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 National Plant Germplasm System

CROP GERMPLASM COMMITTEE FOR MAIZE

ABSTRACT AND RECOMMENDATIONS

Maize (Zea mays L.) is the most important grain crop in the United States and the second (following wheat) most important cereal crop in the world. The United States is the leading exporter of maize in the world. Because of the importance of the crop to the United States economy and to the problem of feeding the world, it is essential that the maize germplasm base be maintained and enhanced. In this report, present germplasm activities are reviewed, status of vulnerability of the maize crop is examined, and germplasm needs (including collection, evaluation, and enhancement) are discussed in some detail.

Since the last report of this committee, a number of important steps have been taken to meet what were then the most important germplasm needs. Many of the at risk Latin American germplasm accessions were increased and seed supplied to the National Plant Germplasm System (NPGS). The Latin American Maize Project (LAMP) was carried through to the final stage of germplasm enhancement. The physical size of the National Seed Storage Laboratory (NSSL) in Fort Collins has doubled. The U.S. Germplasm Enhancement of Maize (GEM) project, cooperative between the private and public sectors, was developed and partial funding obtained.

Despite the major progress which has been made, a number of steps remain to be done to assure the safety of the maize germplasm base and to take full advantage of the germplasm resources available. The following is a list of priority items which remain.

  • Expand the effort on enhancement of germplasm. As one step, increase funding of the GEM project from the present level to the $950,000 per year originally requested. This project will help speed incorporation of tropical germplasm into elite U.S. material.

  • Develop a systematic screening effort to evaluate accessions in the Plant Introduction system for pest resistance. Modest efforts are underway on key insect pests. A joint proposal for systematic evaluation of accessions for disease resistance, requiring modest funding ($30,000 per year), has been made by public and industry corn pathologists.

  • Complete collections of tropical and semitropical maize and of Teosinte where important items are missing. See the section on acquisition for items needed. Continue to acquire public germplasm, including older inbreds, from programs which are being closed.

  • Expand the effort to regenerate and categorize maize accessions. With the increased number of accessions resulting from the expanded regeneration project, increased capacity for regeneration is needed. Based on 1996 dollars, an estimated additional $250,000-$300,000 per year in the North Central Regional Plant Introduction Station Zea genetic resources budget is needed.

CROP GERMPLASM COMMITTEE FOR MAIZE

I. INTRODUCTION

Maize (Zea mays L.) is the most important grain crop in the United States in total area and production. Total U.S. maize production in the last 45 years has increased by 320% with an accompanying reduction of 19% in acreage. This was accomplished by more than tripling 3.22 times) maize yields during that time. This means an increase in yield from an average of 37.1 bu/a in 1946-1952 to a 123.6 bu/a average from 1992-1994. These dramatic increases inyields were the result of a combination of genetic improvements in hybrids and improvements in husbandry and management practices. Russell, 1986 estimated that nearly 60% of the improvements were the result of genetic improvements. If these yield improvements are to continue, the germplasm base of maize must be maintained and expanded.

Maize is an important component of the national economy and a major contributor in the export trade of agricultural products. In 1994/95, approximately 2 billion bushels of maize were exported. In addition to the export market, refiners process maize into products such as sweeteners, starch, alcohol, oil, and feed ingredients. Since 1986, the date of the figures included in the last report of this committee, total Food and Industrial uses of maize increased by 39%. The largest increases were in Fuel alcohol (84.4%), High Fructose Corn Syrup (38%), and Glucose and Dextrose (35%). Of the average 8.6 billion bushels of corn produced each year from 1992-1994, approximately 1.7 billion bushels were used for food and industrial uses and approximately 2.0 billion bushels were exported (Corn Refiners Association, 1995). Most of the rest was used for livestock feed.

The United States is the leading exporter of maize in the world. Given the anticipated increased demand for maize in the Peoples Republic of China and other countries, the demand for maize in the export market will continue to increase. The continued increase in demand for corn sweeteners and fuel alcohol will also maintain strong demand. The development of high oil corn as a feed for poultry and swine is likely to further increase demand. Given the increased need for control of soil erosion, which likely will require increased use of rotations and forage crops, the acreage planted to maize in the U.S. is unlikely to increase by large amounts in the future. Thus, if the increased demand for maize is to be met by U.S. farmers, yields on a per acre basis must continue to increase. Given the national concern about water quality, increases in Nitrogen fertilizer rates are unlikely. This means more efficient exploitation of the germplasm available to breeders and the increased use of more diverse germplasm are essential.

In succeeding sections of this report, the status of crop vulnerability, germplasm activities presently underway to take advantage of diverse germplasm, and the present status and needs of maize germplasm collections are assessed.

II. STATUS OF CROP VULNERABILITY

The original landrace varieties included in the continental United States represented a vast array of maize germplasm, ranging from the tall, late, white southern dents concentrated in the southeast and the early flints located 42 to 45 degrees north latitude. Brown and Goodman (Brown and Goodman, 1977) assigned the pre-hybrid U.S. maize germplasm to nine broad racial complexes. The productive U.S. Corn Belt dents were derived in the 19th century from the crossing of the Southern Dents with the Northern Flints. Because of the highly productive conditions in the area designated as the U.S. Corn Belt, the Corn Belt Dents received greatest attention, and productive landrace varieties (open-pollinated varieties) were developed by seedsmen and farmers throughout the area. Lancaster Sure Crop, Reid Yellow Dent, Leaming, Hays Golden, Krug, Midland, Osterland Yellow Dent, Iodent, etc., became recognized varieties and were distributed and used both locally and throughout the U.S. Corn Belt. Many strains of the originally named open-pollinated varieties also were developed locally and assigned names either by the originator (e.g. Krug, Black's Yellow Dent) or the local area (e.g., Boone County White, Iowa Ideal, Pride of Saline). The open-pollinated varieties were used before the advent of hybrids and provided the basic germplasm for U.S. breeding programs.

After Shull (Shull, 1909) suggested the pure line-hybrid concept, breeding efforts were focused on testing the concept to provide a practical method for developing lines for use in hybrids. Shull's concept was extensively tested and developed to the point where it is considered one of plant breeding's greatest achievements. By the 1930s, double-cross hybrids were available for farmer use. Hybrids were rapidly accepted and by 1950 most of the U.S. Corn Belt was planted to hybrid maize. Inbred lines developed for use in the original hybrids were derived from the open-pollinated varieties. This approach was a natural one because the landrace varieties were the primary source of adapted, improved germplasm. Subsequently, pedigree selection was initiated in segregating populations derived from inbred line crosses, and the relative importance of the landraces as breeding germplasm decreased rapidly (Jenkins, 1978).

Breeding methods to exploit the pure line-hybrid concept of maize breeding reduced the interest and emphasis given to the original landrace varieties. It was soon found that repeated samplings of the landrace varieties were nonproductive, as would be expected if the original samplings were adequate. Hence, pedigree selection among progeny of crosses of elite lines was emphasized. The long-term potential value of the landraces was not appreciated and in some instances they were lost or were inadequately sampled for storage in germplasm banks.

After the initial pause in hybrid improvement, development of successive generations of recycled inbred lines resulted in continuing increases in yield and general performance of hybrids. Interest in the genetic causes of improved performance and the hope for continued improvement stimulated quantitative genetic and selection studies within and between maize populations (Gowen, 1950). Most of the evidence suggested that the genetic variation was primarily due to additive genetic effects with partial to complete dominance of the alleles at segregating loci. There were some exceptions: with F2 populations derived from inbred lines, over dominance was detected but this was found to be due to repulsion phase linkage bias; and significant nonadditive effects were detected from analyses of generation means and types of hybrids. Generally, it seems additive genetic effects are of greater importance, and breeding and selection schemes that effectively capitalize on selection method studies demonstrated that sufficient genetic variability was present in maize germplasm to allow continued genetic progress. (Hallauer and Miranda, 1981).

Genetic variability within maize breeding programs was extensively discussed after the southern leaf blight epidemic in 1970. The southern leaf blight epidemic was caused by Helminthosporium maydis, Race T, which infected plants with Texas male sterile cytoplasm (Tcms). The use of Tcms was found to be a very efficient method for the production of hybrid seed without detasseling. Other cytoplasmic male sterile systems were available and had been used but the Tcms system was more broadly useful and reliable than the others (Duvick, 1965). Consequently, it is estimated that as much as 90% of the hybrid seed available to farmers in 1970 was produced with aid of Tcms. In 1970 the right weather, the host and the pathogen were all present at the same time, and H. maydis, Race T spread rapidly from the Southeast U.S. to the central Corn Belt. The severity and wide range of this disease in 1970 generated wide-spread concern about the genetic vulnerability of the U.S. maize crop. Although the H. maydis Race T epidemic was due to cytoplasmic uniformity, concern was also expressed about the possibility of excessive nuclear uniformity. The H. maydis race T epidemic of 1970 was correctly diagnosed and within one year the problem was corrected either by producing seed on normal cytoplasm or using other male sterile cytoplasms (Duvick, 1972). All Tcms inbreds were also available in normal cytoplasm, because the normal, pollen fertile lines were maintainers of the male sterile counterparts.

The 1970 H. maydis Race T epidemic focused attention on the genetic variability of major U.S. crop species, including maize (NRC, 1972). Based on surveys of the most widely used public lines included to produce hybrid seed, it seemed that the genetic base, at least for the lines used for producing hybrid seed, of the U.S. maize crop was becoming more restricted over time (NRC, 1972 p. 97-118, Zuber and Darrah, 1980). B37 (26%) and B14 and its derivatives (19%), for example, were frequently included in the parentage of commercial hybrids during the 1960s (Sprague, 1971). Subsequently, B73 was one parent of 16% of the hybrid seed produced for use in 1980 (Darrah and Zuber, 1986). These data were only for lines developed by publicly supported breeding programs and did not include either recovered or new lines developed by privately supported breeding programs. Thus the possibility existed that a variety of unrelated inbred lines were used in 1980 to make single cross hybrids with B73 as one parent, thereby increasing genetic variability in hybrids on the farm.

The concern of the danger of genetic vulnerability of the U.S. corn crop is valid but the dangers of uniformity in the maize crop may not be as great as originally supposed in the early 1970s. There are two important locations of useful genetic diversity: 1) among the hybrids that are produced and made available for production to the growers; and 2) among the genetic materials, both lines and hybrids, that are available in breeding programs. Most commercial firms have a complete product line of hybrids available to meet the customers' needs, but if a particular hybrid is superior to others, the customer will demand the best and not the second, third, or fourth best. This can result in great uniformity in some areas. However, farmers usually wish to spread their risk and buy top-performing hybrids from several different companies, or the best hybrids in two or three maturity groups from a single company. Further, the genetic variability among hybrids will increase as the production area expands because of the need to emphasize greater drought tolerance, specific pest resistance, or hybrids with different maturities in the different areas of the U.S. Corn Belt. The pedigrees among hybrids change in both the east-to-west and north-to-south directions in the United States, due to needs for adaptation to different environments. Hence, there is an automatically enforced degree of genetic diversity among hybrids between regions, within regions, and also in time (as old hybrids are replaced by new ones every few years) (Duvick, 1975, Duvick, 1984).

Dynamic breeding programs continually develop new lines and hybrids that are superior to those currently used. Although a few elite lines may be used extensively, the breeder has an arsenal of other lines that can rapidly replace those currently used. Although hybrids are used 5 to 7 years on the average, there are lines available to produce different hybrids immediately, if those currently used become susceptible to a pest. The genetic base of breeding programs is always greater than the hybrids made available to growers, for only a small sample of tested hybrids and inbreds are chosen for release. Breeders are continually seeking new alleles or new genetic arrangements to increase stability of performance including resistance to anticipated pests.

Breeding efforts have been concentrated within adapted elite germplasm, however. Goodman (Goodman, 1985) estimated foreign (exotic) germplasm currently accounted for less than 1% of the U.S. maize germplasm base and only about 4% of the total U.S. maize acreage. He suggested that the percentage of exotic germplasm in U.S. corn hybrids would increase very little during the next 50 years. Concern over the slow progress in introducing exotic germplasm into the U.S. hybrid corn base has prompted initiation of a program especially designed to introgress exotic germplasm into elite U.S. germplasm pools. The program, called GEM (Germplasm Enhancement of Maize) is carried out by a consortium of public breeders and seed companies (Salhuana, et al., 1993/1994).

Adequate and useful genetic variation is of concern to all involved with maize germplasm, breeding, and production. The issues of genetic vulnerability have been discussed and inventories made of the materials used and available. Because of the nature of maize gerrnplasm, breeding methods used, cultivars available for production, and the level of research activity conducted in maize, genetic variation is available to minimize risks of a widespread catastrophe. Although hybrids in each era tend to concentrate on a few inbred lines and their derivatives, and less than 5% of the world's maize germplasm has been used by U.S. breeders, the genetic variation included in most U.S. breeding programs seems adequate to insure the safety of future maize production. In years to come the ancestral base of U.S. maize hybrids will increase, as introgression of exotic germplasm slowly takes place. However, there is urgent need for new surveys of the kind conducted by Darrah and Zuber, by Duvick, and by Goodman, to establish the amount and sources of diversity and the pace of changes in diversity in U.S. maize hybrids and breeding pools.

In addition to surveys of breeders, sophisticated new technologies can be used to estimate the amount of genetic diversity among maize hybrids (Smith and Smith, 1992). Isozymes, protein characterization, and molecular markers can be used to detect the degree of relatedness of maize hybrids, even without knowledge of their pedigrees (Smith, 1994). Smith analyzed 138 hybrids available for use in the U.S. Corn Belt (Smith, 1988). He used 21 isozyme loci and chromatographic zein profiles to estimate the relative commonality among the 138 hybrids. He concluded that 40% of the hybrids released by private companies were unique and 25% released by foundation seed companies were unique. The data also indicated that nearly 60% of the hybrids released from private firms either included B73, Mol7, and A632 as direct parents or as one of the major contributors of germplasm in their pedigrees. Smith concluded that B73, A632, Mol7, and Oh43 continued to be major contributors to U.S. maize production either directly or through derivatives. Recycling to develop improved versions of popular inbred lines will develop lines with subtle differences from their parents; the unanswered question is how important these differences will be in contributing to useful genetic diversity to protect against widespread epidemics and infestations, or unfavorable weather (NRC, 1993).

III. PRESENT GERMPLASM ACTIVITIES

Maize germplasm ranges from elite inbred lines to composite varieties developed by intermating land race varieties. Some of the germplasm may be currently used in pedigree selection programs whereas other sources may have had limited use after the initial collections were placed in germplasm storage banks. The relative use of the different categories of germplasm depends on the primary objectives of the research programs. Applied breeding programs emphasize selection within elite germplasm sources to enhance the opportunities of developing lines and hybrids superior to those currently available. Other types of germplasm are considered either as possible backup sources to meet unexpected emergencies or for seeking additional genetic variability. Basic genetic studies depend on specific genetic stocks for identified markers either for a particular locus or a specific chromosomal arrangement. Programs that emphasize genetic enhancement use either adapted land race varieties or synthetic varieties derived by intercrossing materials that meet certain standards for specific traits. Recent programs have been initiated to consider the evaluation, adaptation, and enhancement of germplasm that is not adapted to temperate environments.

A. Breeding programs

Germplasm sources are an important component of the germplasm included in breeding programs, ranging from F 2 populations derived from crosses of elite inbred lines to genetically broad-based composites. The types of germplasm included depend on the major objectives of the breeding programs. Bauman (1981) in a survey of U.S. maize breeders reported that 39% of their effort for inbred line development included germplasm that originated from double crosses (2%), single crosses (22%), and related line crosses (15%). Only 15% of their effort was devoted to genetically broad-based populations. For programs that do not have primary responsibility for inbred line development, greater emphasis may be given to evaluating and adapting exotic germplasm sources, genetic enhancement of adapted germplasm, and evaluation of breeding methods. But Hallauer (1979) in a survey of publicly and privately supported breeding programs did not find any detectable differences in the types of germplasm used by the public and private breeders.

Since 1970, there has been concern that the genetic variation within U.S. maize breeding programs has become too restricted. Because of the concentration of maize production in the North Central Region of the United States, the North Central Corn Breeding Research Committee (NCR-2, 1977) surveyed the maize breeding programs within the North Central Region to determine the extent of the maize improvement work in progress and the populations available within the publicly supported breeding programs. The survey included 446 maize populations. Of these, 226 were included in selection and breeding programs whereas 200 populations were inactive. The active populations were generally genetically broad based and included open pollinated varieties (11%), synthetic varieties that included adapted lines (64%), and varieties that included some exotic germplasm (25%). Of the inactive populations, 21% were open-pollinated varieties or direct derivatives thereof. Single-trait selection was used in 75% of the active populations with yield (18%) and disease resistance (27%) receiving greatest emphasis. Mass (56.9%) and SI progeny selection (14.6%) were the more prevalent forms of selection.

The NCR-2 (1977) study demonstrates the breadth of germplasm available to maize breeders. Although the survey was restricted to publicly supported programs of the North Central Region, the information suggests the types of materials available in other programs and other regions. Private breeding programs emphasize selection in more restricted populations, but they also have available reserve germplasm either undergoing mild selection or in storage. The NCR2 report indicated there were 24 active selection programs being conducted in Stiff Stalk Synthetic. Selection, however, was for different traits at the same or different stations. Selection would affect allele frequencies in the different strains of Stiff Stalk Synthetic to create additional genetic variability within Stiff Stalk Synthetic.

Since the 1977 survey, changes have occurred which could impact the survey conclusions: 1) several public sector maize breeding programs in the North Central Region either have been discontinued or downsized; and 2) a significant expansion in the number and size of the private sector maize breeding programs has occurred. Both changes could reduce the genetic variability of germplasm emphasized in maize breeding. The number of populations under selection in public sector breeding programs surely is reduced since 1977. Also, the extent and number of traits under selection have been reduced. By contrast, the number of populations under selection has significantly increased within the private sector breeding programs, but the populations considered in selection are generated from elite line crosses. There are fewer elite lines being released by public breeding programs, and, therefore, externally available lines for use in private breeding programs are reduced. Again, suggesting that the total genetic variability available to maize breeding programs has been reduced during the past 20 years.

The possible changes that could reduce genetic variability may be counter-balanced by increased emphasis on genetic enhancement of genetically broad-based populations by private breeders, the Latin American Maize Project (LAMP) to rescue and to evaluate maize germplasm, and the Gennplasm Enhancement of Maize Project (GEM) for the genetic enhancement of germplasm identified in LAMP. Each of these programs has long-term goals but should ensure potential sources of elite germplasm for the future. For the present, the genetic variation of the germplasm used in breeding programs is less than in the past, but adequate genetic variability seems to be available for continued genetic gain. Duvick (1981a,b) summarized the status of genetic resources and urged that maize breeders develop and put into practice precise methods to increase useful genetic diversity in maize. The recent LAMP and GEM programs suggest genetic diversity should not be a limiting factor in the future (Hallauer, 1995).

B. Zea germplasm management sites

1. The NCRPIS --The NPGS active collection of more than 14,000 accessions of Zea (maize and the teosintes) is managed by the NPGS's Zea Curator at the USDA/ARS North Central Regional Plant Introduction Station (NCRPIS) at Iowa State University, Ames, IA. The active collection includes inbred lines (mostly from the U.S.), breeding populations (mostly from mid- to high-latitudes), land races from North and Latin America, and the most comprehensive collection of teosintes (the wild relatives of maize) outside of Mexico. Managing the active collection involves acquiring, maintaining, distributing, evaluating, and characterizing this germplasm, and administering associated passport, characterization, and evaluation data maintained on the Germplasm Resources Information System (GRIN).

Since the last Maize Germplasm Report in 1987, the NPGS collection has acquired Prof. H. Iltis's teosinte accessions, which substantially augmented the NPGS's holdings of these plants. The active collection of maize has been transformed from a collection of 6,000 accessions, dominated by mid-latitude land races and lines, to a collection of more than 14,000 accessions, with a broad ecogeographical scope and relatively strong Latin American holdings. "Authentic sources" for 44 U. S. field corn public inbred lines were acquired for PVP certification purposes. The Crookham sweet corn collection and the Mangelsdorf-Galinat collection (South American maize collected during the 1940s) were also incorporated into the NPGS collection.

Seed for distribution is maintained at 5 oC and 30% rh, but since the last Maize Germplasm Report, all the original seed and the ca. 1600 accessions of the Mangelsdorf-Galinat collection have been moved to a new -20 oC walk-in freezer at the NCRPIS. Seed is periodically assayed for its health and germinability. More than 68% of the active collection is backed-up in the base collection at the USDA/ARS National Seed Storage Laboratory (NSSL). The active collection of Zea is used heavily, with an average of more than 3,000 seed packets per year distributed during 1988-1995, ca. 75% going to scientists in the U.S. and the remainder to other countries. About a third of the distributions to U.S. requesters go to the private sector, with the remaining two-thirds requested by university and government researchers. The number of accessions distributed annually averages + 15% of the total available for distribution.

During 1987-1995, an average of ca. 200 mid-latitude accessions were regenerated per year in Ames, with another 100 tropical accessions regenerated per year in Puerto Rico, yielding a total of ca. 2,700 Zea accessions regenerated during that period. Maize land races are regenerated via a special paired-plant controlled-crossing scheme that maximizes the effective population size for the accession. "Balanced samples" were produced from maize populations so that a maximum of genetic diversity is retained for future use. Several seed companies, such as Northrup King, Limagrain, and Pioneer Hi-Bred, regenerated several hundred tropical and mid-latitude maize accessions for us at no charge. Teosinte accessions were regenerated in positive pressure chambers in glasshouses at Ames very successfully. Priorities for regeneration were set according to the seed inventory and demand for the accession.

Since the last Maize Crop Germplasm Report, substantial progress has been made in regenerating endangered Latin American land race accessions which are, or may soon be, part of the NPGS and the Centro Internacional de Mejoramiento de Maiz y Trigo (CIMMyT) Zea collections. This regeneration effort was mounted by a series of administratively separate projects with very similar goals: 1) A joint USDA/ARS-North Carolina State University (NCSU) cooperative project, coordinated by Prof. M. Goodman at NCSU, regenerated more than 5,000 land race accessions from Colombia, Mexico, and Peru. Most of the regenerated seed was retained in the country of origin, and a duplicate sample was provided to the NPGS, but not to CIMMyT; 2) Dr. W. Salhuana of Pioneer HI-Bred International, Inc. regenerated almost 2,000 Latin American accessions from the CIMMyT collection, and provided samples to both CIMMyT and the NPGS; 3) Dr. S. Taba of CIMMyT coordinated a project funded by the U.S. Agency for International Development (USAID), the USDA/ARS, and CIMMyT, that regenerated more than 7,000 accessions from 13 Latin American countries. Most of the regenerated seeds were retained by the country of origin, but duplicate samples were provided to the CIMMyT active and base collections, which are backed-up at the NSSL; 4) As noted earlier, more than 900 Latin American maize land race accessions in the NPGS collection were regenerated by NCRPIS and other NPGS personnel in Puerto Rico or at the quarantine site at St. Croix, VI.

Characterization is a particular priority with the Zea collection, because many maize accessions have not been identified even to the level of race. Characterization data can help verify racial affinity, assist with quality assurance, provide information potentially valuable for defining core subsets or test arrays, and otherwise improve curatorial efficiency and encourage utilization. Fifteen morphological and general agronomic features are recorded from ca. 200 Zea accessions per year in Ames (one location, one replication) and from 100 accessions per year in Puerto Rico (one location, one replication). Maize ears regenerated in Ames and Puerto Rico are photographed (or "imaged," see below) for archival and quality assurance purposes. Based on the variational patterns of agronomic characters, a core subset of the CIMMyT maize collection is being assembled by CIMMyT personnel. A provisional test array composed of ca. 1500 New World accessions of maize has been assembled for the NPGS active collection; its utility is currently being assessed at the NCRPIS.

Since 1991, the USDA/ARS National Program Staff has funded an ongoing specific cooperative agreement between the NCRPIS and Iowa State University (ISU) Seed Science Center to develop an integrated computerized system for managing images of maize ears for curatorial applications. The project has essentially attained its goals of developing an optimal system composed of various software and hardware for capturing and storing these images.

Since the last Maize Gerrnplasm Report, the NCRPIS began to develop its Zea genetic marker characterization capabilities by hiring a grad uate student and a technician and by purchasing additional chemicals and equipment. Isozyme and DNA marker analyses will assist various genetic resource-management tasks: resolving systematic and genetic affinities of poorly known accessions and providing quality assurance for our regeneration programs will be among their most important roles.

During 1987-95, ca. 1000 maize accessions per year were evaluated at the NCRPIS for host-plant resistance (hpr) to 1st generation European Corn Borer (ECB). About 200 accessions per year were screened for hpr to corn earworm feeding. Of the ca. 9,000 accessions assessed for hpr to ECB, eleven Peruvian accessions were identified as highly resistant to ECB feeding, but lacking in high concentrations of DIMBOA. At present, there is no replicated plot screening program at the NCRPIS for hpr to maize pathogens, but accessions regenerated in Ames are observed in the field for phytosanitary certification, during which time resistance/susceptibility to the following diseases is noted: Puccinia spp. (rust), Ustilago maydis (smut), Cochliobolus and Exserohilum spp. (leaf blights), Erwinia stewartii (Stewart's wilt), and Kabatiella zeae (eyespot). The epidemiology of Stewart's bacterial wilt ("sbw") in maize and its transmission via kernels, the primary quarantine issue affecting international shipment of U.S. seedcorn, was studied at the NCRPIS with ELISA assays and by statistical estimation of seed infection rates. The results of this research suggested that the rate of sbw transmission via seeds was essentially zero for high-quality seed lots.

2. The MGSC-SC-- The approximately 3,000 genotypically different accessions composing the NPGS active collection of maize genetic stocks are managed by the Maize Genetic Stock Curator at the USDA/ARS Maize Genetics Cooperation-Stock Center at the University of Illinois, Urbana, IL. The Stock Center, which acquires, maintains, and distributes maize genetic stocks of specific loci, linkage groups, and chromosomal aberrations, is very important nationally and internationally for providing researchers with specific combinations of mutant loci and cytological abnormalities for genetic, physiological, breeding, and molecular research. Since the last Maize Germplasm Report, responsibility for the daily operations of the MGSC-SC has been assumed by USDA/ARS personnel.

The MGSC-SC acquires not only recently-discovered mutants, but also "orphaned" collections from deceased or retired researchers. Information associated with these orphaned collections, such as the notebooks and reprint collections of M.M. Rhoades, G.F. Sprague, and E.G. Anderson, are also maintained. During 1993-1995, it annually distributed an average of more than 2,600 samples in response to nearly 300 requests per year. Almost 80% of the distributions were within the U.S., whereas the remainder were international. A broad spectrum of researchers requested germplasm, with university (72%) and industry (20%) researchers comprising most of the clientele, followed by government institutions (6%) and other [including K-12] institutions (2%).

While regenerating genetic stocks, the MGSC-SC may improve them by introgressing mutants into adapted inbred lines that are easier to manage in the field and in storage. This is necessary for relatively few mutants, because most can be maintained in a somewhat nondescript genetic background, with occasional outcrossing followed by re-extraction to invigorate the stock and adapt it more widely (especially if the mutant is derived from tropical or extreme northern material).

Various projects for characterizing maize mutants have been conducted at the MGSC-SC since the last Maize Germplasm Report. The identity and allelic relationships of particular mutants are determined by classic allelism tests, the details of which depend on the mutants' genetic control (multigenic vs. single genes, dominant vs. recessive, homozygous lethals, etc.). The best genetic backgrounds for enhancing the expression of particular mutant alleles are determined by crossing the mutant into various inbred lines with different genotypes.

Several new mutants are being characterized, including one for starch biosynthesis generated via the Mutator system, a reverse germ orientation mutant in which homozygous plants yield ears upon which the kernel germs face toward the base of the ear, nuclear pollen sterile mutants (very valuable for studying pollen/anther development), a mutable allele of a novel viviparous locus with a lemon endosperm (with yellow sectors) and albino seedlings with green revertant stripes, suggesting a defect in the carotenoid pathway, and null globulin1 alleles that were isolated from the Illinois high-low oil/protein selection stocks.

3. The NSSL--The NPGS base collection of more than 8,800 accessions of Zea is managed by th e NSSL, Ft. Collins, CO. Since the last Maize Germplasm Report, the physical size of the NSSL has been doubled, and high security long-term storage vaults constructed. The use of advanced technologies for long-term seed storage (e.g., cryopreservation) has been increased.

After maize kernels are dried to equilibrium at ca. 5 oC and 25% rh, they are stored at 18 oC in moisture-resistant packages in a high-security vault. In addition to preserving the NPGS base collection, the NSSL also safeguards in "security backup" duplicate seed samples of the ca. 10,000 Latin American maize accessions comprising the CIMMyT active and base collections for Zea. A selected subset of the CIMMyT collection will eventually be incorporated into the NPGS collection.

During the last decade, scientists at the NSSL have been developing strategies for rescuing via embryo culture deteriorated maize kernels from "orphaned collections," from poorly maintained collections, or even from recent fieldwork. The deterioration may have resulted from suboptimal field conditions during cultivation or harvest, suboptimal harvesting and drying practices, and suboptimal storage or transport conditions. These kernels may germinate very poorly in the field and greenhouse, thus hampering experiments or efforts to regenerate the germplasm in the field or greenhouse with standard techniques. Excised maize embryos from deteriorated kernels were grown in tissue culture under a variety of antibiotic, hormonal and nutrient regimens. The embryo culture protocol increased the germination percentage and growth rate of aged embryos, as compared to normal germination methods, so it is a promising technique for rescuing maize germplasm.

C. Latin American Gene Banks

In addition to holdings in the U.S. germplasm system, gene banks in Latin American countries are holding large numbers of accessions from Central and South America. The largest number of germplasm accessions are in INIFAP in Mexico and ICA in Colombia, both were initiated by support of the Rockefeller Foundation. The germplasm bank in Mexico is holding accessions from Mexico, Central America and the Caribbean. Some of the foreign accessions are not duplicated in any other germplasm bank. Colombia is holding collections from Colombia, Caribbean, and South America, some of which are not duplicated in other gene banks. Other Latin American countries are holding germplasm which was collected in their own country. Many of these storage facilities are small and the equipment is very old. Thus, it is important to inventory collections in the various countries, collate the inventories, and make provision for duplicating collections not held in other storage facilities.

According to the reports of the maize curators of Latin American countries (CIMMYT, 1989. Proceedings of the Global Maize Germplasm Workshop) the total number of accessions held in these gene banks is 31,159. An estimated 3,996 of these are duplicates. Thus, the total number of accessions is approximately 27,763.

CIMMYT, NSSL, and NCRPIS need to work together to develop information to know which materials are missing and develop a coordinated plan to fill any gaps.

IV. GERMPLASM NEEDS

A. Acquisition

Germplasm is incorporated into the NPGS collection according to 1) curatorial judgment regarding gaps in the NPGS collection's representation of the genetic diversity within Zea, 2) recommendations from the Maize CGC and other scientists regarding Zea germplasm of specific interest to the user community, and 3) the NPGS guidelines for plant acquisition (see Diversity 5(4): 30). Judging from the preceding criteria, we estimate that the NPGS Zea collection will ultimately number probably 18,000-20,000 accessions. Cold-storage space at the NCRPIS or at the NSSL is not currently a limiting factor for managing that number of accessions.

At present, we do not foresee acquiring significantly more teosinte accessions, due to the strength of our current collection, but additional samples of Guatemalan populations would be desirable. However, this is not an appropriate time, politically, to collect in Guatemala. Adequate of 100-150 teosinte accessions from Mexico and Guatemala are lacking. Many of these have been collected by the national program (INIFAP) in Mexico but are not readily available to workers in the U.S. The NPGS should offer to backup these teosinte holdings. Although the NPGS Zea collection includes relatively comprehensive holdings of maize land races from North America, and from certain Latin American countries (e.g., Mexico, Peru), land races from the following New World regions should be acquired: Bolivia, Brazil, Central America (especially Costa Rica, Guatemala, and Panama), certain Caribbean locations (e.g., Coastal Tropical Flint from Dominica), and the Amazon lowlands. Furthermore, accessions designated as "typical" representatives of Latin American land races in the maize racial bulletins should be acquired.

The NPGS collection includes representative holdings of the maize of Europe and mid-latitude Asia. Certain ecogeographical regions in Asia and Africa are poorly represented in the NPGS collection, including the Himalayan foothills, the Philippines, former Portuguese colonies/enclaves in Africa (e.g., Mozambique), India (e.g., Goa), and Southeast Asia (e.g., Macao). As quarantine regulations allow, NPGS will try to acquire these land races via CIMMyT, other germplasm banks (e.g., the SADCC bank in Lusaka, Zambia), or by field collection. The depth and breadth of these future acquisitions will take into account the holdings at CIMMyT, and the activities of its maize germplasm bank and of other banks.

In response to the user community's expressed needs, NPGS will acquire the following, more elite germplasm for distribution: 1) additional inbred lines from discontinued U.S. public maize breeding programs, especially historically-important U.S. "southern breeders' lines." The U.S. inbred lines are among the most frequently requested accessions in the NPGS collection, 2) selected tropical inbred lines from CIMMyT and the University of Hawaii program, and selected tropical hybrids (public and private), especially derived from Caribbean germplasm, and 3) selected U.S. popcorn and sweet corn germplasm.

The Zea curator will continue to assign P.I. numbers to LAMP and other Latin American accessions entering the NSSL, while concurrently scanning the NSSL and CIMMyT inventories for germplasm needed for the NPGS collection. Bit by bit, sub-samples of Zea accessions held at the NSSL but not at the NCRPIS will be secured; concurrently, the unduplicated accessions at the NCRPIS will be backed-up at NSSL.

As mentioned earlier, storage space at the NCRPIS or the NSSL for new accessions is not a limiting factor for germplasm acquisition, but funds for exchanging and processing accessions are limited. Additional monies (see budget) will be required for 1) clerical assistance to inventory and process newly acquired accessions; ii) additional plantings in quarantine nurseries; iii) importing and exchanging germplasm.

B. Preservation--(Maintenance and Regeneration)

To keep up with the NPGS Zea collection's growth, the NCRPIS must increase the number of accessions regenerated per year at Ames to ca. 300-400. A particular priority is regenerating the ca. 1600 accessions in the Mangelsdorf-Galinat whenever additional glasshouse space and technical assistance become available. In addition to our Puerto Rico winter nursery, we continue to investigate low-latitude, high-elevation sites, such as at ca. 800 in elevation on the "Big Island" of Hawaii, for regenerating Andean accessions. The number of NPGS accessions regenerated in the tropics in the future will be determined by available funds and low latitude nursery sites, future collaborations with the private sector, and with reference to CIMMyT's maize collection and that institution's activities.

The funds available for regenerating accessions are currently a limiting factor for the NPGS Zea management program. Additional funds (see budget) must be allocated on a per year basis in order to regenerate 300-400 accessions per year at Ames, and more or less the same number in the tropics. Ideally, more seed companies will collaborate in the effort to regenerate maize accessions, especially tropical germplasm.

C. Characterization

When new funds become available (see budget), the NCRPIS plans to at least double the genetic-marker program, to test how efficiently and effectively our management program is conserving purity and genetic diversity of Zea accessions. Isozyme and DNA analyses will become increasingly important to curating the NPGS collection, as the NCRPIS assumes managerial responsibility for additional special gerrnplasm such as the "PVP-authentic source" inbred lines. By the end of 1996, the computerized-imaging project will have provided the NCRPIS with a tool, complementary to genetic-marker analyses, for defining test arrays, core

subsets, and increasing our characterization program's efficiency and efficacy. Additional funds will be needed to purchase more computers, scanners, etc., to make full use of this tool.

D. Evaluation

1. NCRPIS--The evaluation effort at the NCRPIS would in the future emphasize encouragement and coordination of Zea evaluation efforts, and "curation" of the ensuing evaluation data, such as those from cooperative insect and disease-evaluation programs. The NCRPIS would provide the seeds for these programs, so the regeneration-maintenance effort would need to be increased accordingly. As core subsets and test arrays are identified in conjunction with the Maize CGC, these subsets of accessions will have priority for evaluation. To expand significantly the Zea pathogen evaluation program, additional funding will be needed for operating this program, and for supporting cooperative evaluation programs with the public and private sectors.

The recently expanded physical facilities at the NCRPIS will not limit the maize hpr evaluation effort in the future. The evaluation programs for host plant resistance (hpr) to 2nd generation ECB, additional corn earworm screening, and perhaps screening for hpr to fall army worm, corn root worm, and Southwestern corn borer could be studied in conjunction with USDA/ARS scientists in Georgia, Missouri, and/or Mississippi if additional funding were available. Elucidating the genetic control for any novel sources of hpr encountered, and the reaction of tropical maize to temperate zone insect pests, would be emphasized.

Over the years, a substantial body of Zea evaluation data (e.g., general agronomic merit, hpr, etc.) has accumulated without being entered into GRIN or otherwise disseminated. As funds become available, at least an additional $15,000 per year should be allocated for hiring at least one additional full-time clerk/data entry specialist for data entry and "information curation" of Zea evaluation data, and any additional LAMP or GEM data which will be forthcoming. Should comprehensive Zea evaluation projects be initiated, funding increases for maintenance regeneration would be needed to provide seed for those programs.

2. LAMP--In addition to the ongoing evaluation effort at NCRPIS, the first coordinated international project to deal with the evaluation of the genetic resources of a major world crop occurred in maize. This project, the Latin American Maize Project (LAMP), was initiated when Pioneer Hi-Bred International provided $1.5 million to the USDA-ARS in 1987 for carrying out a five stage maize evaluation protocol. LAMP was based on the cooperative efforts of 12 countries: Argentina, Bolivia, Brazil, Colombia, Chile, Guatemala, Mexico, Paraguay, Peru, United States, Uruguay, and Venezuela. LAMP evaluated over 12,000 accessions (74% of total maize races) in locations divided into five homologous areas covering latitudes from 34 oS to 41 oN, longitudes from 44 o to 101 oW, and altitudes from 2900-3300 m above sea level.

In 1991, a catalog and CD-ROM of data of 12,113 accessions evaluated in LAMP's first stage, and 2,794 selected (primarily on yield) accessions evaluated in the second stage in 59 different locations of 32 regions of the 12 countries was published. Based on that data the Principal Investigators in each country selected a total of 268 elite accessions that were crossed with the best testers of each region. Thirty-one testers were used for crossing with the elite accessions. Within a homologous area, Principal Investigators exchanged testcrosses among other Principal Investigators in the same homologous area, so that testcross evaluations were done in more than one country. Data from the testcross evaluations were published in a catalog and updated LAMP CD-ROM in 1995.

LAMP's agronomic evaluations proved to be an efficient method to screen a large number of accessions, select superior genotypes for breeding, determine the precise status of germplasm banks in Latin America, determine accessions that need regeneration and establish the adaptability of races and accessions. LAMP was also effective in establishing a network for effective cooperation among countries.

Evaluations on a smaller scale occur at CIMMyT, primarily in major races to establish core collections, and at other germplasm banks such as NCRPIS, primarily for disease and insect resistance. Public and private researchers often conduct small germplasm evaluations for yield or other traits, but results are not always published or widely available.

It is estimated that about 10,000 accessions from 17 Latin American countries did not have seed available when LAMP began, but due to the Latin American Maize Regeneration Project are now available for evaluation. A LAMP II has been proposed to evaluate these additional accessions, but has no funding at this time.

E. Enhancement

In the final stage of LAMP, each Principal Investigator was to enhance selected germplasm to meet particular breeding objectives, yet minimal funding was available. In the U.S. it was recognized that for enhancement of the LAMP germplasm to proceed a cooperative effort needed to be organized among public and private breeders. Member companies of the American Seed Trade Association (ASTA) expressed their conviction for an enhancement effort by pledging in-kind support in the form of winter and summer nursery rows, yield trial plots, and disease observation rows worth about $450,000 annually. A sub-committee from the Corn & Sorghum Basic Research Committee of ASTA lobbied key legislators for permanent base funding to ARS to support the public effort at ARS and university locations. In 1995, $500,000 of permanent funding was appropriated to support coordination of the enhancement effort at the ARS location in Ames, IA, a satellite location at the ARS location in Raleigh, NC, and support for public cooperators at other ARS and university locations.

The objective of this enhancement effort, named U.S. Germplasm Enhancement of Maize (GEM), is to provide to the corn industry materials developed using germplasm enhancement of useful exotic gennplasm. GEM is an ongoing project, but to initiate enhancement 50 elite tropical and temperate LAMP accessions were chosen, plus 7 commercial tropical hybrids provided by DeKalb Genetics. The enhancement protocol is for one of the private cooperating companies to cross an exotic material by a proprietary inbred line to make a 50% exotic breeding cross, then for another private cooperator to cross the 50% cross with their proprietary line of the same heterotic pattern to make a 25% exotic breeding cross. All breeding crosses are evaluated for yield as testcrosses, and the best used to develop breeding lines by cooperators. Because proprietary germplasm is used to make breeding crosses, access to breeding materials and data collected on them is limited to GEM cooperators, but the opportunity to become a cooperator is available to all. GEM enhanced lines and synthetics, and all associated data, will be freely available through NCRPIS after their release. Traits targeted for improvement are agronomic productivity, disease and insect resistance, and value-added characteristics.

At this time, 21 companies are involved as private cooperators, and over 30 public scientists have either conducted evaluations, become involved in the breeding effort, or expressed interest in cooperating in the future. Over two hundred exotic by proprietary inbred line Stiff Stalk or non-Stiff Stalk breeding crosses have been developed (both 25 and 50% exotic). The initial accessions and/or breeding crosses either have or will be evaluated for yield as testcrosses, total composition (%starch, oil and protein), oil quality, starch quality, protein quality, wet milling characteristics, gray leaf spot, corn rootworm, European corn borer, and in addition to many other diseases and insects.

A sub-committee of the Corn & Sorghum Basic Research Committee of ASTA is continuing to lobby key legislators for full funding for GEM, which would be an additional appropriation of $450,000 ($950,000 total). This would enable the Iowa and North Carolina ARS locations to purchase equipment and fund staff necessary to carry out the GEM objectives. It would also provide funding for the increased germplasm evaluation and breeding necessary to test and enhance the initial GEM accessions, plus other exotic materials.

V. RECOMMENDATIONS

Major recommendations are summarized in the abstract and repeated here. Details of requested needs are provided in the body of the report.

  • Expand the effort on enhancement of germplasm. As one step, increase funding of the GEM project from the present level to the $950,000 per year originally requested. This project will help speed incorporation of tropical gennplasm into elite U.S. material.

  • Develop a systematic screening effort to evaluate accessions in the Plant Introduction system for pest resistance. Modest efforts are underway on key insect pests. A joint proposal for systematic evaluation of accessions for disease resistance, requiring modest funding ($30,000 per year), has been made by public and industry corn pathologists.

  • Complete collections of tropical and semitropical maize and of Teosinte where important items are missing. See the section on acquisition for items needed. Continue to acquire public germplasm, including older inbreds from programs which are being closed.

  • Expand the effort to regenerate and categorize maize accessions. With the increased number of accessions resulting from the expanded regeneration project, increased capacity for regeneration is needed. Based on 1996 dollars, an estimated additional $250,000-$300,000 per year in the North Central Regional Plant Introduction Station Zea genetic resources budget is needed.


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