Showing papers in "Maydica in 2006"

Dissection of physiological processes underlying grain yield in maize by examining genetic improvement and heterosis

Abstract: Maize (Zea mays L.) grain yield in the USA has increased by about 100 kg ha-1 year-1 or 2% year-1 from the start of large-scale adoption of hybrids by maize growers in the late 1930s until the first decade of the 21st century and about 75% of the yield improvement has been attributed to genetic gain. Yield improvement has been associated with increased stress tolerance and het- erosis, but the nature of the enhanced stress tolerance re- mains elusive and heterosis does not appear to be related to rate of yield improvement per se since the introduction of commercial hybrids. Yield improvement is the result of changes in underlying physiological processes and we have examined the physiological component processes that are associated with genetic gain and heterosis in maize. Changes in two important processes appear to be involved in both genetic improvement in grain yield and in yield differences between hybrids and their parental in- bred lines. First, sustaining leaf photosynthesis during the grain-filling period appears to be a major contributor to increases in dry matter accumulation. Second, increased partitioning to the kernels during the sensitive period of kernel number determination appeared to be the major factor in the increase in kernel number. Two 'processes' that have influenced genetic improvement and heterosis differently are maximum leaf area per plant and harvest index. Genetic gain is not associated with a change in harvest index because the increase in kernel number and the increase in dry matter accumulation during the grain- filling period have been proportional, whereas in hetero- sis the increase in kernel number is much greater relative to increase in dry matter accumulation during the grain- filling period.

Changes in drought tolerance in maize associated with fifty years of breeding for yield in the US Corn Belt [Zea mays L.]

Abstract: Understanding the changes underlying past breeding progress may help to focus research efforts and accelerate future genetic gains. The major abiotic stress affecting maize production on a worldwide basis is drought. We addressed the improvements in drought tol- erance over a 50-year period of hybrid breeding by evalu- ating, under targeted stress conditions, a set of 18 Pio- neer-brand hybrids that had been released during the 1953-2001 period. Stress treatments were designed as overlapping windows of water deficit covering the pre- flowering to late grain filling development stages. Data were collected on grain yield, yield components and an- thesis-silking interval (ASI), and were analyzed using a linear mixed model approach. Genetic gain was mea- sured as the slope of the regression of the trait on the year of hybrid release. Significant, positive genetic gains of varying magnitude were observed for grain yield in all windows of stress evaluated. The largest genetic gains for grain yield were observed under conditions of full irriga- tion and severe flowering stress. ASI and barrenness, es- pecially under stress at flowering, were significantly re- duced by selection. Though flowering remains the most susceptible stage to drought in maize, selection has re- duced its negative effects, and susceptibility during early grain filling is now of similar importance in many modern hybrids. Yield under drought at flowering has more than kept pace with the increase in yield potential because of the emphasis breeders have placed on improved floral synchrony.

More on the introduction of temperate maize into Europe: large-scale bulk SSR genotyping and new historical elements

Abstract: The commonly accepted scenario for the spread of maize into Europe is a single introduction fol- lowed by a slow adaptation to temperate climates. With the purpose of clarifying the origins and modalities of maize introduction in Europe, we conducted an extensive survey of 275 maize populations from both American and European origins by using microsatellite (SSR) analysis on pools of individuals. Our data strongly support two major sources, one from the Caribbean and one from northeast- ern America, giving rise to most of the open pollinated varieties cultivated in Europe. A detailed historical analy- sis confirms a first introduction of maize in southern Eu- rope by Columbus, and suggests that introduction(s) of temperate northeastern American maize should have oc- curred at the beginning of the 16th Century at the time of Spanish or French expeditions. In addition, our results re- veal that maize varieties cultivated at middle latitudes in Europe likely resulted from hybridization between the southern and northeastern European varieties.

Grain composition and amino acid content in maize cultivars representing 80 years of commercial maize varieties

Abstract: In order to determine how modern hybrids have impacted grain composition and amino acid content of the corn crop, we characterized a set of cultivars that were widely grown in different eras from the 1920s through 2001. Grain composition exhibited clear trends with time, with protein decreasing and starch increasing. The effects of different plant densities were examined. The grain protein content of modern hybrids responds to plant density and environment differently than the protein content of older varieties. These differences are consistent with a model in which protein content is modulated by different growth conditions. These differences may ex- plain, in part, the mechanism by which modern hybrids maintain yield in different environments, i.e. reduction of protein content in stressful environments frees resources that are used to maintain yield. We examined the content of the nutritionally limiting essential amino acids lysine, methionine and tryptophan in grain of these cultivars. On a per tissue mass basis, the levels of these amino acids dropped with time while on a per protein basis, their lev- els were not significantly changed. We conclude that the development of modern hybrids has resulted in maize with reduced protein content, but the nutritional quality of this protein has not changed.

Genetic analysis of performance of maize inbred lines selected for tolerance to drought under low nitrogen

Abstract: Low soil nitrogen (low N) limits maize (Zea mays L.) production in sub-Saharan Africa (SSA). Knowl- edge of inheritance of tolerance to low N in inbred lines selected for drought tolerance would be useful for devel- oping hybrids adapted to low N conditions. The objectives of this study were to determine the (i) effect of zero, one and two doses of drought tolerance in inbred lines on per- formance of hybrids under low N, (ii) type of gene action conditioning performance under low N, and (iii) relation- ship between per se performance of parental inbred lines and their hybrids. Ninety-six hybrids were formed from 24 inbred lines using the Design II mating scheme. The parental lines and hybrids were evaluated under both low and high N in separate trials side by side during the rainy season at one location in Nigeria for two years. Most of the hybrids with at least one drought tolerant parent had higher yields than those having two drought susceptible parents. Mean squares for GCA-males, GCA-females and SCA effects were significant for grain yield under both low and high N, whereas their interactions with year was sig- nificant only for GCA-male effects under low N. Non-addi- tive gene action was slightly higher than additive gene ac- tion for grain yield under low N. Average heterosis for grain yield was 129% under low N and 114% under high N. Grain yield, plant and ear heights of inbred lines con- tributed positively to grain yields of hybrids. However, grain yields of inbred lines accounted for less than 15% of the total variation in grain yield among hybrids at each N level. Five of the 12 drought tolerant lines and two of the 12 susceptible inbred lines had significant positive GCA ef- fects for grain yield under low N. Use of inbred lines, such as KU1409, 4058, 1824 and 9432 with consistently positive GCA effects for grain yield under low-N in a breeding pro- gram would be desirable to further improve grain yield and associated traits for low N environments.

The development and importance of zein protein bodies in maize endosperm

Abstract: Research over the last fifty years has shown that the vitreousness of the maize kernel is influenced by the formation of zein protein bodies. Here we summarize what is known about the structure and importance of protein bodies, beginning with their first detailed charac- terization by Donald Duvick in the 1950s. We describe the proteins that compose zein protein bodies and ex- plain how available data describing zein gene expression and zein protein interactions suggest a model for their ini- tiation, expansion and structure. We also describe maize mutants with reduced kernel hardness and show that they can be explained as a result of a perturbation to zein pro- tein structure and its effect on protein body formation. However, there are other soft kernel mutants that appear to be unrelated to zein proteins, indicating that much re- mains to be learned about the factors influencing the tex- ture of mature maize endosperm.

Latin american maize project (lamp) and germplasm enhancement of maize (gem) project: generating useful breeding germplasm 1

Abstract: Finding and incorporating new maize genet- ic resources into productive varieties cannot be accom- plished without long-term sustained effort. The wide ge- netic diversity of the crop has been maintained in germplasm banks. Using this diversity in breeding pro- jects is difficult and has met with little success in the past. In 1987, a 5-year maize evaluation project named the Latin American Maize Project (LAMP) used coordinated national and international efforts to evaluate many maize accessions. Most selected LAMP germplasm is unadapted to temperate locations and requires a long-term approach of conversion and selective adaptation to the major corn growing areas of the U.S. To accomplish this, an unprece- dented public/private research effort to utilize the genetic diversity from LAMP and other elite exotic sources was initiated as the Germplasm Enhancement of Maize project (GEM). This project is a unique example of collaboration in which public entities and private seed companies work together with the objective of increasing the productivity and genetic diversity of maize grown in the U.S. The re- sults indicate that alleles from exotic sources can be com- bined with genes of elite Corn Belt lines to generate ma- terials useful in breeding programs. We think the ap- proaches taken by LAMP and GEM solved, in part, some of the greatest difficulties experienced by plant breeders using exotic germplasm for improving quantitative traits.

Multivariate analyses of the genetic diversity of forty-seven striga resistant tropical early maturing maize inbred lines

Abstract: Grouping of inbred lines on the basis of phenotypic values of genetic traits is highly desirable in maize breeding in designing guidelines for use in devel- oping heterotic pools, populations, and synthetic vari- eties. Forty-seven Striga resistant, early maturing inbred lines derived from five different source materials were evaluated during the 2000 rainy season and the 2001 dry season at Ferkessedougou in Cote d'Ivoire under artificial Striga infestation. Principal component analysis (PCA) and the Ward's minimum variance cluster analysis of the inbred lines based on the similarity of quantitative charac- ters produced four major clusters. The four clusters de- rived from the Ward's cluster procedure showed remark- able correspondence with the PCA. The grouping of the inbred lines was independent of the genetic background from which they were derived or the endosperm color, indicating that the inbred lines had diverse genetic back- grounds and could be used to develop broad-based pools and populations. Based on the results of this study, it may be predicted that superior specific hybrid combinations could be obtained by crossing inbred lines between rather than within clusters. Eight promising inbred lines were identified that combined high grain yield with re- duced Striga damage symptom and numbers of emerged Striga plants. These could be used as sources of Striga re- sistance for the development of synthetics and for use in the national maize breeding programs in West and Central Africa (WCA).

Past and prospects of forage maize breeding in europe. ii. history, germplasm evolution and correlative agronomic changes

Abstract: Although maize was early recognized as an excellent forage plant soon after its introduction in Eu- rope, during a long time it was only bred for grain traits. However, the first recommendations of maize varieties for specific forage use are probably those given in the French VILMORIN-ANDRIEUX catalogues as early as the second mid of the 19th century. The 1940 Dutch variety list distin- guished several types of maize varieties and was already recommending three varieties for silage use. Whereas US hybrids were introduced in Europe in the early 1950s, the significant extension of silage maize cropping began after the release of early flint x dent hybrids such as INRA258 (1958) and a little later Brillant DK202, Capella, LG11, and Blizzard G188 (between 1965 and 1975). The increase went on until 1990, with a decrease or stabilization fol- lowing. The first generation of early European maize hy- brids was mostly often based on crosses between flint La- caune and dent Minnesota13 lines. The registration of Dea (1980) in France and a few years later Golda in Ger- many both illustrated tremendous changes in maize dent, and to a lesser extent flint, germplasm and marked the onset of a second era in European maize hybrid breeding. Iodent and BSSS origins were thus substituting for Min- nesota13. Correlatively, the era of 1980s was also marked by significant improvement of hybrid earliness. The actual maize breeding is characterized by a significantly greater introgression of medium late germplasm into early dent and flint maize lines. The beginning of this last period may be dated by the registration of Banguy (1992). The average genetic improvement in whole plant yield was close to 0.10 t/ha.year during the period between 1958 and 1988, but reached 0.17 t/ha.year between 1986 and 2004. In early maize, highly significant improvements of stalk standability, stalk rot and lodging resistance have been achieved between 1950 and 2004 in Europe. Physio- logical changes associated to these improvements are at least delayed senescence of leaves and stems, higher grain filling rate, and higher stress tolerance. Conversely to agronomic value, a steady decline in the average cell wall digestibility of hybrids was observed since the 1950s, and maize of the next future have to give a better balance between agronomic and feeding value traits.

The Gametophyte-1 locus and reproductive isolation among Zea Mays subspecies

Abstract: Fourteen paired populations of annual teosinte and an associated maize landrace were character- ized for allelic composition of the potential crossing barri- er locus ga1. The four populations of subspecies parvig- lumis teosinte carried predominately ga1 or Ga1-m, both of which are receptive to ga1 pollen. The sole population of subspecies huehuetenangensis was polymorphic for Ga1-m and for Ga1-s, which is unreceptive to ga1 pollen. Ga1-s predominated among the six weedy populations of subspecies mexicana whereas the three ruderal/weedy populations of this subspecies contained ga1 or Ga1-m. This distribution suggests a role for Ga1-s enabling weedy populations to grow in intimate association with maize. However, the predominant allele in sympatric maize landraces proved to be Ga1-m, which regularly fer- tilized Ga1-s in controlled crosses. The Ga1-s pistil barrier thus would seem unable to prevent teosinte from being fertilized by sympatric maize. Nevertheless, Ga1-s pollen fertilized Ga1-s Ga1-s plants somewhat more effectively than Ga1-m when both were present. This preference may be strengthened by modifier gene differences be- tween teosinte and maize, thereby providing partial re- productive isolation between the two.

Effects of long-term selection on seedling traits and ABA accumulation in commercial maize hybrids

Abstract: Retrospective studies investigating historical series of the best genotypes released in the past decades can provide important clues to evaluate the changes asso- ciated with the breeding work and to gain a better under- standing of the involved physiological processes. This study, using hydroponics, evaluated changes in seedlings of a historical series of maize (Zea mays L.) hybrids re- leased in the U.S. Corn Belt from 1930 to 2000 and previ- ously evaluated for agronomic traits and yield in field tri- als. We investigated to what extent the past decades of breeding have been accompanied by changes in a num- ber of morpho-physiological characteristics of the root and shoot of seedlings, including accumulation of abscisic acid (ABA) in response to conditions of water deficit arti- ficially induced through the addition of polyethylene gly- col (PEG) to the growing solution in hydroponics. A sig- nificant, negative linear trend was associated with the year of release for most of the seedling root and shoot traits, thus indicating that the hybrids' mean values for such traits decreased progressively over the years. A sig- nificant linear decrease across years was also detected for the ABA concentration in the seedling leaf measured 24 hours after PEG treatment. A number of factors that could account for the observed historical trends in seedling morphology and physiology are discussed.

Phylogeography of the wild subspecies of zea mays

Abstract: Z. mays ssp. parviglumis and ssp. mexicana are the closest wild relatives to domesticated maize. Using isozyme and chloroplast evidence, this study examined how populations of these subspecies are related to one another, and how geography has structured the relation- ships between them. As some lines of evidence indicate that ssp. mexicana is a derived clade of ssp. parviglumis, dispersal, isolation by distance (IBD), and altitudinal hy- potheses were tested to explain the genetic differentiation of ssp. mexicana and parviglumis populations. Simple dispersal hypotheses explained most of this genetic varia- tion, while IBD and altitudinal models explained very lit- tle of the variation. The origin of this dispersal appeared to be the middle and lower elevation regions of Guerrero. These dispersal events are discussed in light of Late Pleis- tocene and Holocene climate change and maize domesti- cation.

Temporal trends in ssr allele frequencies associated with long-term selection for yield of maize 1

Abstract: In association with improved grain yield and agronomic performance of maize hybrids developed by Pioneer Hi-Bred for the central U.S. corn-belt, there have been detectable changes in the organization of ge- netic diversity. Genetic diversity was measured using 1698 Simple Sequence Repeat (SSR) markers distributed over the 10 chromosomes of maize. A subset comprising of 361 SSRs was used to assay the 88 inbred parents of a se- quence of 53 successful maize hybrids released by Pio- neer Hi-Bred from 1934 to 2004. Patterns of change in SSR allele frequencies over the history of the breeding program are complex and indicative of a large open breeding system. Many of the temporal trends for individ- ual SSR alleles over the history of the breeding program are unique and likely result from the combined influences of many factors that operated during the history of the breeding program. Both random sampling of alleles and selection within the pedigree relationships of the germplasm, from the founding ancestors to the modern inbreds, can explain some of the trends in genetic diversi- ty. In addition, new germplasm with unique genetic di- versity was introduced into the breeding effort at different times. Selection has been applied to organize the germplasm into heterotic groups with unique allele com- binations at the SSR loci and to increase the frequencies of some alleles within the heterotic groups. The modern commercial maize hybrids of today combine the diversity between the heterotic groups and enable the deployment of this diversity within the U.S. corn-belt.

Genetic analysis of traits correlated with maize ear length

Abstract: Ear length is a component of maize (Zea mays L.) grain yield. Thirty generations of selection for in- creased ear length, however, failed to increase grain yield in the Iowa Long-Ear Synthetic (BSLE). Negative correla- tions between ear length and other yield-related traits complicated selection for grain yield. This investigation was conducted to map quantitative trait loci (QTL) for grain yield and traits correlated with ear length. A popula- tion developed from inbreds divergent for ear length (de- rived from the long-ear and short-ear generation 24 sub- populations of BSLE), and previously used to map ear length QTL, was used for this investigation. Genotypes and phenotypes of 188 F 2 plants and their F 2:3 progeny replicated twice in four environments were used for QTL analyses. More QTL were mapped for kernel-row number (10 in the F 2 ; 12 in the F 2:3 ) and kernel depth (7; 6) than in prior studies. QTL in the F 2:3 explained more than 50% of the kernel-row number and kernel depth variation, and most alleles had additive effects. Only three QTL in each generation were mapped for grain yield. Collectively, 52% of the kernel-row number, kernel depth, and grain yield QTL mapped in the F 2:3 were previously identified in the F 2 . The number of coincidental QTL followed the trends of heritability. Genetic regions associated with trait corre- lations were identified. The cluster of QTL on chromo- some 5 exemplified the genetic basis for the failure of ear length selection to increase grain yield in BSLE because of repulsion-phase linkage between QTL of the two traits. QTL on chromosome 6 may partially explain the positive correlation between ear length and grain yield.

Effect of random mating on marker-qtl associations for grain quality traits in the cross of illinois high oil x illinois low oil 1

Abstract: Altered concentrations of oil, protein or starch in maize (Zea mays L.) kernels can provide value- added products. Progeny developed from the cross be- tween the long-term divergently selected strains, Illinois High Oil (IHO) and Illinois Low Oil (ILO), cycle 70, were used to detect quantitative trait loci (QTL) for protein, starch, and oil concentration, and kernel weight. Recom- binant inbred lines (RIL) were derived from an F 2 created from one cycle of random mating the F1 (RM1) or after four cycles of random mating the RM1 (RM5). Both per se and corresponding testcrossed lines (TC1 and TC5) were evaluated in four environments. Genetic variance was lower in the testcrosses than in the per se progenies for all traits. Random mating was associated with a reduction in genetic variance for oil concentration between TC1 and TC5. Recombination maps constructed for the RM1 and RM5 revealed an expansion of 2.05 fold after random mating, close to the expected 1.83 fold expansion. Fewer loci were significant in the random mated generations, re- flecting recombination between the markers and the quantitative trait loci (QTL) or between linked QTL. Using composite interval mapping (CIM), kernel oil concentra- tion QTL were detected in chromosomal bins 2.09, 3.04, 6.04, 8.04, 8.05, and 8.07 of RM1 or TC1. The oil QTL de- tected in bin 6.04 in RM1 was also detected in TC1 and in previous research, but not in RM5 or TC5. This may be due to break up of marker-QTL associations and/or re- combination between QTL.

Maize population hybrids : An exploitation of U.S. temperate public genetic diversity in reserve

Abstract: Public maize (Zea mays L.) breeding pro- grams provide genetic diversity in reserve through con- tinuous genetic improvement of elite genetically broad- based breeding populations. These elite populations have been moderately utilized as improved adapted sources of elite inbred lines for quantitative traits. The number of successful heterotic combinations currently available is limited because public genetic diversity in re- serve has not been fully exploited. Maize population hy- brids can exploit heterosis in a fashion similar to single- cross hybrids through a 'population-hybrid' concept. To date, heterotic patterns of population hybrids with rela- tively high mid-parent heterosis values have not been ex- ploited. In addition, few reciprocal recurrent selection programs have been developed and utilized for this pur- pose. Our purpose was to identify genetically broad- based U.S. temperate maize populations and population hybrids derived from breeding programs with continuous and extensive use of recurrent selection that may have an impact on public and private breeding programs. Elite maize population hybrids that averaged 28.2% high-par- ent heterosis was detected. Based on these data we be- lieve further efforts are needed to determine the practical use of genetically broad-based germplasm and whether these efforts justify the existence of public maize breed- ing programs. Re-direction of public breeding efforts may be needed.

Consensus quantitative trait maps in maize: a database strategy

Abstract: Experimenters who seek to apply the many and diverse studies on quantitative trait loci (QTL) face complex problems in summarizing, interrelating, and inte- grating them. We report a strategy for consensus QTL maps that leverages the highly curated data in MaizeGDB, in particular, the numerous QTL studies and maps that are integrated with other genome data on a common coordi- nate system. In addition, we exploit a systematic QTL nomenclature and a hierarchical categorization of over 400 maize traits developed in the mid 90's; the main nodes of the hierarchy are aligned with the trait ontology at Gramene, a comparative mapping database for cereals. Consensus maps are presented for one trait category, in- sect response (80 QTL); and two traits, grain yield (71 QTL) and kernel weight (113 QTL), representing over 20 separate QTL map sets of 10 chromosomes each. The strategy is germplasm-independent and reflects any trait relationships that may be chosen. Whether the goal of the experimenter is to understand processes of growth, devel- opment, or stress response; to define and isolate genes specific to traits; or to mark QTL segments for selection in maize improvement, the elements of the strategy can be applied equally well.

Stability of QTLs involved in heterosis in maize when mapped under several stress conditions

Abstract: Quantitative Trait Loci (QTLs) are regions of the genome associated with the expression of quantitative traits. This paper presents results from a study designed to assess the stability of QTLs in maize (Zea mays L.) involved in the expression of heterosis for yield, ear height, and days to pollen shed (DPS) when mapped under several environmental regimens. A recombinant inbred (RI) population (n = 202), derived from a Mo17 x B73 F 1 cross and genotyped at 184 distinct marker loci, was backcrossed to both parents. Both backcross (BC) populations were grown in four year-locations under eight stress regimens: all combinations of adequate or low soil moisture; normal or low levels of nitrogen fertilizer; and high or low planting density (32 environmental combinations). Based on different line x stress ANOVAs, the genotype x stress (environment) interaction effects were small (P>0.99) for each trait and BC. Composite Interval Mapping (CIM) was used to map QTLs for each trait in each BC population, using the trait value for each line averaged across all 32 environments, and detected 17 QTLs for yield, 11 for ear height, and 8 for DPS. The least allelic effect that was significant at an experiment-wise α-level = 0.05 was 239 kg/ha for yield, 3.0 cm for ear height, and 0.4 days for DPS. The yield QTLs showed positive dominant gene action, whereas the ear height and DPS QTLs exhibited behavior that ranged from negative dominance to additivity to positive dominance. The trait values for each line also were averaged across the 16 environmental combinations within individual stresses (moisture, nitrogen, or density) and analyzed using CIM. Only a few QTLs were affected by stress: three yield QTLs were affected by moisture, one by nitrogen, and one by planting density; four ear height QTLs were affected by moisture. In every case where a QTL was affected by stress, increased stress lowered the allelic effect of the QTL, and no QTLs were mapped for the stress conditions that were not mapped for the non-stress conditions. If these results are typical of other maize populations, they imply that maize breeders could conduct selection under non-stress conditions even though the selected material would be grown in less favorable environments. Each line from the RI population used in this study was descended by selfing from an F 3 line that itself was a member of the population (n = 264) used in an earlier study to map QTLs involved in heterosis for yield. In that study, QTLs were mapped using Interval Mapping (IM) and single factor analysis, and most yield QTLs seemed to act in an overdominant manner. Those results are in contrast with the present study in which every mapped yield QTL showed dominant gene action. We investigated this discrepancy by making a direct comparison using only lines and markers that were common in the two generations. Because the F 3 generation was grown under non-stress conditions, the yield data used from the present study for this comparison were only from those plants grown under non-stress conditions. In the BC to Mol7, eight QTLs affecting yield were found to be in common between the two generations in at least one statistical test. In the BC to B73, five QTLs were found to be in common between the two generations. In both BCs, two QTLs were scored in the F 3 that were not found in the RI population; however, if the more dense marker map that was developed for the RI population was used, QTLs were found in the RI population at those positions in all but one case. In general, CIM defined QTLs to more narrow regions than IM. The discrepancy between the gene action associated with the QTLs mapped in the two studies can, therefore, be attributed largely to the more dense marker map used in the RI study and also to the use of CIM as opposed to IM, which defined QTLs in the RI study so that they did not overlap; therefore, they did not display (pseudo-) overdominance as in the F 3 study.

Characterization of floral morphology and synchrony among Zea species in Mexico

Abstract: Pollen and silk attributes as well as floral synchrony are the most important morphological features that influence pollination and gene flow in the genus Zea. To characterize floral morphology and synchrony, 115 teosinte populations, 60 maize accessions representing 53 landraces, three improved open pollinated varieties and 14 commercial hybrids were evaluated. A high level of variation among the Zea populations was found. The pollen size in landraces and commercial hybrids was larg- er (103-105 µm) than in teosinte (84 µm). Teosinte pollen grains of several accessions collapsed at pollen shed but for some populations viability was retained from 30 min- utes to one hour. Silks in maize were twice as long as teosinte silks (16 to 27 cm vs 7.2-11.4 cm). Trichome den- sity was five times higher in teosinte silks than in maize silks. Most indigenous teosinte (ssp. mexicana) popula- tions from the Valley of Mexico and Central Plateau areas flowers synchronously with landraces and commercial hy- brids. In contrast, tropical teosinte (ssp. parviglumis) flow- ers 2-3 weeks later than maize. The results of this study are important components to estimate the likelihood of gene flow among species of the genus Zea in Mexico.

Isozyme and morphological variation in maize of five andean countries 1

Abstract: To determine the genetic diversity and rela- tionships among the races of maize of five Andean coun- tries, 439 accessions representing 131 races were ana- lyzed for 13 enzyme systems encoded by 23 loci. Most data on morphology, altitude, latitude and longitude were obtained from the original studies of the accessions pub- lished by the National Science Foundation. A very high level of variation among the Andean races was found. Most of the 131 Andean races studied fall into 10 reason- ably well-defined groups. A total of 228 alleles was recorded for the races of the five countries, with an aver- age of ten alleles per locus. Pointed White Popcorns, North Andean Mid-elevation Flints, Tropical Dents, Tropi- cal Northern Popcorns have the highest diversity values; the lowest diversity values were observed in the complex- es from the highlands of Central Andes.

A genetic bridge to utilize Tripsacum germpiasm in maize improvement

Abstract: Eastern gamagrass (Tripsacum dactyloides L.) is known to possess genes conferring tolerance to corn rootworm (Diabrotica sp.), drought, and adaptation to acidic soils without incurring damage from aluminum toxicity. Plant breeders, however, have generally been unable to utilize these and other valuable gamagrass traits for effective maize improvement because of sterility in maize-gamagrass hybrids. By crossing Eastern gamagrass with diploid perennial teosinte (Zea dipioperennis Iltis, Doebley and Guzman), a closer wild relative that is cross-fertile with maize, Eubanks obtained fully fertile recombinants that are cross-fertile with maize. The gamagrassteosinte recombinants provide a new means for moving valuable genes from gamagrass into maize with conventional plant breeding methods. Progress developing gamagrass-introgressed, elite breeding lines with rootworm, drought, and acid-soil tolerance is described.

Linkage disequilibrium between ssr markers in six pools of elite lines of an european breeding program for hybrid maize 1

Abstract: D etailed knowledge about the patterns and distribution of linkage disequilibrium (LD) is of funda- mental importance for mapping of genes by whole- genome association studies. In this study, we (i) analyzed the molecular genetic diversity, (ii) investigated the pat- terns and distribution of LD in elite maize inbreds, and (iii) assessed the prospects of association mapping meth- ods in breeding materials. Six germplasm pools with a to- tal of 497 elite lines of a commercial breeding program in Europe were fingerprinted by 81 SSR markers covering the entire maize genome. The extent of LD among mark- er loci was estimated by different measures of LD. Across all germplasm pools, the SSR markers were highly poly- morphic, and four groups of inbred lines were detected by principal component and Bayesian cluster analysis. We detected genome-wide LD among pairs of loci. LD be- tween linked markers decreased not only with distance but also with a smaller number of alleles and lower gene diversity. In conclusion, association mapping via a genome-wide scan may be applied to detect associations between marker and traits, but LD between unlinked loci will most likely result in high rates of false positives.

Genetic distance based on SSR and grain yield of inter and intrapopulational maize single cross hybrids

Abstract: The objective of this work was to correlate the genetic distances between the progenitors obtained by microsatellite markers with the grain yield of inter and intrapopulational maize single cross hybrids. Three s 0 populations derived from commercial single cross hybrids were used to obtain 163 hybrids (110 interpopulational and 53 intrapopulational). The two best hybrids and two worst hybrids of each the inter- and intrapopulational crosses were selected and their progenitors maintained through self-pollination of the second ear of each s 0 plant, genotyped with 47 SSRs. The Modified Roger's Distance (MRD) between each pair of s 1 inbred lines, the number of alleles and the polymorphic information content (PIC) of each primer were estimated. The genetic distances between progenitors were correlated with the grain yield of the inter- and intrapopulational hybrids. The number of obtained alleles was 186, with a mean of 3.96 alleles. The PIC varied from 0.49 to 0.80, with a mean of 0.65. The mean genetic distance between all s 1 inbred lines was 0.75, varying from 0.40 to 0.89, indicating the existence of variability between the s 1 inbred lines. The correlation between MRD and grain yield was high and significant for the interpopulational crosses (r = 0.84, P ≤ 0.01) and low and not significant (r = 0.18, P ≥ 0.05) for intrapopulational crosses.

Mapping of factors for resistance to leaf-blade feeding by european corn borer (ostrinia nubilalis) in maize 1

Abstract: The European corn borer (ECB) Ostrinia nu- bilalis Hubner is an important pest of maize in the U.S. Breeding for resistance to leaf-feeding by ECB can be more efficient if the genetic basis for resistance is better understood. The objectives of this study were (i) to locate and characterize the genetic factors controlling resistance to leaf blade feeding by ECB larvae in temperate lines and (ii) to compare the locations of the quantitative trait loci (QTLs) detected in the F 2:3 and F 6:8 generations of the same maize population derived from the cross of inbred lines Mo17 and H99. One hundred and fifty F 2:3 lines and the parents and 185 F 6:8 lines and the parents were grown in replicated trials in 1989 and 1995, respectively. Plots were infested with ECB larvae and visual leaf blade feed- ing ratings were obtained for each plot. Lines were geno- typed with RFLP and SSR loci and a linkage map was de- veloped. QTL analysis was performed with composite in- terval mapping. The heritability of family means was 0.77 in the F 2:3 generation and 0.91 in the F 6:8 generation. The QTLs explained around 50% of the phenotypic variance in both generations. Five QTL were detected in each genera- tion. A QTL on linkage group 4 near umc123 and a QTL on linkage group 7 near umc110 were detected in both generations. Significant epistatic interactions among QTL were detected in both generations. Several loci that showed significant interactions did not have significant main effects, indicating that important epistatic interactions will be missed if only interactions between QTLs with sig- nificant main effects are tested. Marker-assisted selection will be most efficient when main effects and interaction effects of QTL are included in the selection criteria.

Pathogenicity, aggressiveness, and virulence of three species of Cercospora associated with gray leaf spot of maize

Abstract: In the eastern United States, three genetical- ly distinct species of Cercospora, the two sibling species of C. zeae-maydis (types I and II) and C. sorghi var. may- dis, are routinely isolated from maize leaves exhibiting symptoms of gray leaf spot (GLS). To determine if resis- tance to GLS is species specific, we conducted field trials comparing four commercially available maize hybrids for resistance to four isolates of C. zeae-maydis type I, three isolates of C. zeae-maydis type II, two isolates of C. sorghi var. maydis, as well as isolates of C. sorghi from Sorghum spp., and C. kikuchii from soybean. Isolates within each of the two sibling species of C. zeae-maydis were highly variable in their aggressiveness on the hy- brids. Resistance in maize to GLS appears to be equally effective against both type I and type II of C. zeae-may- dis. Neither of the two isolates of C. sorghi var. maydis isolated from maize, the C. sorghi isolates, nor C. kikuchii were pathogenic on maize. These results indicate that ag- gressive isolates, regardless of which sibling species of C. zeae-maydis, should be used to select for GLS resistance in field trials.

Selecting among available, elite tropical maize inbreds for use in long-term temperate breeding 1

Abstract: The narrowness of the temperate maize (Zea mays L.) germplasm base has long been recognized, and there are many available, elite tropical lines that might be used to profitably broaden it. However, there are few comparative yield-trial data by which to choose which line(s) might be most useful. As the investment re- quired for using a tropical line in a temperate breeding program is large, line-choice is critical. Here we report the results of testing a group of potentially-useful tropical lines in topcrosses grown in North Carolina. Results for 50%-exotic topcrosses and for 25%-exotic topcrosses are compared, and the 50%-exotic topcrosses with a broad- based tester (here, LH132.LH51) appear to be most effi- cient for initial screening. In addition, virtually all crosses suggested that any superior tropical line could be used equally well with either Stiff Stalk or non-Stiff Stalk germplasm. Of the 22 lines tested, CML258 and Tzi9 ap- pear to be the most promising, if yield improvement is a major criterion. None of the lines appeared to have seri- ous lodging, maturity or moisture problems in either 25% or 50%-tropical crosses.

Incorporation of elite subtropical and tropical maize germplasm into elite temperate germplasm

Abstract: Elite germplasm and adequate genetic varia- tion are essential in successful plant breeding programs. Maize (Zea mays L.) breeding programs in the U.S. Corn Belt have realized significant genetic advance with a limit- ed sample of the available maize germplasm. The objec- tive of our study was the incorporation of elite subtropi- cal and tropical germplasm into elite U.S. Corn Belt germplasm. Crosses, backcrosses, and testcrosses were evaluated to identify superior progenies that were inter- mated to form germplasm sources that include 75% tem- perate germplasm and either 25% subtropical or 25% tropical germplasm. Backcrosses (25%) and testcrosses (12.5%) with 25% exotic germplasm had maturities and agronomic traits similar to the elite temperate germplasm sources. The populations formed by intermating the supe- rior backcross progenies, based on backcross and test- cross data, will provide additional germplasm resources for future use in temperate maize breeding programs.

Evaluation of heterotic patterns of Iowa Stiff Stalk Synthetic and Non-Stiff Stalk Synthetic maize populations [Zea mays L.]

Abstract: Interest has increased as to how adapted exotic maize (Zea mays L.) populations combine with elite temperate populations to broaden the germplasm base of temperate maize breeding programs. A Design II mating design was used to evaluate eight U.S. Corn Belt populations and four exotic maize populations selected for adaptation to temperate environments. All populations have been under recurrent selection for improved yield, greater root and stalk strength, and earlier harvest maturi- ty. The populations evaluated were placed into either Iowa Stiff Stalk Synthetic (BSSS) or non-BSSS heterotic groups, based on data from other studies. Objectives of study were to determine mid-parent heterosis of popula- tion crosses between BSSS and non-BSSS populations and if significant maternal effects reside in the population crosses. Data collected at six Iowa locations indicated ma- ternal effects were not significant for the eight U.S. Corn Belt and the four adapted exotic populations included in the crosses. Only 3 of the 35 reciprocal crosses had signif- icant (P ≤ 0.05) differences between whether the popula- tions were used as either male or female parents. Average high parent heterosis was 1.79 t ha-1 (25.7%); average yield of the 35 crosses, including reciprocals, was 6.97 t ha-1 vs. 5.18 t ha-1, average yield of the 12 parents. Aver- age midparent heterosis was 34.6%. BS13(S)C9 (2.30 t ha- 1) and BSSS(R)C14 (2.5 t ha-1) had the greatest average midparent heterosis for the BSSS heterotic group and BS11(FR)C14 (1.78 t ha-1) and BSCB1(R)C14 (2.58 t ha-1) for the non-BSSS heterotic group. For individual popula- tion crosses, BSSS(R)C14 x BSCB1(R)C14 (3.43 t ha-1 or 78.9%) and BS13(S)C9 x BSCB1(R)C14 (3.15 t ha-1 or 69.0%) had the greatest midparent heterosis. BS29(S)C3 (an adapted strain of Suwan-1) had the greatest estimate of average midparent heterosis (1.61 t ha-1) and a positive estimate for general combining ability in crosses with BSSS populations. Based on estimates of midparent het- erosis for grain yield, BS16(S)C2 (an adapted strain of ETO Composite) and BS29(R)C3 had greater midparent heterosis with BS13(S)C9 and BSSS(R)C14 compared with BS28(R)C3 (an adapted strain of Tuxpeno germplasm). BS28(R)C3 exhibited greater midparent heterosis with BSCB1(R)C14 (57%) and BS11(FR)C14 (30%) than with BS29(R)C3 (17%). BS29(S)C3 had a significantly positive estimate of general combining ability in crosses with pop- ulations of the BSSS heterotic group, but a significantly negative estimate of specific combining ability crossed with BS28(S)C3.

Genetic diversity among maize hybrids widely grown in contrasting regional environments in the United States during the 1990s

Abstract: Geographic regions can be classified accord- ing to broad parameters of climatic conditions that can be expected from historical data. Biotic and abiotic stress conditions that cultivated varieties will encounter depend in large part upon climatic conditions. Genetic diversity ar- rayed within cultivars might therefore be expected to vary across geographic regions and to be associated with differ- ent climates. Our goals were therefore to utilize pedigree data to examine the extent and patterns of genetic diversi- ty among maize hybrids that were widely cultivated dur- ing the 1990s in each of ten U.S. states that collectively represent contrasting climatic conditions. Forty-eight hy- brids were selected on the basis that they represented the ten most widely grown hybrids in each of ten states for which pedigree data were available to us. The hybrids had 53 founders. Patterns of genetic diversity among hybrids were associated with climatic differences among regions. Multivariate analysis of pedigree data clustered states into four groups that reflected their contrasting climates. Most founders showed differences in their pedigree contribu- tions to hybrids among different regions. Abilities of breeders and farmers to access a base of genetic diversity are necessary to allow continued adaptation of improved varieties and to help provide insurance against crop failure associated with inclement weather, pests or diseases.

Cold tolerance improvement of sugary enhancer1 hybrids of sweet corn

Abstract: The ears of su1se1 hybrids have a higher quality for human consumption and maintain the eating quality for a longer period than su1 cultivars. The main problem of su1se1 hybrids when compared to su1 hy- brids is reduced emergence and poor early vigor, espe- cially in cold soils. Inbreds su1, inbreds su1se1, hybrids su1se1, and hybrids su1 × su1se1 were evaluated for traits associated with cold resistance (NW Spain, 1999 and 2000). The plants were grown in the field and in a growth chamber at cold and optimum temperatures with the fol- lowing objectives: 1) to study the relationships between emergence and early vigor in the field and in a chamber at cold temperature; 2) to find sources of favorable alleles among adapted su1 inbred lines to improve cold toler- ance (emergence and early vigor) in su1se1 hybrids; 3) to develop a combined method using a laboratory test and field evaluation to obtain cold-resistant genotypes. In this study we found a lack of correspondence between emer- gence and early vigor, and also a low correlation between the performances of the genotypes under field versus cold chamber conditions. So, when choosing a cold-toler- ant genotype, both traits (emergence and early vigor) and growth in the chamber and in the field should be consid- ered. We also found that the best genotypes under cold chamber conditions had an acceptable performance un- der field conditions, but the opposite was not true. There- fore, to obtain cold resistant-genotypes, genotypes show- ing the best performance in the chamber should be cho- sen first and then among these, select the best genotypes in the field. Finally, according to our results we can con- clude that emergence and seedling vigor of the su1se1 hybrids at cold temperature can be improved by the su1 inbreds.