International Agricultural Research Centers and a number of countries have assembled large collections of genetic resources managed in ex situ genebanks. In addition, some countries are managing in situ collections of selected germplasm. Pressure to reduce public funding of all government activities continues to force genebank managers to he more efficient and effective while maintaining quality factors such as high germinability, genetic integrity, and freedom from diseases and insect damage. Quality seed begins in the field at pollination time when flowers of allogamous species are bagged or caged to exclude foreign pollen. Complete seed maturation and drying before harvest provides material that will have optimal storage life. Properly maintained cold room conditions (4°C and 25–30% RH for most seeds) will ensure maximum longevity and reduce the number of regenerations necessary for each accession.

The Latin American Maize Program (LAMP) evaluated nearly 15,000 maize accessions (mostly landrace collections) in 11 Latin American countries and the U.S. to identify germplasm that can be used as source materials to improve performance and expand genetic diversity of commercial hybrids and varieties. We reviewed breeding theory, methodologies, and procedures to provide users information and suggestions as they proceed with the utilization of these elite accessions. The development of a common heterotic pattern is recommended to enhance cooperative research and exchange of derived inbred lines with the limited resources that are available. We reviewed different methods of selection that are available to improve performance of the population cross, hybrids based on inbred lines derived from the improved breeding populations, and varieties developed from the advanced generation of the population cross.

The main goals of genetic resource management are to acquire, maintain, distribute, characterize, regenerate, preserve, evaluate, and utilize the genetic diversity of crops and their wild relatives. The objectives of this study for ex-situ conservation of maize (Zea mays L.) are to review and describe: (1) practical regeneration methods that are based on population genetic theory; (2) practical problems encountered in choosing core subsets of a maize collection. Whenever possible, regeneration procedures should control the number of pollen parents (male gametes; through controlled hand pollination) and the number of female parent gametes (by harvesting equal numbers of kernels from each seed plant). When the number of pollen and seed parents are controlled during regeneration, the effective population size (Ne) is twice the size of the original population (N). Examples of practical methods for controlling the number of male and female parents are presented. The procedure involves random-paired plant crosses and taking equal numbers of seeds from each maize ear. To form a core subset, accessions of a maize race are subdivided through a stratified sampling procedure. Delineation of a core subset from a Tuxpeño racial collection is described as an example.

One objective of the regeneration of genetic populations is to maintain at least one copy of each allele present in the original population. Genetic diversity within populations depends on the number and frequency of alleles across all loci. The objectives of this study on outbreeding crops are: (1) to use probability models to determine optimal sample sizes for the regeneration for a number of alleles at independent loci; and (2) to examine theoretical considerations in choosing core subsets of a collection. If we assume that k-1 alleles occur at an identical low frequency of p0 and that the k(th) allele occurs at a frequency of 1-[(k-1)p0], for loci with two, three, or four alleles, each with a p0 of 0.05, 89-110 additional individuals are required if at least one allele at each of 10 loci is to be retained with a 90% probability; if 100 loci are involved, 134-155 individuals are required. For two, three, or four alleles, when p0 is 0.03 at each of 10 loci, the sample size required to include at least one of the alleles from each class in each locus is 150-186 individuals; if 100 loci are involved, 75 additional individuals are required. Sample sizes of 160-210 plants are required to capture alleles at frequencies of 0.05 or higher in each of 150 loci, with a 90-95% probability. For rare alleles widespread throughout the collection, most alleles with frequencies of 0.03 and 0.05 per locus will be included in a core subset of 25-100 accessions.

Abstract Through natural evolutionary processes, thousands of plant species have developed. Only a small percentage of these have been selected and utilized as agronomic, horticultural, ornamental, or forestry crops. Hence, most plant genetic resources exist in natural ecosystems according to the principle of “survival of the fittest,” with no inventory and no managed preservation. As the world’s population continues to expand, the areas required for intensive agriculture and forestry will increase at the expense of habitats for other plant species. Because in situ preservation may not be adequate in the future, ex situ preservation must be expanded.

Evaluation of the results of six years of selection in the Kitale maize breeding methods study proved reciprocal recurrent selection to be an effective interpopulation improvement method. Ear-to-row selection was effective in improving ‘Kitale Composite A’ (KCA). Data from a diallel of the cycle-6 ear-to-row substrains showed that where less than 10 lines were selected, inbreeding appeared to depress gain, but differences among the various ear-to-row experiments were not significant. Intererossing the substrains was predicted to reduce the effect of inbreeding. Yield gains in reciprocal recurrent selection and ear-to-row selection were associated with increases in ears per 100 plants. The reciprocal recurrent selection variety cross yield gain was estimated at 3.5% per year (7%/cycle) and that of the best ear-to-row procedure at 2.6% per year. The variety cross syn-2 from reciprocal recurrent selection was predicted to improve at the same rate as the best ear-to-row procedure (E7) with one long growing season per year. Under a cropping system with two similar seasons per year, however, ear-to-row selection should be more effective than reciprocal recurrent selection or its derived syn-2. Mass selection at 10% selection intensity produced significant improvement, but not at 2% selection intensity.

Tests were designed to determine the possibilities of obtaining resistant lines from breeding populations as an alternative for insecticides for the control of the greenbug, Schizaphis grominum (Rondani). The KP2BR Sorghum bicolor (L.) Moench breeding population, developed from diverse germplasm, was random mated before selecting for resistance to the greenbug among seedlings in the greenhouse. To produce 135 S5 lines, selection was done in each of five generations (half‐sib or S0, S1, S2, S3, and S4). For comparison, six F5 lines derived from individual resistant ✕ susceptible crosses were selected in three generations (F2, F3, and F4). The S5 lines from KP2BR had as high greenbug resistance as the better parental sources, but none of the F5 lines were as resistant. Withinplot variance for some S5 lines exceeded the estimate of within plot environmental variance, which suggested residual genetic variation. Breeding populations offer an alternative to the widely used practice of introducing insect (or other pest) resistance into new parental lines by backcrossing.

We evaluated three maize (Zea mays L.) crosses among BSCBI(R)C5, BSSS(R)C5, and BSSS(HT)C6 populations in six environments. BSCBI(R)C5 and BSSS(R)C5 improved populations from reciprocal recurrent selection, and BSSS(HT)C6 was obtained from testcross selection with a double‐cross tester. Also, we used five selected S3 lines from each of BSCBI(R)C5 and BSSS(R)C5 and five selected S3 or S4 lines from BSSS(HT)C5 in three sets of line ✕ line crosses among the sources. Average yields of the population crosses were essentially equal, which was unexpected for BSSS(R)C5 ✕ BSSS(HT)C6 Most of the variation among crosses in each set was accounted for by average line performance (general combining ability) in all traits.Average yield of line crosses in a set was significantly higher than the population cross in each set and in the BSCBI(R)C5 ✕ BSSS(R)C5 and BSCBI(R)C5 ✕ BSSS (HT)C6 sets, the best cross yielded 35% more than the population cross. Two BSCBI(R)C5 ✕ BSSS(R)C5 crosses yielded significantly higher than B37 ✕ Oh43, and one of these crosses was equal to B37 ✕ Oh43 for lodging resistance. None of the crosses in the other two sets was equal to B37 ✕ Oh45 for all traits, mainly because of lodging. A modified reciprocal recurrent selection procedure using an inbred tester is proposed as a more efficient method of simultaneously improving breeding populations and developing elite single crosses.

Expected genetic responses were used to compare the simultaneous improvement of several agronomic traits for three selection index methods in recurrent selection programs. Observations for various agronomic traits in maize (Zea mays L.) were obtained from 144 S1 lines of the BSSS2 maize population grown in several locations during the 1970‐72 growing seasons. These traits were European corn borer (Ostrinia nubilalis (Hubner)) resistance, cold tolerance (percentage of emergence, emergence index, and seedling dry weight), grain yield, grain moisture, root and stalk lodgings, and corn root worm (Diabrotica spp.) resistance (root damage, root size, and secondary root growth). Seven of them, grain yield, corn borer resistance, percentage emergence, emergence index, seedling dry weight, root size, and secondary root growth, were used to construct three types of index selection for S1 selection.The conventional indices constructed from two sets of arbitrarily assigned relative economic weights did not give satisfactory predicted improvement for all traits included in individual indices. Most of the predicted improvement went to grain yield and percentage of emergence. Base indices, in which relative economic weights were used as index coefficients, were 95 and 97% as efficient as conventional selection indices. Hence, the application of the base index would be preferable with S1 selection when precise relative economic values of the traits included in the indices are known. When modified selection indices (based on desired gains) were used, final goals from recurrent selection of the traits considered (S1 testing and 10% selection intensity) were predicted to be attained after 14 cycles or 28 years of selection. In another scheme of recurrent selection, corn borer resistance and cold tolerance traits would be evaluated in the S1 generation, and yield, root size, and secondary root growth would be tested hi the S2 generation. With 44 and 8% selection intensities in the S1 and S2 generations, respectively, improvement of all traits was expected to reach final goals after 10 cycles or 30 years of selection. Assuming that the two sets of the arbitrarily assigned relative economic weights were realistic, modified selection indices (based on their contributions to aggregate genetic advance) were shown to be only 46 and 61% as efficient as conventional indices for the specified desired gains and relative economic weights considered. Modified selection index with the S1 testing scheme was expected to give 53% of improvement for yield, compared with single trait selection based on yield alone. Use of the modified index selection is recommended for the simultaneous improvement of several agronomic traits in recurrent selection programs when the relative economic values of the traits are difficult to determine.Results from comparisons of predicted gain from recurrent selection with S1 testing and a combination of S1 and S2 testings suggested that S1 testing would be preferable, because fewer resources would be required and the construction of selection indices would be less complicated.

Crop ScienceVolume 14, Issue 2 cropsci1974.0011183X001400020064x p. 341-342 Registration of Germplasm Registration of Maize Germplasm1 (Reg. No. GP 26 to GP 34) Arnel R. Hallauer, Arnel R. HallauerSearch for more papers by this authorS. A. Eberhart, S. A. EberhartSearch for more papers by this authorW. A. Russell, W. A. Russell Research Geneticists, ARS, USDA, and Professors of Plant Breeding, Iowa State University, Ames, IA 50010; and Professor of Plant Breeding, Iowa State University, Ames, IA 50010.Search for more papers by this author Arnel R. Hallauer, Arnel R. HallauerSearch for more papers by this authorS. A. Eberhart, S. A. EberhartSearch for more papers by this authorW. A. Russell, W. A. Russell Research Geneticists, ARS, USDA, and Professors of Plant Breeding, Iowa State University, Ames, IA 50010; and Professor of Plant Breeding, Iowa State University, Ames, IA 50010.Search for more papers by this author First published: 01 March 1974 https://doi.org/10.2135/cropsci1974.0011183X001400020064xCitations: 14 1 Registered by the Crop Science Society of America. Joint Contribution: Agricultural Research Service, USDA, and Journal Paper No. J-7600 of the Iowa Agricultural and Home Economics Experiment Station, Ames, IA 50010, Project 1897. AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article.Citing Literature Volume14, Issue2March–April 1974Pages 341-342 RelatedInformation

The performance of a group of 49 hybrids from seven elite maize (Zea mays L.) lines selected from each of the two populations, BSK(S)C5 and BSSS(R)C5, was compared with the varietal-cross hybrid of the source populations and with elite single-cross hybrids. Both populations had been improved by five cycles of recurrent selection. BSK(S)C5 was improved by S1 selection from BSK; whereas BSSS(R)C5 was improved by reciprocal recurrent selection from BSSS with BSCB1 as the tester.

This study evaluated recurrent selection for specific combining ability for yield in two maize (Zea mays L.) populations and provided information on the types of gene action involved in the yield improvement. The populations for the recurrent selection program were an open‐pollinated variety, ‘Alph,’ and the F2 of WF9 ✕ B7. Inbred B14 was the tester for both populations. The principal basis of selection was grain yield, and the selection intensity was 10 to 13% in each of five cycles. Ten S1 lines were selected in each cycle and recombined to give a new population in each source. We evaluated progress by measuring yield in B14 ✕ Alph Cn and B14 ✕ (WF9 ✕ B7)Cn populations, Cn populations per se, related and unrelated testers ✕ Cn populations, and Cn ✕ Cn population crosses in six environments. In all types of populations the rates of gain per cycle were significant, and the gains were greater in Alph than in WF9 ✕ B7. Rates of gain per cycle (q/ha) in some comparisons were: B14 ✕ Alph Cn, 3.09; B14 ✕ (WF9 ✕ B7)Cn, 1.32; BSBB ✕ Alph Cn, 3.63; BSBB ✕ (WF9 ✕ B7)Cn, 1.51; and Alph Cn ✕ (WF9 ✕ B7)Cn, 4.09. In all types populations except B14 ✕ (WF9 ✕ B7)Cn, the number ears per 100 plants increased. There were no significant changes for date of silk emergence or lodging, but there were some significant increases for percentage moisture at harvest and plant and ear heights. We concluded that overdominance and overdominant types of epistasis were relatively unimportant in the changes in yield potential of the two populations because the BSBB topcrosses improved as rapidly as the BI4 topcrosses, the Cn ✕ Cn crosses increased more rapidly than the B14 topcrosses, and the heterosis of the Cn ✕ Cn Crosses increased.

Evaluation of progress from five cycles of reciprocal recurrent selection (R) in the BSSS(R) and BSCBI(R) maize (Zea mays L.) populations indicated that the improvement in grain yield of the population cross had been linear at the rate of 2.73 q/ha (4.6%) per cycle. significant changes, however, were detected in the parents. BSSS(R) and BSCBI(R) were developed from Stiff Stalk Synthetic and Corn Borer Synthetic #1, respectively. Topcrosses of improved populations to BSBB did not yield significantly more than topcrosses of the original synthetic populations.Improvement by half‐sib selection with the Iowa 13 double‐cross tester (HT) in BSSS(HT) also had been linear at 1.65 q/ha (2.6%) per cycle. The population cross BSSS(HT)Cn ✕ BSCBI(R)Cn had been improved 2.31 q/ha (3.8%) per cycle, and BSSS(HT) had been proved 0.74 q/ha (1.4%) per cycle, where Cn is the cycle of selection.Heterosis increased from 15% in the CO ✕ CO to 37% for BSSS(R)C5 ✕ BSCBI(R)C5 and to for BSSS(HT)C7 ✕ BSCBI(R)C5. Because only 10 lines were recombined each cycle, the estimated inbreeding of the C5 populations (22%) and the C7 population (29%) probably accounted for part of the increased heterosis and lack of progress in the parental populations.The improvement in yield of the population crosses was obtained with no change in ear height or maturity, and stalk lodging was reduced. Improved population crosses compared favorably with elite single‐cross checks; therefore, BSSS(R)C5, BSSS(HT)C7, BSCBI(R)C5 should be excellent sources of additional inbred lines for developing new improved single‐cross hybrids. Two new lines, B73 and B78, have already been developed from the C5 and C6 of BSSS(HT), respectively; whereas B14 and B37 were developed from the original Stiff Stalk Synthetic.

(1973). Genotype by Environment Interactions in Maize in Eastern Africa. East African Agricultural and Forestry Journal: Vol. 39, No. 1, pp. 61-71.