th century rival the development of hybrid maize (Brummer, 1999). Yields in maize increased dramatically as breeders moved away from open-pollinated cultivars and began developing [...]

Genetic diversity in maize (Zea mays L.) plays a key role for future breeding progress. The main objectives of our study were to (i) investigate the genetic diversity within and among CIMMYT maize populations by simple sequence repeat (SSR) markers, (ii) examine genotype frequencies for deviations from Hardy-Weinberg equilibrium (HWE) at individual loci, and (iii) test for linkage disequilibrium (LD) between pairs of loci. Twenty-three maize populations and pools established in 1974, which mostly comprise germplasm from different racial complexes adapted to tropical, subtropical intermediate-maturity, subtropical early-maturity, and temperate megaenvironments (ME), were fingerprinted by 83 SSR markers covering the entire maize genome. Across all populations, 27% of the SSR markers deviated significantly from HWE with an excess of homozygosity in 99% of the cases. We observed no evidence for genome-wide LD among pairs of loci within each of the seven tropical populations analyzed. Estimates of genetic differentiation ([G.sub.ST]) between populations within MEs averaged 0.09 and revealed that most of the molecular variation was found within the populations. Principal coordinate analysis based on allele frequencies of the populations revealed that populations adapted to the same ME clustered together and, thus, supported clearly the ME structure., GENETIC DIVERSITY in maize is a valuable natural resource and plays a key role for future breeding progress. Germplasm collections as a source of genetic diversity must be well characterized [...]

Heterotic groups and patterns are of fundamental importance in hybrid breeding of maize (Zea mays L.). The major goal of this study was to investigate the relationship between heterosis and genetic distance determined with simple sequence repeat (SSR) markers. The objectives of our research were to (i) compare the genetic diversity within and between seven tropical maize populations, (ii) test alternative hypotheses on the relationship between panmictic midparent heterosis (PMPH) and genetic distances determined with SSR markers, and (iii) evaluate the use of SSR markers for grouping of germplasm and establishing heterotic patterns in hybrid breeding of tropical maize. Published data of a diailel of seven tropical maize populations evaluated for agronomic traits in seven environments were reanalyzed to calculate PMPH in population hybrids. In addition, 48 individuals from each population were sampled and assayed with 85 SSR markers covering the entire maize genome. A total of 532 alleles in the 7 x 48 genotypes assayed were detected. The analysis of molecular variance (AMOVA) revealed that 89.8% of the variation was found within populations and only 10.2% between populations. The correlation between PMPH and the squared modified Roger's distance (MRD) based on SSR markers was significantly positive (P < 0.05) only for grain yield (r = 0.63). With SSR analyses, it was possible to assign Population 29 (Pop29) to the established Heterotic Group A and propose new heterotic groups (Pop25, Pop43). We conclude that SSR markers provide a powerful tool for grouping of germplasm and are a valuable complementation to field trials for identifying groups with satisfactory heterotic response., GENETIC DIVERSITY in maize plays a key role for future breeding progress. The development of molecular markers provides a tool for assessing the genetic diversity at the DNA level in [...]