Marker-based estimates of identity by descent and alikeness in state among maize inbreds

Molecular markers are useful for determining relationships and similarity among inbreds, especially if the proportion of marker loci with alleles common to inbreds i and j is partitioned into: (1) the probability that marker alleles are identical by descent (_Mf_ij); and (2) the conditional probability that marker alleles are alike in state, given that they are not identical by descent ( _ij). Our objectives were to: develop a method, based on tabular analysis of restriction fragment length polymorphism marker data, for estimating _Mf_ij, _ij, and the parental contribution to inbred progeny; validate the accuracy of the method with a simulated data set; and compare the pedigree-based coefficient of coancestry (f_ij) and _Mf_ij among a set of maize (Zea mays L.) inbreds. Banding patterns for 73 probeenzyme combinations were determined among 13 inbreds. Iterative estimation of _Mf_ij, _ij, and the parental contribution to progeny was performed with procedures similar to a tabular analysis of pedigree data. Deviations of _Mf_ij from pedigree-based f_ij ranged from 0.002 to 0.288, indicating large effects of selection and/or drift during inbreeding for some inbreds. Differences between marker-based estimates and expected values of parental contribution to inbred progeny were as large as 0.205. Results for a simulated set of inbreds indicated that tabular analysis of marker data provides more accurate estimates of _Mf_ij and _ij than other methods described in the literature. Tabular analysis requires the availability of marker data for all the progenitors of each inbred. When marker data are not available for the parents of a given inbred, _Mf_ij and _ij may still be calculated if parental contributions to the inbred are assumed equal to their expectations.     

Molecular marker diversity among current and historical maize inbreds

Advanced-cycle pedigree breeding has caused maize (Zea mays L.) inbreds to become more-elite but more-narrow genetically. Our objectives were to evaluate the genetic distance among current and historical maize inbreds, and to estimate how much genetic diversity has been lost among current inbreds. We selected eight maize inbreds (B14, B37, B73, B84, Mo17, C103, Oh43 and H99) that largely represented the genetic background of current elite inbreds in the U.S. seed industry. A total of 32 other inbreds represented historical inbreds that were once important in maize breeding. Cluster analysis of the inbreds, using data for 83 SSR marker loci, agreed well with pedigree information. Inbreds from Iowa Stiff Stalk Synthetic (BSSS), Reid Yellow Dent, and Lancaster clustered into separate groups with only few exceptions. The average number of alleles per locus was 4.9 among all 40 inbreds and 3.2 among the eight current inbreds. The reduction in the number of alleles per locus was not solely due to sample size. The average genetic distance (D _ij ) was 0.65 among the eight current inbreds, 0.67 among the 32 historical inbreds, and 0.67 among all 40 inbreds. These differences were statistically insignificant. We conclude that genetic diversity among current inbreds has been reduced at the gene level but not at the population level. Hybrid breeding in maize maintained, rather than decreased, genetic diversity, at least during the initial subdivision of inbreds into BSSS and non-BSSS heterotic groups. We speculate, however, that exploiting other germplasm sources is necessary for sustaining long-term breeding progress in maize. Received: 21 August 2000 / Accepted: 5 January 2001     

Relationship between single-cross performance and molecular marker heterozygosity

Summary In studies involving isozymes or restriction fragment length polymorphisms (RFLPs), correlations of parental molecular marker diversity with grain yield of maize (Zea mays L.) single-crosses have been too low to be of any predictive value. The relationship of molecular marker heterozygosity (D_ij) with hybrid performance ( _ij) and combining ability was examined. For a simple genetic model involving uncorrelated parental allele frequencies and complete coverage of quantitative trait loci (QTL) by molecular markers, the correlations between _ij and D_ij were 0.25. _ij and D_ij were partitioned into general and specific effects. The expected correlation between specific combining ability and specific molecular marker heterozygosity is high. Expected correlations between general combining ability and general molecular marker heterozygosity are either positive or negative, depending on allele frequencies in the tester lines. Computer simulation was used to investigate a more complex but more realistic genetic model involving incomplete coverage of QTL by molecular markers. All of the following conditions are necessary for effective prediction of hybrid performance based on molecular marker heterozygosity: (1) dominance effects are strong; (2) allele frequencies at individual loci in the parental inbreds are negatively correlated; (3) trait heritability is high; (4) average parental allele frequencies vary only within a narrow range; (5) at least 30–50% of the QTL are linked to molecular markers; and (6) not more than 20–30% of the molecular markers are randomly dispersed or unlinked to QTL.A contribution from Limagrain Genetics, a Groupe Limagrain company     

North American study on essential derivation in maize: inbreds developed without and with selection from F2 populations

An essentially derived variety largely retains the characteristics of a parental or ancestral variety. A consensus has not been reached regarding the threshold for declaring essential derivation in maize (Zea mays L.), partly because benchmark data are lacking. Our objective in this study, commissioned by the American Seed Trade Association, was to determine the range of parental contribution among maize inbreds developed without and with selection. Seed companies in North America contributed existing proprietary data on the molecular marker similarity of 100 or more families, developed without selection from F₂ populations, with each of their parents. The companies also sent us seed samples of elite inbreds, developed with selection from F₂ populations, for analysis using 60 RFLP marker loci and 20 SSR marker loci. Among the families developed without selection, the average parental contribution was close to the expected value of 0.50 for F₂ populations. Specific families received up to 79% of their alleles from one parent. Although selection tended to increase the frequency of such transgressive segregants, it did not necessarily increase the maximum parental contribution in an F₂ population. Parental contributions were consistent between the elite inbreds and their early-generation families. We conclude that inbreds with 70% to nearly 80% of their genome derived from one parent can be obtained from an F₂ population. Further empirical data would be valuable particularly for backcross populations, which were not available in this study. Received: 21 June 2000 / Accepted: 28 July 2000     

The comparison of RFLP,RAPD, AFLP and SSR (microsatellite) markers for germplasm analysis

The utility of RFLP (restriction fragment length polymorphism), RAPD (random-amplified polymorphic DNA), AFLP (amplified fragment length polymorphism) and SSR (simple sequence repeat, microsatellite) markers in soybean germplasm analysis was determined by evaluating information content (expected heterozygosity), number of loci simultaneously analyzed per experiment (multiplex ratio) and effectiveness in assessing relationships between accessions. SSR markers have the highest expected heterozygosity (0.60), while AFLP markers have the highest effective multiplex ratio (19). A single parameter, defined as the marker index, which is the product of expected heterozygosity and multiplex ratio, may be used to evaluate overall utility of a marker system. A comparison of genetic similarity matrices revealed that, if the comparison involved both cultivated (Glycine max) and wild soybean (Glycine soja) accessions, estimates based on RFLPs, AFLPs and SSRs are highly correlated, indicating congruence between these assays. However, correlations of RAPD marker data with those obtained using other marker systems were lower. This is because RAPDs produce higher estimates of interspecific similarities. If the comparisons involvedG. max only, then overall correlations between marker systems are significantly lower. WithinG. max, RAPD and AFLP similarity estimates are more closely correlated than those involving other marker systems.Abbreviations RFLP restriction fragment length plymorphism - RAPD random-amplified polymorphic DNA - AFLP amplified fragment length polymorphism - SSR simple sequence repeat - PCR polymerase chain reaction - TBE Tris-borate-EDTA buffer - MI marker index - SENA sum of effective numbers of alleles     

Genetic differentiation analysis of African cassava (Manihot esculenta) landraces and elite germplasm using amplified fragment length polymorphism and simple sequence repeat markers

Molecular‐marker‐aided evaluation of germplasm plays an important role in defining the genetic diversity of plant genotypes for genetic and population improvement studies. A collection of African cassava landraces and elite cultivars was analysed for genetic diversity using 20 amplified fragment length polymorphic (AFLP) DNA primer combinations and 50 simple sequence repeat (SSR) markers. Within‐population diversity estimates obtained with both markers were correlated, showing little variation in their fixation index. The amount of within‐population variation was higher for landraces as illustrated by both markers, allowing discrimination among accessions along their geographical origins, with some overlap indicating the pattern of germplasm movement between countries. Elite cultivars were grouped in most cases in agreement with their pedigree and showed a narrow genetic variation. Both SSR and AFLP markers showed some similarity in results for the landraces, although SSR provided better genetic differentiation estimates. Genetic differentiation (Fst) in the landrace population was 0.746 for SSR and 0.656 for AFLP. The molecular variance among cultivars in both populations accounted for up to 83% of the overall variation, while 17% was found within populations. Gene diversity (H_e) estimated within each population varied with an average value of 0.607 for the landraces and 0.594 for the elite lines. Analyses of SSR data using ordination techniques identified additional cluster groups not detected by AFLP and also captured maximum variation within and between both populations. Our results indicate the importance of SSR and AFLP as efficient markers for the analysis of genetic diversity and population structure in cassava. Genetic differentiation analysis of the evaluated populations provides high prospects for identifying diverse parental combinations for the development of segregating populations for genetic studies and the introgression of desirable genes from diverse sources into the existing genetic base.     

RFLP markers and predicted testcross performance of maize sister inbreds

 Inbreds selfed from the same F₂ or backcross population are referred to as sister inbreds. In some situations, maize (Zea mays L.) sister inbreds may not have testcross data available for best linear unbiased prediction (BLUP) of single-cross performance. This study evaluated the usefulness of BLUP and restriction fragment length polymorphism (RFLP)-based coefficients of coancestry ( f ) in predicting the testcross performance of sister inbreds. Parental contributions (p) were estimated from 70 RFLP loci for 15 inbreds that comprised three sister inbreds selfed from each of five F₂ populations. Estimates of p were subsequently used to calculate RFLP-based f. Grain yield, moisture, and stalk lodging data were obtained for 2265 single crosses tested by Limagrain Genetics in multilocation trials from 1990 to 1995. Performance of the sister inbreds when crossed to several inbred testers was predicted from the performance of the tested single crosses and RFLP-based f. Correlations between predicted and observed performance, obtained with a delete-one cross-validation procedure, were erratic and mostly low for all three traits. Correspondence was poor between ranks for predicted and observed general combining ability of the sister inbreds. The results suggested that the proportion of the genome derived by a sister inbred from a given parental inbred does not solely determine its testcross performance. The failure of BLUP and RFLP-based f to consistently predict testcross performance indicated that actual field testing will continue to be necessary for preliminary evaluation of sister inbreds. Received : 17 March 1997 / Accepted : 18 April 1997     

Prediction of hybrid performance in maize using molecular markers and joint analyses of hybrids and parental inbreds

The identification of superior hybrids is important for the success of a hybrid breeding program. However, field evaluation of all possible crosses among inbred lines requires extremely large resources. Therefore, efforts have been made to predict hybrid performance (HP) by using field data of related genotypes and molecular markers. In the present study, the main objective was to assess the usefulness of pedigree information in combination with the covariance between general combining ability (GCA) and per se performance of parental lines for HP prediction. In addition, we compared the prediction efficiency of AFLP and SSR marker data, estimated marker effects separately for reciprocal allelic configurations (among heterotic groups) of heterozygous marker loci in hybrids, and imputed missing AFLP marker data for marker-based HP prediction. Unbalanced field data of 400 maize dent × flint hybrids from 9 factorials and of 79 inbred parents were subjected to joint analyses with mixed linear models. The inbreds were genotyped with 910 AFLP and 256 SSR markers. Efficiency of prediction (R ²) was estimated by cross-validation for hybrids having no or one parent evaluated in testcrosses. Best linear unbiased prediction of GCA and specific combining ability resulted in the highest efficiencies for HP prediction for both traits (R ² = 0.6–0.9), if pedigree and line per se data were used. However, without such data, HP for grain yield was more efficiently predicted using molecular markers. The additional modifications of the marker-based approaches had no clear effect. Our study showed the high potential of joint analyses of hybrids and parental inbred lines for the prediction of performance of untested hybrids.     

PEDIGREE ERROR DUE TO EXTRA‐PAIR REPRODUCTION SUBSTANTIALLY BIASES ESTIMATES OF INBREEDING DEPRESSION

Understanding the evolutionary dynamics of inbreeding and inbreeding depression requires unbiased estimation of inbreeding depression across diverse mating systems. However, studies estimating inbreeding depression often measure inbreeding with error, for example, based on pedigree data derived from observed parental behavior that ignore paternity error stemming from multiple mating. Such paternity error causes error in estimated coefficients of inbreeding (f) and reproductive success and could bias estimates of inbreeding depression. We used complete “apparent” pedigree data compiled from observed parental behavior and analogous “actual” pedigree data comprising genetic parentage to quantify effects of paternity error stemming from extra‐pair reproduction on estimates of f, reproductive success, and inbreeding depression in free‐living song sparrows (Melospiza melodia). Paternity error caused widespread error in estimates of f and male reproductive success, causing inbreeding depression in male and female annual and lifetime reproductive success and juvenile male survival to be substantially underestimated. Conversely, inbreeding depression in adult male survival tended to be overestimated when paternity error was ignored. Pedigree error stemming from extra‐pair reproduction therefore caused substantial and divergent bias in estimates of inbreeding depression that could bias tests of evolutionary theories regarding inbreeding and inbreeding depression and their links to variation in mating system.     

Comparison of RFLP and RAPD markers to estimating genetic relationships within and among cruciferous species

Restriction fragment length polymorphism (RFLP) and random amplified polymorphic DNA (RAPD) markers are being used widely for evaluating genetic relationships of crop germplasm. Differences in the properties of these two markers could result in different estimates of genetic relationships among some accessions. Nuclear RFLP markers detected by genomic DNA and cDNA clones and RAPD markers were compared for evaluating genetic relationships among 18 accessions from six cultivated Brassica species and one accession from Raphanus sativus. Based on comparisons of genetic-similarity matrices and cophenetic values, RAPD markers were very similar to RFLP markers for estimating intraspecific genetic relationships; however, the two marker types gave different results for interspecific genetic relationships. The presence of amplified mitochondrial and chloroplast DNA fragments in the RAPD data set did not appear to account for differences in RAPD- and RFLP-based dendrograms. However, hybridization tests of RAPD fragments with similar molecular weights demonstrated that some fragments, scored as identical, were not homologous. In all these cases, the differences occurred at the interspecific level. Our results suggest that RAPD data may be less reliable than RFLP data when estimating genetic relationships of accessions from more than one species.     

Genetic basis of resistance to sugarcane mosaic virus in European maize germplasm

 Sugarcane mosaic virus (SCMV) causes considerable damage to maize (Zea mays L.) in Europe. The objective of the present study was to determine the genetic basis of resistance to SCMV in European maize germplasm and to compare it with that of U.S. inbred Pa405. Three resistant European inbreds D21, D32, and FAP1360A were crossed with four susceptible inbreds F7, KW1292, D408, and D145 to produce four F₂ populations and three backcrosses to the susceptible parent. Screening for SCMV resistance in parental inbreds and segregating generations was done in two field trials as well as under greenhouse conditions. RFLP markers umc85, bnl6.29, umc10, umc44, and SSR marker phi075 were used in F₂ populations or F₃ lines to locate the resistance gene(s) in the maize genome. Segregation in the F₂ and backcross generations fitted to different gene models depending on the environmental conditions and the genotype of the susceptible parent. In the field tests, resistance in the three resistant European inbreds seems to be controlled by two to three genes. Under greenhouse conditions, susceptibility to SCMV in D32 appears to be governed by one dominant and one recessive gene. Allelism tests indicated the presence of a common dominant gene (denoted as Scm1) in all three resistant European inbreds and Pa405. Marker analyses mapped two dominant genes: Scm1 on chromosome 6S and Scm2 on chromosome 3. Received: 17 November 1997 / Accepted: 25 November 1997     

Genetic diversity and its relationship to hybrid performance in maize as revealed by RFLP and AFLP markers

 The challenge to maize breeders is to identify inbred lines that produce highly heterotic hybrids. In the present study we surveyed genetic divergence among 13 inbred lines of maize using DNA markers and assessed the relationship between genetic distance and hybrid performance in a diallel set of crosses between them. The parental lines were assayed for DNA polymorphism using 135 restriction fragment length polymorphisms (RFLPs) and 209 amplified-fragment polymorphisms (AFLPs). Considerable variation among inbreds was detected with RFLP and AFLP markers. Moreover AFLPs detect polymorphisms more efficiently in comparison to RFLPs, due to the larger number of loci assayed in a single PCR reaction. Genetic distances (GDs), calculated from RFLP and AFLP data, were greater among lines belonging to different heterotic groups compared to those calculated from lines of the same heterotic group. Cluster analysis based on GDs revealed associations among lines which agree with expectations based on pedigree information. The GD values of the 78 F₁ crosses were partioned into general (GGD) and specific (SGD) components. Correlations of GD with F₁ performance for grain yield were positive but too small to be of predictive value. The correlations of SGDs, particularly those based on AFLP data, with specific combining-ability effects for yield may have a practical utility in predicting hybrid performance. Received: 15 August 1997 / Accepted: 19 September 1997     

Evaluation of soybean RFLP marker diversity in adapted germ plasm

Summary Soybean RFLP markers have been primarily developed and genetically mapped using wide crosses between exotic and adapted genotypes. We have screened 38 soybean lines at 128 RFLP marker loci primarily to characterize germ plasm structure but also to evaluate the utility of RFLP markers identified in unadapted populations. Of these DNA probes 70% detected RFLPs in this set of soybean lines with an average polymorphism index of 0.30. This means that only 1 out of 5 marker loci was informative between any particular pair of adapted soybean lines. The variance associated with the estimation of RFLP genetic distance (GD_R) was determined, and the value obtained suggested that the use of more than 65–90 marker loci for germ plasm surveys will add little precision. Cluster analysis and principal coordinate analysis of the GD_R matrix revealed the relative lack of diversity in adapted germ plasm. Within the cultivated lines, several lines adapted to Southern US maturity zones also appeared as a separate group. GD_R data was compared to the genetic distance estimates obtained from pedigree analysis (GD_P). These two measures were correlated with r = 0.54 for all 38 lines, but the correlation increased to r = 0.73 when only adapted lines were analyzed.     

Molecular mapping of the Oregon Wolfe Barleys: a phenotypically polymorphic doubled-haploid population

A phenotypically polymorphic barley (Hordeum vulgare L.) mapping population was developed using morphological marker stocks as parents. Ninety-four doubled-haploid lines were derived for genetic mapping from an F₁ using the Hordeum bulbosum system. A linkage map was constructed using 12 morphological markers, 87 restriction fragment length polymorphism (RFLP), five random amplified polymorphic DNA (RAPD), one sequence-tagged site (STS), one intron fragment length polymorphism (IFLP), 33 simple sequence repeat (SSR), and 586 amplified fragment length polymorphism (AFLP) markers. The genetic map spanned 1,387 cM with an average density of one marker every 1.9 cM. AFLP markers tended to cluster on centromeric regions and were more abundant on chromosome 1 (7H). RAPD markers showed a high level of segregation distortion, 54% compared with the 26% observed for AFLP markers, 27% for SSR markers, and 18% for RFLP markers. Three major regions of segregation distortion, based on RFLP and morphological markers, were located on chromosomes 2 (2H), 3 (3H), and 7 (5H). Segregation distortion may indicate that preferential gametic selection occurred during the development of the doubled-haploid lines. This may be due to the extreme phenotypes determined by alleles at morphological trait loci of the dominant and recessive parental stocks. Several molecular markers were found to be closely linked to morphological loci. The linkage map reported herein will be useful in integrating data on quantitative traits with morphological variants and should aid in map-based cloning of genes controlling morphological traits. Received: 23 August 2000 / Accepted: 15 December 2000     

RFLP analyses of early-maturing European maize germ plasm

Summary Thirty inbred lines representing a wide range of early-maturing European elite germ plasm of maize (Zea mays L.) were assayed for RFLPs using 203 clone-enzyme combinations (106 DNA clones with restriction enzymes EcoR1 and HindIII). The genetic materials comprised 14 flint, 12 dent, and 4 lines of miscellaneous origin. Objectives were to (1) characterize the genetic diversity for RFLPs in these materials, (2) compare the level of genetic diversity found within and between the flint and the dent heterotic groups, and (3) examine the usefulness of RFLPs for assigning inbreds to heterotic groups. All but two DNA clones yielded polymorphism with at least one restriction enzyme. A total of 82 and 121 clone-enzyme combinations gave single-banded and multiple-banded RFLP patterns, respectively, with an average of 3.9 and 7.7 RFLP patterns per clone-enzyme combination across all 30 inbreds, respectively. Genetic similarity (GS) between lines, estimated from RFLP data as Dice's similarity coefficient, showed considerable variation (0.32 to 0.58) among unrelated inbreds. The mean GS for line combinations of type flint x dent (0.41) was significantly smaller than for unrelated flint lines (0.46) and dent lines (0.46), but there was considerable variation in GS estimates of individual line combinations within each group. Cluster and principal coordinate analyses based on GS values resulted in separate groupings of flint and dent lines in accordance with phylogenetic information. Positioning of lines of miscellaneous origin was generally consistent with expectations based on known breeding behavior and pedigrees. Results from this study corroborated that RFLP data can be used for assigning inbreds to heterotic groups and revealing pedigree relationships among inbreds.     

Estimation of coefficient of coancestry using molecular markers in maize

Summary The coefficient of coancestry (f_AB) between individuals A and B is the classical measure of genetic relationship. f_AB is determined from pedigree records and is the probability that random alleles at the same locus in A and B are copies of the same ancestral allele or identical by descent (ibd). Recently, the proportion of molecular marker variants shared between A and B (S_AB) has been used to measure genetic relationship. But S_AB is an upwardly-biased estimator of f_AB, especially between distantly-related lines. f_AB, S_AB, and adjusted (to remove bias) estimates of molecular marker similarity (f _AB ^M ) were compared. RFLP banding patterns at 46 probe-restriction enzyme combinations were obtained for 23 maize inbred lines derived from the Iowa Stiff Stalk Synthetic (BSSS) maize (Zea mays L.) population, and for 4 non-BSSS lines. f _AB ^M was estimated as , where _A (or _B) was the average proportion of RFLP variants shared between inbred A (or inbred B) and the non-BSSS lines. The average f_AB among 253 pairwise combinations of BSSS lines was 0.212, whereas the average S_AB was 0.397. The average f _AB ^M was 0.162, indicating that the upward bias in S_AB was effectively removed. S_AB and f_AB were significantly different ( = 0.05) in 76.3% of the comparisons, whereas 24.9% of the f _AB ^M values differed significantly from f_AB. The latter result suggests that selection and/or drift were present during inbred line development and that f_AB may not be an accurate measure of the true proportion of ibd alleles between two lines. Cluster analyses based on S _AB ^M and f _AB ^M grouped lines according to pedigree, although several exceptions were noted. The presence of shared molecular marker variants between unrelated lines must be considered when setting S_AB-based minimum distances for varietal protection. Under simplified conditions, more than 250 molecular marker loci are necessary to obtain sufficiently precise estimates of coefficient of coancestry using molecular markers.A contribution from Limagrain Genetics, a Group Limagrain company     

Genetic diversity for restriction fragment length polymorphisms and heterosis for two diallel sets of maize inbreds

Summary Changes that may have occurred over the past 50 years of hybrid breeding in maize (Zea maize L.) with respect to heterosis for yield and heterozygosity at the molecular level are of interest to both maize breeders and quantitative geneticists. The objectives of this study were twofold: The first, to compare two diallels produced from six older maize inbreds released in the 1950's and earlier and six newer inbreds released during the 1970's with respect to (a) genetic variation for restriction fragment length polymorphisms (RFLPs) and (b) the size of heterosis and epistatic effects, and the second, to evaluate the usefulness of RFLP-based genetic distance measures in predicting heterosis and performance of single-cross hybrids. Five generations (parents, F₁; F₂, and backcrosses) from the 15 crosses in each diallel were evaluated for grain yield and yield components in four Iowa environments. Genetic effects were estimated from generation means by ordinary diallel analyses and by the Eberhart-Gardner model. Newer lines showed significantly greater yield for inbred generations than did older lines but smaller heterosis estimates. In most cases, estimates of additive x additive epistatic effects for yield and yield components were significantly positive for both groups of lines. RFLP analyses of inbred lines included two restriction enzymes and 82 genomic DNA clones distributed over the maize genome. Eighty-one clones revealed polymorphisms with at least one enzyme. In each set, about three different RFLP variants were typically found per RFLP locus. Genetic distances between inbred lines were estimated from RFLP data as Rogers' distance (RD), which was subdivided into general (GRD) and specific (SRD) Rogers' distances within each diallel. The mean and range of RDs were similar for the older and newer lines, suggesting that the level of heterozygosity at the molecular level had not changed. GRD explained about 50% of the variation among RD values in both sets. Cluster analyses, based on modified Rogers' distances, revealed associations among lines that were generally consistent with expectations based on known pedigree and on previous research. Correlations of RD and SRD with f₁ performance, specific combining ability, and heterosis for yield and yield components, were generally positive, but too small to be of predictive value. In agreement with previous studies, our results suggest that RFLPs can be used to investigate relationships among maize inbreds, but that they are of limited usefulness for predicting the heterotic performance of single crosses between unrelated lines.Joint contribution from Cereal and Soybean Research Unit, USDA, Agricultural Research Service and Journal Paper no. J-13929 of the Iowa Agric and Home Economics Exp Stn, Ames, IA 50011. Projects no. 2818 and 2778A.E.M. is presently at the Iowa State University on leave from University of Hohenheim, D-7000 Stuttgart 70, Federal Republic of Germany