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An overview is given for an ongoing maize breeding program that improves populations, inbreds, and hybrids in the Midwestern USA. Breeding and selection occurred under biodynamic conditions in Wisconsin, on an organic winter nursery in Puerto Rico, a biodynamic winter nursery in Hawaii, and a conventional winter nursery in Chile. Emphasis is on improving protein quality, carotenoid content, competitiveness with weeds, nitrogen (N) efficiency/N2 fixation, and cross incompatibility to pollen from genetically engineered (GE) maize. Philosophy is that the plant species is a responding partner in the breeding process. Adaptation and selection emphasizes vigor and yield under N limited conditions. The Ga1 and Tcb1 alleles were utilized to induce cross incompatibility. The program resulted in inbreds and hybrids with increased N efficiency and protein quality coupled with softer grain texture, more chlorophyll in foliage, and densely branched root growth in the topsoil relative to conventionally bred cultivars under N limited conditions. Grain protein quality was improved by utilizing opaque kernels that emerged in populations during the course of the program in surprisingly high frequencies. N efficiency was accentuated by breeding with landraces that may fix N2 with microbes coupled with selection for response traits under N-limited conditions. When grown next to conventional hybrids, the best hybrids from this program have exhibited 30% more methionine and 16% more protein in grain and more protein/ha.

International audience; Tropical strains of maize (Zea mays subsp. mays L.) flower very late in temperate environments. This is a barrier to maize diversification and improvement in regions where a large share of the world's corn production takes place. For investigating early flowering time adaptation, a tightly controlled parallel selection experiment spanning a 28 degrees latitudinal range (similar to 3,100 km) across the United States was conducted. First, a tropical synthetic population (TropicS-G0) (Reg. no. GP-605, PI 698625) of maize was created from seven inbred parents. The molecular genetic diversity in TropicS-G0 is representative of tropical inbreds that are differentiated from the prevailing germplasm used for hybrid production in the United States. Admixture analysis and genome simulation showed that breeding of TropicS-G0 captured the parental genomes mostly at random, as intended prior to selection. With TropicS-G0 as a common base population, a standardized protocol was used to recurrently select for early flowering time at eight locations for two generations, giving rise to location-specific lineages (TropicS-G1-PR, Reg. no. GP-621, PI 698641; TropicS-G2-PR, Reg. no. GP-622, PI 698642; TropicS-G2-FL, Reg. no. GP-620, PI 698640; TropicS-G1-cTX, Reg. no. GP-618, PI 698638; TropicS-G2-cTX, Reg. no. GP-619, PI 698639; TropicS-G1-nTX, Reg. no. GP-616, PI 698636; TropicS-G2-nTX, Reg. no. GP-617, PI 698637; TropicS-G1-NC, Reg. no. GP-614, PI 698634; TropicS-G2-NC, Reg. no. GP-615, PI 698635; TropicS-G1-DE, Reg. no. GP-610, PI 698630; TropicS-G1-IA, Reg. no. GP-608, PI 698628; TropicS-G2-IA, Reg. no. GP-609, PI 698629; TropicS-G1-WI, Reg. no. GP-606, PI 698626; TropicS-G2-WI, Reg. no. GP-607, PI 698627). Additional generations of selection were performed for the DE lineage (TropicS-G3-DE, Reg. no. GP-611, PI 698631; TropicS-G4-DE, Reg. no. GP-612, PI 698632; TropicS-G5-DE, Reg. no. GP-613, PI 698633). The parallel-selected maize population is a novel resource for breeders and those seeking to investigate adaptation.

The use of gametophyte factors to protect specialty-type maize has long been advocated, but as of yet, they have made very little impact on preventing pollen contamination due to the complications associated with breeding with these materials, mainly the additive nature of the alleles. A dominant gametophyte factor (DGF) overcomes this problem, allowing for less time consuming production of gametophytic hybrids, but effectively utilized sources do not exist. Tcb1-s, a known DGF, is a teosinte introgression into maize and the leading candidate for utilization, however, it has several issues that limit its effective use in expediting the breeding process for gametophytic hybrids. The use of maize for a source of DGFs may overcome this problem; with the idea years of selection by farmers would likely have minimized any segregation for yield associated with these alleles, making their use for production of gametophytic hybrids an appealing option for modern breeders. Through screening and backcrossing selected maize accessions, we identified DGFs in seven accessions from race Maiz Dulce, which we document here as a starting point for identification of additional maize-derived DGFs. These accessions did not appear to segregate for yield, a marked improvement over existing DGFs. Additionally, we assessed the compatibility of identified maize-derived DGFs from one accession, and showed that, while lines are generally compatible, they are not obligately so since a single accession may segregate for multiple gametophyte factors. There is, therefore, a need to consider the compatibility of pairs of DGFs early in the inbreeding process. Maize-derived DGFs provide a more effective method of producing gametophytic hybrids, making their production economical enough to be brought to market. The use of DGFs has wider potential to benefit any producers interested in preventing pollen contamination with gametophytic hybrids through the same benefits provided to breeders for organic and other specialty systems. In combination with Ga1-m resistance, maize-derived DGFs provide a long-term gametophytic solution to pollen contamination, in a more expeditious way.

Made available in DSpace on 2018-11-26T17:51:58Z (GMT). No. of bitstreams: 0 Previous issue date: 2018-04-01. Added 1 bitstream(s) on 2019-10-09T18:27:29Z : No. of bitstreams: 1 S1984-70332018000200148.pdf: 411594 bytes, checksum: f843ea3db934b080a1247f9bd035c2ee (MD5) The objectives of this study were to identify resistant and susceptible corn inbred lines based on their stability and adaptability for resistance to disease symptoms of gray leaf spot and northern leaf blight, suggest resistant inbred lines aimed at producing synthetics, and identify the sowing dates with higher occurrence of diseases to use these dates for identification of genetic resistance. We evaluated 41 corn inbred lines in randomized block designs with three replications and 11 sowing dates. Evaluations were performed 30 days after silking. Stability and adaptability analyses were performed by the regression method. The highest severities of disease occurred in sowing from June to September. On average, all the inbred lines were classified as resistant; however, considering adaptability and stability, five inbred lines were found suitable to produce dent synthetics resistant to both diseases. For flint synthetics resistant to both diseases, inbred lines IVF1-7,IVF1-10, 2F, 9F, and 10F are most suitable. Univ Estadual Paulista, Dept Biol & Zootecnia, Campus Ilha Solteira, BR-15385000 Ilha Solteira, SP, Brazil North Carolina State Univ, Crop Sci, Raleigh, NC USA Univ Estadual Paulista, Dept Biol & Zootecnia, Campus Ilha Solteira, BR-15385000 Ilha Solteira, SP, Brazil

Due to maize’s wind-driven pollination, non-target pollen contamination is problematic for producers and breeders. Maize gametophyte factors, specifically gametophyte factor 1 (ga1), have long been used to produce selectively pollinating phenotypes. The use of these factors is a cornerstone of commercial popcorn production, and they are used for a large range of other purposes, including preventing contamination by genetically modified pollen in organic production. However this system is at great risk from another allele at the ga1 locus, Ga1-m, which overcomes the selectively pollinating phenotypes. To further complicate this problem, the risk posed by this allele has been under-assessed. Here we reinterpret the key study on Ga1-s and report genetic resistance to the Ga1-m allele in maize lines that carry dominant gametophyte factors. We identified genetic resistance to the allele segregating in lines derived from four landraces, showed the resistance is heritable, and that it acts in full-strength and attenuated versions. Additionally, we have suggested the validity of evolutionary-based inquiry into our plant genetic resources, and provided some validation of this effort. Our results provide the first report of effective genetic resistance to pollination by the Ga1-m allele, providing an option to continue the use of genetic barriers to non-target pollination. A source of resistance to the Ga1-m allele allows research to be conducted about the allele itself, allowing for research into the possible existence of multiple versions of the allele and their distributions. We anticipate our research will be a starting point for identification of additional sources of resistance to the Ga1-m allele, specifically in popcorn production, where it is most immediately needed to prevent pollen contamination, as well as the eventual localization and mapping of the resistance alleles. We also believe the suggestion of evolutionary-based inquiry into plant genetic resources will provide a highly effective method for identification of specific traits, but will need more extensive validation.

In this study we examined the effects of two quantitative trait loci (QTL) for southern leaf blight (SLB) resistance on several agronomic traits including disease resistance and yield. B73–3B and B73–6A are two near-isogenic lines (NILs) in the background of the maize (Zea mays L.) inbred B73, each carrying one introgression (called 3B and 6A respectively) encompassing a QTL for SLB resistance. Sets of isohybrid triplets were developed by crossing B73, B73–3B, and B73–6A to several inbred lines. A subset of these triplets for which the B73–3B and/or B73–6A hybrid was significantly more SLB resistant than the B73 check hybrid was selected and assessed in multi-environment yield trials with and without disease. In the presence of SLB, 3B was associated with an approximately 3% yield increase over B73. 6A was associated with a yield advantage in the presence of SLB in specific pedigrees where the 6A resistance phenotype was highly expressed. Results suggested that both introgressions might confer a yield cost in the absence of SLB, but only introgression 6A was associated with a statistically significant reduction. We present evidence to suggest that the yield cost is associated with the resistance phenotype rather than with linkage drag. J.H. Santa-Cruz and P.J. Balint-Kurti, Dep. of Plant Pathology, N.C. State Univ., Raleigh, NC 27695; K.L. Kump, M.M. Goodman, M.D. Krakowsky, and J.B. Holland, Dep. of Crop Science, N.C. State Univ., Raleigh, NC 27695; C. Arellano, Dep. of Statistics, N.C. State Univ., Raleigh, NC 27695; M.D. Krakowsky, J.B. Holland, and P.J. BalintKurti, USDA-ARS, Plant Science Research Unit, Raleigh NC 27695. J.H. Santa-Cruz and K.L. Kump contributed equally to this work. Received 19 Aug. 2013. *Corresponding author (pjbalint@ncsu.edu ). Abbreviations: CL, NCSU Central Crops Research Station in Clayton, NC; CTAB, cetyltrimethylammonium bromide; dQTL, diseaseresistance quantitative trait locus (loci); DTA, days to anthesis; DTS, days to silking; KI, Cunningham Research Station in Kingston, NC; LE, Peanut Belt Research Station in Lewiston, NC; NIL, near isogenic line; PCR, polymerase chain reaction; SNP, single nucleotide polymorphism; QDR, quantitative disease resistance; QTL, quantitative trait locus (loci); R, resistance; sAUDPC, standardized area under the disease progress curve; SH, Sandhills Research Station in Jackson springs, NC; SLB, southern corn leaf blight; SSR, simple sequence repeat. Published in Crop Sci. 54:882–894 (2014). doi: 10.2135/cropsci2013.08.0553 © Crop Science Society of America | 5585 Guilford Rd., Madison, WI 53711 USA All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permission for printing and for reprinting the material contained herein has been obtained by the publisher. Published March 21, 2014

Crop science is a highly integrative science employing expertise from multiple disciplines to broaden our understanding of agronomic, turf, and forage crops. A major goal of crop science is to ensure an adequate and sustainable production of food, feed, fuel, and fiber for our world's growing population. The Crop Science Society of America (CSSA) identified key Grand Challenges which, when addressed, will provide the tools, technologies, and solutions required to meet these challenges. The Grand Challenges are: (i) Crop adaptation to climate change: Increase the speed with which agriculture can adapt to climate change by using crop science to address abiotic stresses such as drought and heat. (ii) Resistance to biotic stresses: Increase durability of resistance to biotic stresses that threaten yield and quality of major crops. (iii) Management for resource limited systems: Create novel crop cultivars and management approaches designed for problem soils and low-input farming to increase economic prosperity for farmers and overcome world hunger. (iv) Crop management systems: Create novel crop management systems that are resilient in the face of changes in climate and rural demographics. (v) Biofuels: Develop sustainable biofuel feedstock cropping systems that require minimal land area, optimize production, and improve the environment. (vi) Bioresources: Genotyping the major crop germplasm collections to facilitate identification of gene treasures for breeding and genetics research and deployment of superior genes into adapted germplasm around the globe. These challenges are intended to be dynamic and change as societal needs evolve. Available funding and national prioritization will determine the rate that they will be addressed.

This chapter contains sections titled: - Introduction - What Kind of Maize do Organic Farmers Want? - Are There Viable Alternatives to Single Cross Hybrids? - Testing and Using Alternative Hybrids - Are There Benefits for Breeding under Organic Conditions? - For Which Traits Is It Necessary to Test under Organic Conditions? - Choice of Parents for Breeding Programs - Breeding Programs - Future Directions - Notes - References

Codifying Maize Modifications Maize, one of our most important crop species, has been the target of genetic investigation and experimentation for more than 100 years. Crossing two inbred lines tends to result in “better” offspring, in a process known as heterosis. Attempts to map the genetic loci that control traits important for farming have been made, but few have been successful (see the Perspective by Mackay ). Buckler et al. (p. 714 ) and McMullen et al. (p. 737 ) produced a genomic map of maize that relates recombination to genome structure. Even tremendous adaptations in very diverse species were produced by numerous, small additive steps. Differences in flowering time in maize among inbred lines were not caused by a few genes with large effects, but by the cumulative effects of numerous quantitative trait loci—each of which has only a small impact on the trait.

Because of the economic importance of maize and its scientific importance as a model system for studies of domestication, its evolutionary history is of general interest. We analyzed the population genetic structure of maize races by genotyping 964 individual plants, representing almost the entire set of ∼350 races native to the Americas, with 96 microsatellites. Using Bayesian clustering, we detected four main clusters consisting of highland Mexican, northern United States (US), tropical lowland, and Andean races. Phylogenetic analysis indicated that the southwestern US was an intermediary stepping stone between Mexico and the northern US. Furthermore, southeastern US races appear to be of mixed northern flint and tropical lowland ancestry, while lowland middle South American races are of mixed Andean and tropical lowland ancestry. Several cases of post-Columbian movement of races were detected, most notably from the US to South America. Of the four main clusters, the highest genetic diversity occurs in highland Mexican races, while diversity is lowest in the Andes and northern US. Isolation by distance appears to be the main factor underlying the historical diversification of maize. We identify highland Mexico and the Andes as potential sources of genetic diversity underrepresented among elite lines used in maize breeding programs.

While maize (Zea mays L.) is a highly diverse species, this diversity is not well represented in U.S. maize production acreage. Increased genetic diversity can be obtained through breeding with exotic germplasm, especially tropical-exotic sources. However, the pool of available tropical germplasm is large and diverse, making choices of tropical parents diffi cult. The maize breeding program at North Carolina State University has initiated a large-scale screening effort to evaluate elite exotic maize inbreds, most of which are tropical-exotic in origin. Here we report screening results for 88 inbreds obtained from various international breeding programs. These lines were tested in replicated yield trials in North Carolina as 50% exotic topcrosses by crossing them to a single-cross U.S. tester of stiff-stalk (SS) by nonstiff-stalk (NSS) origin. The more promising lines additionally entered 25% tropical topcrosses with SS and NSS testers and were further evaluated in yield-trials. A handful of tropical inbred lines— CML10, CML108, CML157Q, CML274, CML341, CML343, and CML373—performed well overall. It was further determined that topcrossing to a single SS by NSS tester will suffi ce for initial screening purposes, allowing for elimination of the poorest performing lines. Topcrossing to additional SS and NSS testers may be of value when determining where, in terms of heterotic patterns, the better-performing lines will fi t into a breeding program.

Improving grain quality of maize (Zea mays L.), including endosperm hardness and density, is often a breeding objective. Dense seed is preferred by dry millers and for alkaline processing, and can command a price premium at market. This study attempted to increase kernel density in a backcrossing program for two inbreds of maize using two selection techniques, specific gravity of kernels and the percentage of sinking kernels in a salt solution (or sinkers). Two inbreds, B73G and A632, were crossed with synthetics exhibiting apparent high kernel density, and several generations of backcrossing and self-pollination followed. Examples of mean comparisons of backcross-derived inbreds with the recurrent parents, B73G and A632 are as follows: B73G-Specific gravity, 1.251 and 1.206; Sinkers, 62.3 and 14.9%; and A632-Specific gravity, 1.266 and 1.250; Sinkers 45.4 and 29.1%. Both the specific gravity and sinkers techniques were successful for increasing kernel density during backcrossing.

*Summary Crop improvement and the dissection of complex genetic traits require germplasm diversity. Although this necessary phenotypic variability exists in diverse maize, most research is conducted using a small subset of inbred lines. An association population of 302 lines is now available – a valuable research tool that captures a large proportion of the alleles in cultivated maize. Provided that appropriate statistical models correcting for population structure are included, this tool can be used in association analyses to provide high-resolution evaluation of multiple alleles. This study describes the population structure of the 302 lines, and investigates the relationship between population structure and various measures of phenotypic and breeding value. On average, our estimates of population structure account for 9.3% of phenotypic variation, roughly equivalent to a major quantitative trait locus (QTL), with a high of 35%. Inclusion of population structure in association models is critical to meaningful analyses. This new association population has the potential to identify QTL with small effects, which will aid in dissecting complex traits and in planning future projects to exploit the rich diversity present in maize.

Tropical maize (Zea mays L.) represents the most diverse readily available source of germplasm to broaden the limited genetic base of temperate maize in the USA. One objective of this study was to determine if exotic-derived alleles contributing to enhanced testcross agronomic performance were maintained in semiexotic lines created by inbreeding and pedigree selection. A second objective was to determine if first-generation semiexotic lines could produce hybrids with agronomic performance comparable to commercial U.S. hybrids. One hundred sixty-four semiexotic inbred lines were developed from crosses between temperate-adapted inbred line Mo44 and 23 Latin American maize accessions. Mo44 and each semiexotic line were testcrossed to temperate hybrid LH132 × LH51 for evaluations. In first-stage replicated yield trials, testcrosses of 18 semiexotic lines, representing six different races, had significantly greater grain yields than the Mo44 testcross. Advanced yield evaluations were performed on check entries and 33 selected semiexotic line testcrosses in three additional environments. Across 10 environments, 12 semiexotic line testcrosses exhibited significantly greater grain yield than the Mo44 testcross, indicating recovery of favorable exotic alleles. Semiexotic testcrosses were not competitive with commercial hybrids for grain yield but were similar to or better than commercial hybrids for grain moisture and lodging resistance. Many superior accessions represent relatively recent introductions into regions from which they were collected. Tropical landraces seem to be a good source of exotic germplasm that can be used to broaden the genetic base of modern U.S. maize production and improve productivity.

Exotic maize ( Zea mays L.) germplasm may allow for increased flexibility and greater long-term progress from selection if it can be incorporated at high rates into U.S. breeding programs. Crosses were made between a temperate line, NC262A, and each of eight different lines consisting of 100% temperate-adapted tropical germplasm. Pedigree selection was used to generate a set of 148 F(5)S(2) lines that were evaluated in testcrosses with FR992/FR1064 in nine North Carolina environments. Several entries had grain yield, grain moisture content and standability that were comparable to three commercial checks. The best testcrosses outyielded the cross NC262A x FR992/FR1064 by 9.5 to 10.9%, suggesting that a significant amount of tropical germplasm was retained in these lines and that this germplasm combined well with the Stiff Stalk tester. Previous researchers had suggested that tropical alleles could be rapidly lost during inbreeding in populations derived from tropical x temperate bi-parental crosses, leading to the development of lines that possess significantly less than 50% tropical germplasm. F(5)S(5) sub-lines corresponding to the 14 best testcrosses were genotyped at 47 to 49 polymorphic simple sequence repeat (SSR) loci across all ten chromosomes to estimate the amount of tropical germplasm that was retained. The estimated genetic contribution from the tropical parent ranged from 32 to 70%, with the average being 49%. Only two of the 14 lines deviated significantly from a 50%-tropical/50%-temperate ratio, suggesting limited overall selection against germplasm from the tropical parents. These experiments collectively demonstrated that tropical maize germplasm can be incorporated at high rates into a temperate line via pedigree breeding methods in order to derive new inbred lines with acceptable agronomic performance.

The use of genetically resistant maize hybrids is the preferred means of control of gray leaf spot, caused by Cercospora zeae-maydis. One problem faced by maize breeders attempting to breed for resistance to gray leaf spot is the high degree of genotype-environment interactions observed in disease trials. In North Carolina gray leaf spot trials conducted at four locations in the western part of the state, we found consistent hybrid-location interactions over the 1995 and 1996 growing seasons. Isolates of C. zeae-maydis from those test locations were evaluated on the same hybrids used in the multilocation testing at a location in central North Carolina that does not have a history of gray leaf spot. The hybrid-isolate interactions observed in the isolate trial mirrored the hybrid-location effects seen in the multilocation testing. Most of the interactions arose from changes in the magnitude of differences between hybrids when inoculated with the isolates rather than from any change in hybrid ranking. Analysis of internal transcribed spacer-restriction fragment length polymorphisms (RFLPs) and mitochondrial rDNA RFLPs of those isolates and others revealed that both type I and type II sibling species of C. zeae-maydis, as well as C. sorghi var. maydis, are isolated from typical gray leaf spot lesions. Breeders should use the most aggressive isolates of C. zeae-maydis to maximize discrimination between genotypes in gray leaf spot trials.

To evaluate the performance of microsatellites or simple sequence repeats (SSRs) for evolutionary studies in Zea, 46 microsatellite loci originally derived from maize were applied to diverse arrays of populations that represent all the diploid species of Zea and 101 maize inbreds. Although null phenotypes and amplification of more than two alleles per plant were observed at modest rates, no practical obstacle was encountered for applying maize microsatellites to other Zea species. Sequencing of microsatellite alleles revealed complex patterns of mutation including frequent indels in the regions flanking microsatellite repeats. In one case, all variation at a microsatellite locus came from indels in the flanking region rather than in the repeat motif. Maize microsatellites show great variability within populations and provide a reliable means to measure intraspecific variation. Phylogeographic relationships of Zea populations were successfully reconstructed with good resolution using a genetic distance based on the infinite allele model, indicating that microsatellite loci are useful in evolutionary studies in Zea. Microsatellite loci show a principal division between tropical and temperate inbred lines, and group inbreds within these two broad germplasm groups in a manner that is largely consistent with their known pedigrees.

Association studies based on linkage disequilibrium (LD) can provide high resolution for identifying genes that may contribute to phenotypic variation. We report patterns of local and genome-wide LD in 102 maize inbred lines representing much of the worldwide genetic diversity used in maize breeding, and address its implications for association studies in maize. In a survey of six genes, we found that intragenic LD generally declined rapidly with distance ( r 2 < 0.1 within 1500 bp), but rates of decline were highly variable among genes. This rapid decline probably reflects large effective population sizes in maize during its evolution and high levels of recombination within genes. A set of 47 simple sequence repeat (SSR) loci showed stronger evidence of genome-wide LD than did single-nucleotide polymorphisms (SNPs) in candidate genes. LD was greatly reduced but not eliminated by grouping lines into three empirically determined subpopulations. SSR data also supplied evidence that divergent artificial selection on flowering time may have played a role in generating population structure. Provided the effects of population structure are effectively controlled, this research suggests that association studies show great promise for identifying the genetic basis of important traits in maize with very high resolution.

Commercial maize (Zea mays L.) in the USA has a restricted genetic base as newer hybrids are largely produced from crosses among elite inbred lines representing a small sample (predominantly about 6- to 8-base inbreds) of the Stiff stalk and Lancaster genetic backgrounds. Thus, expansion of genetic diversity in maize has been a continuous challenge to breeders. Tropical germplasm has been viewed as a useable source of diversity, although the integration of tropical germplasm into existing inbred line and hybrid development is laborious. The present study is an evaluation of the potential of tropical germplasm for temperate maize improvement. All possible single-, three-way-, and double-cross hybrids among three largely temperate and three temperate-adapted, all-tropical inbred lines were evaluated in yield-trial tests. Single-cross hybrids containing as much as 50–60% tropical germplasm produced 8.0 t ha-1 of grain yield, equivalent to the mean yield of the commercial check hybrids. On the other hand, three-way and double-cross hybrids with the highest mean yield contained lower amounts of tropical germplasm, 10–19% and 34–44%, respectively. Overall, hybrids containing 10–60% tropical germplasm yielded within the range of the commercial hybrid checks. Hybrids with more than 60% tropical germplasm had significantly lower yields, and 100% tropical hybrids yielded the least among all hybrids evaluated. The results indicate that inbred lines containing tropical germplasm are not only a useful source to expand the genetic diversity of commercial maize hybrids, but they, also are competitive in crosses with temperate materials, producing high-yielding hybrids. These experimental hybrids exhibited good standability (comparable to the commercial check hybrids) but contained 1–2% higher grain moisture, leading to delayed maturity. Recurrent selection procedures are being conducted on derivatives of these materials to extract lines with superior yield, good standability, and reduced grain moisture which can be used for commercial exploitation.

Among maize (Zea maize L.) breeders, there is a heightened awareness of the necessity for both maintaining genetic diversity for crop improvement and improving the quality of genetic resource management. Restriction fragment length polymorphisms (RFLPs) and isozymes can serve as genetic markers for estimating divergence or diversity ; however, the limited number of polymorphic isozyme loci available and the labor intensive and time consuming nature of RFLPs make their use for this purpose prohibitive. Simple sequence repeats (SSRs), when resolved using agarose gels, may be a viable and cost-effective alternative to RFLPs and isozymes. Ninety-four elite maize inbred lines, representative of the genetic diversity among lines derived from the Corn Belt Dent and Southern Dent maize races, were assayed for. polymorphism at 70 SSR marker loci using agarose gels. The 365 alleles identified served as raw data for estimating genetic similarities among these lines. The patterns of genetic divergence revealed by the SSR polymorphisms were consistent with known pedigrees. A cluster analysis placed the inbred lines in nine clusters that correspond to major heterotic groups or market classes for North American maize. A unique fingerprint for each inbred line could be obtained from as few as five SSR loci. The utility of polymerase chain reaction (PCR)-based markers such as SSRs for measuring genetic diversity, for assigning lines to heterotic groups and for genetic fingerprinting equals or exceeds that of RFLP markers, a property that may prove a valuable asset for a maize breeding program.

The inheritance of resistance to southern rust (caused by Puccinia polysora Underw.) was investigated in two F2:3 populations derived from crossing two temperate-adapted, 100% tropical maize (Zea mays L.) inbred lines (1416-1 and 1497-2) to a susceptible Corn Belt Dent hybrid, B73Ht×Mo17Ht. The inbred lines possess high levels of resistance to southern rust and may be unique sources of resistance genes. Heritability for resistance was estimated as 30% and 50% in the two populations from regression of F2:3 family mean scores on F2 parent scores, and as 65% and 75% from variances among F2:3 families on a single-plot basis. RFLP loci on three chromosomal regions previously known to possess genes for resistance to either southern rust or common rust (P. sorghi Schw.) were used to localize genes affecting resistance to southern rust in selected genotypes of both populations, and to estimate their genetic effects. A single locus on 10S, bnl3.04, was associated with 82–83% of the variation among field resistance scores of selected F2:3 families in the two populations. Loci on chromosomes 3 (umc26) and 4 (umc31) were significantly associated with resistance in the 1497-2 population, each accounting for 13–15% of the phenotypic variation for F2:3 field scores. Multiple-marker locus models, including loci from chromosomes 3, 4, and 10 and their epistatic interactions, accounted for 96–99% of the variation in F2:3 field scores. Similar results were obtained for resistance measured by counting pustules on juvenile plants in the greenhouse. An attempt was made to determine if the major gene for resistance from 1416-1 was allelic to Rpp9, which is also located on 10S. Testcross families from the cross (1416-1×B37Rpp9)×B14AHt were evaluated for resistance to southern rust in Mexico. Neither source of resistance was completely effective in this environment, preventing determination of allelism of the two genes; however, both sources of resistance had better partial resistance to southern rust than did B14AHt.

Twenty maize breeding germplasm lines were developed cooperatively by the USDA GEM (Germplasm Enhancement of Maize) project (Reg. no. GP-407 to GP-426, PI 639037 to PI 639056). These lines were developed by selfing and selecting variable F1s from variable source × US inbred crosses in North Carolina under standard nursery conditions, followed by a second selfing-selection season in Homestead, Florida, and a third selfing-selection season in a selection nursery in Raleigh (F2S2). The germplasm lines were selected on the basis of resistance to Fusarium ear rot (Gibberella moniliformis and Fusarium proliferatum) and anthracnose (Colletotrichum graminicola), resistance to lodging, early flowering, synchrony of silk and pollen production, and reduced plant and ear height. In trials conducted in 2001 and 2002, the germplasm lines recorded grain yields ranging from 11197 to 13596 kg/ha (compared with 11009 kg/ha for the control) and grain moisture content ranging from 185 to 212 g/kg (compared with 190 g/kg for the control).

Late maturity and high grain moisture content at harvest have been major limitations to the use of tropical maize (Zea mays L.) germplasm in temperate regions. The objective of this study was to determine if selection for reduced grain moisture content at harvest in a tropical maize population indirectly influenced grain yield potential. Two hundred sixteen temperately-adapted S 4 lines were derived in 1991 at Raleigh, NC, from Cycle 9 of recurrent phenotypic selection for reduced grain moisture at harvest in the tropical maize population TROPHY, and these were testcrossed in 1992 at Raleigh, NC, to the temperate hybrid B73Ht x Mo17Ht. Selected subsets of these testcrosses were evaluated for their agronomic potential in seven environments over two years, and their performance was compared with that of Cycle 0 S 0 testcrosses as well as three public and three commercial F 1 hybrids. Selected Cycle 9 S 4 testcrosses had higher mean grain yield (7.14 Mg ha -1 ) and lower mean grain moisture at harvest (184 g kg -1 ) than the Cycle 0 S 0 testcrosses (6.77 Mg ha -1 , and 189 g kg -1 respectively). The highest yielding Cycle 9 S 4 testcrosses were comparable to the commercial hybrid LH132 x LH51. This study supported the conclusion that agronomically competitive inbred lines with acceptable grain moisture content at barvest can be derived from 100% tropical germplasm.

Few data exist regarding the agronomic utility of tens of thousands of available tropical maize (Zea mays L.) racial accessions. A long-term, multi-stage evaluation project has been developed at North Carolina State University to evaluate the breeding potentials in the temperate USA of typical Latin American maize germplasm collections. Agronomic evaluations of accessions per se were performed in daylength neutral environments. Superior accessions were crossed to a temperate-adapted inbred line and converted to photoperiod-insensitive 50%-exotic and 75%-exotic populations by selection during 1986 to 1990. These semi-exotic populations were evaluated as testcrosses to Corn Belt testers in North Carolina during 1993 and 1994. Testcrosses of many 75%-exotic families were higher-yielding than the hybrid tester, B73Ht x Mo17Ht, providing evidence that agronomically superior Latin American maize accessions possess favorable genes for yield that apparently are absent from Corn Belt germplasm. Yields of accessions evaluated as 100%-exotic and 50%-exotic populations were significantly correlated (r = 0.62, P = 0.005) with each other, but not with yields of 75%-exotic populations. Some accessions exhibited extreme ranking changes across different testing stages. Multi-stage evaluations of maize racial accessions successfully identified superior germplasm for use by U.S. breeders, but there is no guarantee that all of the most valuable accessions were advanced to later testing stages. Evaluations in many environments and minimal culling levels at each stage could be used to improve the probabilities of successfully identifying useful accessions in future multi-stage evaluation procedures.

Tropical maize (Zea mays L.) is a germplasm resource that may contribute genes for high yield and disease resistance to temperate breeding programs. Yet, this resource has not been utilized extensively, mainly because of the poor agronomic performance and photoperiod sensitivity that often accompany tropical germplasm. The objective of this study was to determine the agronomic performance of 95 inbred lines derived in North Carolina from 100% tropical germplasm. Ninety-five lines were derived from seven tropical commercial hybrids and intercrosses among them, using two complete cycles of pedigree selection. Visual selection emphasized early flowering, silk-tassel synchrony, low ear placement, standability, prolificacy, and ear quality. The inbreeding coefficient of plants within these lines was 0.96 to 0.98. Agronomic performance was evaluated in inbred yield trials during 1990 at three locations. Grain yields ranged from 0.68 to 3.76 Mg ha⁻¹. The highest yielding check inbred, NC252 (an improved B73), yielded 3.65 Mg ha⁻¹, slightly less than the best experimental inbred. Mean percent stalk lodging ranged from 0 to 16%, and root lodging was infrequent. Grain moisture at harvest was mostly within the range of the checks. The genetic correlation between grain yield and number of ears per plant was 0.72, while the correlation between grain yield and moisture was not significant. Several fines possessed excellent combining ability and inbred performance. The best line, 1497-2, produced testcrosses competitive with the best commercial hybrids and as a line per se performed as well as the best public inbreds available for North Carolina. Contribution of North Carolina Agric. Res. Serv.

To supplement minimal information regarding the utility of tropical maize (Zea mays L.) germplasm to temperate maize breeding programs, agronomic evaluations of typical accessions of the Latin American races were made. Based on data from previous evaluation stages, 40 accessions exhibiting superior agronomic performance in tropical environments were chosen for photoperiod conversion and combining-ability evaluations with U.S. germplasm in temperate environments. Accessions were converted to photoperiod insensitive semiexotic populations by crossing each to Mo44, a temperate-adapted inbred unrelated to either of two major U.S. heterotic groups. Four families from each semiexotic population were crossed to two U.S. testers. Testcrosses were evaluated in three North Carolina locations for 2 yr. Based on these results, 29 agronomically superior semiexotic testcrosses were tested a third year. The variation for combining ability for yield was estimated to be two times greater among vs. within accessions, suggesting that sampling among accessions be given priority compared with sampling within to maximize genetic diversity for combining ability. Family yields were highly correlated across testers (r=0.78), suggesting that a single temperate tester would be sufficient for evaluating large samples of Latin American accessions. Grain yields, resistance to gray leaf spot disease (incited by Cercospora zeae-maydis Tehon & E.Y. Daniels), and other agronomic traits of superior semiexotic testcrosses were competitive with the public U.S. hybrid Mo17 × B73. The best accessions should be a promising source of useful genes for commercial U.S. maize breeding programs

Tropical maize (Zea mays L.) has been suggested as a germplasm source to improve yield and disease resistance for U.S. breeding programs. Our objective was to compare the performance of testcrosses between tropical-derived lines and an elite U.S. tester with the performance of adapted commercial hybrids. A group of 190 lines was derived from seven tropical commercial hybrids and intercrosses among them using two cycles of pedigree selection for adaptation to North Carolina. Ear-to-row sib-mating and selfing were used to develop the lines. Visual selection of each generation concentrated on silk-tassel synchrony, earliness, standability, lower ear height, and ear quality. No selection for combining ability was made during line development. Lines were topcrossed onto the public U.S. single cross B73Ht/Mo17Ht. Testcrosses were evaluated in yield trials at three locations during 2 yr. Among 190 testcrosses, 16 were within the LSD (0.05) of the commercial checks for yield, standability, and grain moisture. Most genetic variation for yield, standability, and grain moisture was accounted for by differences among pedigrees rather than differences among lines within a pedigree. Seventeen testcrosses yielded more than the tester, B73Ht/Mo17Ht. The inbreds B73 and Mo17 represent two dominant heterotic groups utilized by U.S. maize breeders. Since tropical-derived lines appear to combine well with B73/Mo17, they could be used as a source of disease resistance for either heterotic group. In addition, they have potential to enhance the combining ability of inbreds representing these two heterotic groups

Gray leaf spot (GLS) (caused by Cercospora zeae-maydis Theon & Daniels) is a fungal disease of the maize (Zea mays L.) that can cause significant grain yield reductions. Quantitative resistance to GLS does exist; therefore, one method of controlling the disease is to develop hybrids with genetic resistance. The objective of this study was to identify quantitative trait loci (QTL) and type-of-gene action for GLS resistance in segregating families of three single-cross populations of maize using restriction fragment length polymorphisms (RFLPs) (.)

Optimal exploitation of crop genetic resources requires a knowledge of the range and structure of the variation present in the gene pool of interest. Avena sterilis L., the cultivated oat progenitor, contains a store of genetic diversity that is readily accessible to the oat breeder. The objectives of the present paper were: (1) to evaluate isozyme polymorphisms in a sample of A. sterilis accessions from the U.S. National Small Grains Collection, (2) to analyze the distribution of isozyme diversity across the geographic range of the accessions, (3) to classify the accessions into groups based on isozyme variation, and (4) to suggest strategies for efficient sampling of this germplasm collection. One thousand and five accessions from 23 countries and 679 collection sites were screened for variation using 23 enzyme systems. Due to limited information about the genetic relationship among individual members of families of isozymes in hexaploid oat species, data were recorded solely for band presence. The frequencies of bands in accessions from the various countries were used to calculate the probability of genotypic identity (Ix.y), the probability of a unique genotype (Ux.y), and an adjusted polymorphic index (Hx). Accessions from Turkey and Lebanon had the largest polymorphic index values, Turkish and Moroccan accessions displayed the greatest numbers of bands. Accessions from Iran, Turkey, Iraq, and Lebanon had the largest mean probabilities of containing unique genotypes. Based on isozyme data, Turkey appeared to represent the center of diversity in this germplasm collection. Band frequencies calculated among countries were used in a principal component analysis. Accessions from Israel and Morocco clustered together; accessions from Iran, Iraq, Turkey, and Ethiopia formed another group; and Algerian accessions formed an outlying group. Several isozyme bands had a regional distribution. These results suggested that choosing accessions from countries based on their groupings in the principal component analysis should secure a greater range of diversity than sampling from the collection at random. Cluster analyses based on Jaccard's distances calculated for all pairwise combinations of the 1005 accessions revealed six broad genetic groups of accessions. Groups 1 and 6 contained accessions from many countries and encompassed half of all accessions. Groups 2 and 4 were heavily populated by accessions from Israel and Morocco. Groups 3 and 5 were composed almost exclusively of accessions from Iran, Iraq, and Turkey. By selecting representative accessions from these six groups, oat breeders could most effectively sample the range of genetic variation in this A. sterilis collection.

To determine the relative importance of the genotype, the environment, and their interaction on the expression of morphological characters in maize races, 50 Mexican races and 24 races from Central America, South America, and the U.S. were grown in several locations and seasons in Mexico and 47 characters were measured directly. Estimates of the ratio of variance components, rc = [Vc/(Ve+VCs/], were used as criteria to determine the appropriate characters for racial classification. Twenty-four useful variables were identified. Analysis of the structure of the data matrix facilitated visual examination of correlations among the variables and of the variability represented by each variable. Based on these analyses, a minimum list of 9 characters was suggested to be appropriate variables for racial classification: number of leaves per plant, branched part length/tassel length, central spike internode length, male glume length, kernel width, rachis segment length, pith diameter, ear diameter/length, and kernel width/length.

The last two decades have seen important advances in our knowledge of maize domestication, thanks in part to the contributions of genetic data. Genetic studies have provided firm evidence that maize was domesticated from Balsas teosinte ( Zea mays subspecies parviglumis ), a wild relative that is endemic to the mid- to lowland regions of southwestern Mexico. An interesting paradox remains, however: Maize cultivars that are most closely related to Balsas teosinte are found mainly in the Mexican highlands where subspecies parviglumis does not grow. Genetic data thus point to primary diffusion of domesticated maize from the highlands rather than from the region of initial domestication. Recent archeological evidence for early lowland cultivation has been consistent with the genetics of domestication, leaving the issue of the ancestral position of highland maize unresolved. We used a new SNP dataset scored in a large number of accessions of both teosinte and maize to take a second look at the geography of the earliest cultivated maize. We found that gene flow between maize and its wild relatives meaningfully impacts our inference of geographic origins. By analyzing differentiation from inferred ancestral gene frequencies, we obtained results that are fully consistent with current ecological, archeological, and genetic data concerning the geography of early maize cultivation.

5 pages, 5 figures.-- Supporting information (Materials and Methods, Suppl. figs S1-S75, tabs S1-S5, refs., 88 page PDF file) available at: http://www.sciencemag.org/cgi/content/full/sci;325/5941/714/DC1, Flowering time is a complex trait that controls adaptation of plants to their local environment in the outcrossing species Zea mays (maize). We dissected variation for flowering time with a set of 5000 recombinant inbred lines (maize Nested Association Mapping population, NAM). Nearly a million plants were assayed in eight environments but showed no evidence for any single large-effect quantitative trait loci (QTLs). Instead, we identified evidence for numerous small-effect QTLs shared among families; however, allelic effects differ across founder lines. We identified no individual QTLs at which allelic effects are determined by geographic origin or large effects for epistasis or environmental interactions. Thus, a simple additive model accurately predicts flowering time for maize, in contrast to the genetic architecture observed in the selfing plant species rice and Arabidopsis., This work was supported by U.S. NSF Plant Genome Program (DBI-9872631, DBI-0321467, DBI-0820619 to E.S.B., M.M.G. (98,03), J.B.H. (03,08), M.D.M. (03,08), S.K. (03,08), D.W. (03); DBI-0604923 to T.R.R.; U.S. Department of Agriculture–Agricultural Research Service (to E.S.B., J.B.H., M.D.M.), and Spanish Ministry for Education and Science (AP-2004-6033, fellowship to M.C.R.).

B73 is a historically important maize line with excellent yield potential but high susceptibility to the foliar disease southern leaf blight (SLB). NC292 and NC330 are B73 near-isogenic lines (NILs) that are highly resistant to SLB. They were derived by repeated backcrossing of an elite source of SLB resistance (NC250P) to B73, with selection for SLB resistance among and within backcross families. The goal of this paper was to characterize the loci responsible for the increased SLB resistance of NC292 and NC330 and to determine how many of the SLB disease resistance quantitative trait loci (dQTL) were selected for in the development of NC292 and NC330. Genomic regions that differentiated NC292 and NC330 from B73 and which may contribute to NC292 and NC330s enhanced SLB resistance were identified. Ten NC250P-derived introgressions were identified in both the NC292 and NC330 genomes of which eight were shared between genomes. dQTL were mapped in two F(2:3) populations derived from lines very closely related to the original parents of NC292 and NC330--(B73rhm1 x NC250A and NC250A x B73). Nine SLB dQTL were mapped in the combined populations using combined SLB disease data over all locations (SLB AllLocs). Of these, four dQTL precisely colocalized with NC250P introgressions in bins 2.05-2.06, 3.03, 6.01, and 9.02 and three were identified near NC250P introgressions in bins 1.09, 5.05-5.06, and 10.03. Therefore the breeding program used to develop NC292 and NC330 was highly effective in selecting for multiple SLB resistance alleles.

Isozymatic data taken from 67 Guatemalan collections of maize were subjected to numerical taxonomic analyses to elucidate systematic relationships among the 19 maize races and subraces described for Guatemala by Wellhausen et al. As with Bolivian and Mexican races, isozymatic variation in Guatemalan maize was strongly associated with altitude. Guatemalan lowland races were in general isozymatically distinct from races of higher elevations. Two middle elevation Guatemalan races proved difficult to place taxonomically. As a group, Guatemalan highland races were isozymatically more diverse than races from lower elevations, and were rather weakly differentiated from Mexican highland races. Notably, variational patterns evident from phenetic analyses of isozyme data were generally congruent with those apparent in phylogenetic analyses. The data reported here, and in earlier studies, suggested that divergent combinations of isozymatic, karyotypic, and morphological features have evolved in local maize races from Mexico, Guatemala, and Bolivia, perhaps as the result of the different selective regimens indigenous cultivators have imposed on different regional phylogenetic lineages.

The relative costs and benefits of genetic stock collections and germ plasm collections are discussed. The status of national and international collections is compared with the needs of plant breeders and geneticists. There is an international need for germ plasm systems that emphasize the use and employment of materials rather than acquisition and storage. For base collections to function, they must provide for regeneration, characterization, documentation, and evaluation of their materials. The quality of a germ plasm system should be judged on the basis of the quality of the materials available to scientists. Adequate quantities of high-quality seed that are of known provenience, spanning the range of known genetic diversity, promptly delivered, and well described constitute the minimum that should be expected. All too often such minimal requirements are not met.

Southern rust, caused by Puccinia polysora Underw, is a foliar disease that can severely reduce grain yield in maize (Zea mays L.). Major resistance genes exist, but their effectiveness can be limited in areas where P. polysora is multi-racial. General resistance could be achieved by combining quantitative and race-specific resistances. This would be desirable if the resistance alleles maintained resistance across environments while not increasing plant maturity. Recombinant inbred (RI) lines were derived from a cross between NC300, a temperate-adapted all-tropical line, and B104, an Iowa Stiff Stalk Synthetic line. The RI lines were topcrossed to the tester FR615 x FR697. The 143 topcrosses were scored for Southern rust in four environments. Time to flowering was measured in two environments. The RI lines were genotyped at 113 simple sequence repeat markers and quantitative trait loci (QTL) were mapped for both traits. The entry mean heritability estimate for Southern rust resistance was 0.93. A multiple interval mapping model, including four QTL, accounted for 88% of the variation among average disease ratings. A major QTL located on the short arm of chromosome 10, explained 83% of the phenotypic variation, with the NC300 allele carrying the resistance. Significant (P < 0.001), but relatively minor, topcross-by-environment interaction occurred for Southern rust, and resulted from the interaction of the major QTL with the environment. Maturity and Southern rust rating were slightly correlated, but QTL for the two traits did not co-localize. Resistance was simply inherited in this population and the major QTL is likely a dominant resistant gene that is independent of plant maturity.

Crop improvement and the dissection of complex genetic traits require germplasm diversity. Although this necessary phenotypic variability exists in diverse maize, most research is conducted using a small subset of inbred lines. An association population of 302 lines is now available--a valuable research tool that captures a large proportion of the alleles in cultivated maize. Provided that appropriate statistical models correcting for population structure are included, this tool can be used in association analyses to provide high-resolution evaluation of multiple alleles. This study describes the population structure of the 302 lines, and investigates the relationship between population structure and various measures of phenotypic and breeding value. On average, our estimates of population structure account for 9.3% of phenotypic variation, roughly equivalent to a major quantitative trait locus (QTL), with a high of 35%. Inclusion of population structure in association models is critical to meaningful analyses. This new association population has the potential to identify QTL with small effects, which will aid in dissecting complex traits and in planning future projects to exploit the rich diversity present in maize.

Cereal starch production forms the basis of subsistence for much of the world’s human and domesticated animal populations. Starch concentration and composition in the maize (Zea mays ssp mays) kernel are complex traits controlled by many genes. In this study, an association approach was used to evaluate six maize candidate genes involved in kernel starch biosynthesis: amylose extender1 (ae1), brittle endosperm2 (bt2), shrunken1 (sh1), sh2, sugary1, and waxy1. Major kernel composition traits, such as protein, oil, and starch concentration, were assessed as well as important starch composition quality traits, including pasting properties and amylose levels. Overall, bt2, sh1, and sh2 showed significant associations for kernel composition traits, whereas ae1 and sh2 showed significant associations for starch pasting properties. ae1 and sh1 both associated with amylose levels. Additionally, haplotype analysis of sh2 suggested this gene is involved in starch viscosity properties and amylose content. Despite starch concentration being only moderately heritable for this particular panel of diverse maize inbreds, high resolution was achieved when evaluating these starch candidate genes, and diverse alleles for breeding and further molecular analysis were identified.

Two hundred and sixty maize inbred lines, representative of the genetic diversity among essentially all public lines of importance to temperate breeding and many important tropical and subtropical lines, were assayed for polymorphism at 94 microsatellite loci. The 2039 alleles identified served as raw data for estimating genetic structure and diversity. A model-based clustering analysis placed the inbred lines in five clusters that correspond to major breeding groups plus a set of lines showing evidence of mixed origins. A “phylogenetic” tree was constructed to further assess the genetic structure of maize inbreds, showing good agreement with the pedigree information and the cluster analysis. Tropical and subtropical inbreds possess a greater number of alleles and greater gene diversity than their temperate counterparts. The temperate Stiff Stalk lines are on average the most divergent from all other inbred groups. Comparison of diversity in equivalent samples of inbreds and open-pollinated landraces revealed that maize inbreds capture

Genetic diversity of elite maize germplasm in the United States is narrow relative to the species worldwide. Tropical maize represents the most diverse source of germplasm. To incorporate germplasm from tropical maize landraces into the temperate gene pool, 23 Latin American maize accessions were crossed to temperate inbred line Mo44. During inbred line development, selection was practiced in temperate environments, potentially resulting in the loss of substantial proportions of tropical alleles. Genotyping 161 semiexotic inbreds at 51 simple sequence repeat (SSR) loci permitted the classification of their alleles as either Mo44 or tropical and allowed estimation of the proportion of detectable tropical alleles retained in these lines. On average, the percentage of detectable tropical alleles ranged among lines from 15% to 56%, with a mean of 31%. These are conservative, lower-bound estimates of the proportion of tropical germplasm within lines, because it is not known how frequently Mo44 and the tropical maize accession parental populations shared SSR alleles. These results suggest that substantial proportions of exotic germplasm were recovered in the semiexotic lines, despite their selection in temperate environments. The percent of tropical germplasm in semiexotic lines was not correlated to grain yield or moisture of lines testcrossed to a Corn Belt Dent tester, indicating that the incorporation of a substantial percentage of tropical germplasm in an inbred line does not necessarily negatively impact its combining ability. Thus, tropical maize accessions represent a good source of exotic germplasm to broaden the genetic base of temperate maize without hindering agronomic performance.

There exists extraordinary morphological and genetic diversity among the maize landraces that have been developed by pre-Columbian cultivators. To explain this high level of diversity in maize, several authors have proposed that maize landraces were the products of multiple independent domestications from their wild relative (teosinte). We present phylogenetic analyses based on 264 individual plants, each genotyped at 99 microsatellites, that challenge the multiple-origins hypothesis. Instead, our results indicate that all maize arose from a single domestication in southern Mexico about 9,000 years ago. Our analyses also indicate that the oldest surviving maize types are those of the Mexican highlands with maize spreading from this region over the Americas along two major paths. Our phylogenetic work is consistent with a model based on the archaeological record suggesting that maize diversified in the highlands of Mexico before spreading to the lowlands. We also found only modest evidence for postdomestication gene flow from teosinte into maize.