Plant architecture, phenology and yield components of cultivated plants have repeatedly been shaped by selection to meet human needs and adaptation to different environments. Here we assessed the genetic architecture of 24 correlated maize traits that interact during plant cycle. Overall, 336 lines were phenotyped in a network of 9 trials and genotyped with 50K single-nucleotide polymorphisms. Phenology was the main factor of differentiation between genetic groups. Then yield components distinguished dents from lower yielding genetic groups. However, most of trait variation occurred within group and we observed similar overall and within group correlations, suggesting a major effect of pleiotropy and/or linkage. We found 34 quantitative trait loci (QTLs) for individual traits and six for trait combinations corresponding to PCA coordinates. Among them, only five were pleiotropic. We found a cluster of QTLs in a 5 Mb region around Tb1 associated with tiller number, ear row number and the first PCA axis, the latter being positively correlated to flowering time and negatively correlated to yield. Kn1 and ZmNIP1 were candidate genes for tillering, ZCN8 for leaf number and Rubisco Activase 1 for kernel weight. Experimental repeatabilities, numbers of QTLs and proportion of explained variation were higher for traits related to plant development such as tillering, leaf number and flowering time, than for traits affected by growth such as yield components. This suggests a simpler genetic determinism with larger individual QTL effects for the first category.

To investigate the genetic basis of maize adaptation to temperate climate, collections of 375 inbred lines and 275 landraces, representative of American and European diversity, were evaluated for flowering time under short- and long-day conditions. The inbred line collection was genotyped for 55 genomewide simple sequence repeat (SSR) markers. Comparison of inbred line population structure with that of landraces, as determined with 24 SSR loci, underlined strong effects of both historical and modern selection on population structure and a clear relationship with geographical origins. The late tropical groups and the early “Northern Flint” group from the northern United States and northern Europe exhibited different flowering times. Both collections were genotyped for a 6-bp insertion/deletion in the Dwarf8 (D8idp) gene, previously reported to be potentially involved in flowering time variation in a 102 American inbred panel. Among-group D8idp differentiation was much higher than that for any SSR marker, suggesting diversifying selection. Correcting for population structure, D8idp was associated with flowering time under long-day conditions, the deletion allele showing an average earlier flowering of 29 degree days for inbreds and 145 degree days for landraces. Additionally, the deletion allele occurred at a high frequency (>80%) in Northern Flint while being almost absent (<5%) in tropical materials. Altogether, these results indicate that Dwarf8 could be involved in maize climatic adaptation through diversifying selection for flowering time.

Early vigor, earliness and cold tolerance are the main potential contributions of European maize ( Zea mays L.) for breeding programs for adaptation to areas with short growing seasons and cold springs. The objective of this research was to determine the potential contributions of populations from different European regions to breeding for adaptation. Six Spanish and six French maize populations differing on variability for earliness, vigor and cold tolerance were crossed in a complete diallel without reciprocals. The populations and their crosses were evaluated in the field and in a cold chamber. Minimum temperatures were the main environmental trait affecting genotype × environment interaction, probably due to the cold sensitivity of the genotypes with the best performance in the field. The best population cross, based on specific heterosis for adaptation-related traits in the field, was Viana × Rastrojero, but this cross was cold sensitive. Tuy × Lazcano should be the best choice for a breeding program for adaptation, based on performance in the field and cold tolerance. As conclusions, there was variability for earliness, vigor and cold tolerance among the populations and crosses involved in this study, being tolerant to cold conditions the populations with medium growing cycle originated in areas with short growing seasons. The highest yielding crosses were cold sensitive.

Heterotic patterns among European maize (Zea mays L.) populations are strongly affected by genotype × environment (GE) interactions and no single heterotic pattern has been identified so far that is not influenced by GE interaction. The objectives of this work were to study (i) the mean performance and stability of the heterotic patterns ‘Humid Spain × Southern France’ and ‘Dry Spain × Humid Spain’ and (ii) the influence of some environmental and genotypic covariates on G and E main effects and their interaction. We studied the GE interaction for grain yield in eight environments using Sites Regression (SREG) and factorial regression models. The biplot obtained from the SREG model allowed visual cultivar evaluation. The factorial regression model incorporated genotypic and environmental covariates that enhanced biological interpretation of GE interaction. The heterotic patterns Humid Spain × Southern France and Dry Spain × Humid Spain had similar mean performance across environments, but the former, represented by the cross Lazcano × Millette du Lauragais, was more stable. Effects of G, E, and GE for grain yield were mainly due to earliness, vigor effects, and/or environmental factors related to cold stress. An adequately long vegetative cycle along with early vigor had a great influence on the mean grain yield performance of Lazcano × Millette du Lauragais. Additionally, its intermediate number of days to silking and tolerance to temperature stresses could be related to its stability. Breeding for tolerance to temperature stresses could render more stable maize genotypes.

Since its introduction in Europe five centuries ago, maize spread in Europe and numerous landraces have been cultivated. During the second half of the XXth century, large collections have been established to preserve this genetic diversity. The objectives of this paper are (i) to review recent results on the genetic structuration and the origin Of European maize, (ii) to present the constitution of the representative core-collection of European maize landraces built in RESGEN CT96-088 project, and (iii) to Study the methodology Of use of these handraces in present breeding programs. Based on molecular markers, five Studies found a hi-h allelic richness in landraces from Mediterranean regions Such as Spain, and (for two of them) a strong similarity between several populations from Southern Spain and a group of Caribbean Populations. These studies also attest the originality of Northern Eastern Europe landraces, for which a similarity is observed with American Northern Flint landraces. Historical investigations confirm the hypothesis Of introduction of maize from this origin in the North of Europe, only a few decades after introduction of tropical maize in Southern Spain by Colombus. Starting from a total of 2899 European landraces, we established with the Mstrat software a representative core collection of 96 maize accessions that maximizes allelic richness at Molecular markers and best represents variation at phenotypic traits. This collection is characterized for traits of agronomical interest such as silage quality and insect tolerance. Regarding the transfer of relevant traits to elite material, comparison of F, versus backcross foundation Populations showed that this last strategy leads to a higher population mean while not leading to a decrease in variance, therefore backcross method appears Superior. Preliminary selection of superior material within a landrace did not increase average expected genetic gain but increased stability in variable environments. Molecular markers should prove helpful to extend this back-cross approach to the targeted transfer of donor interesting genomic regions.

The resolution that can be obtained from molecular genetic markers affords new prospects for understanding the dispersion of agricultural species from their primary origin centres. In order to study the introduction and the dispersion of maize in Europe, we have characterised a large and representative set of maize populations of both American and European origins for their variation at 29 restriction fragment length polymorphism loci. Polymorphism was higher for American populations than for European populations (respectively, 12.3 and 9.6 alleles per locus, on average), and only a few alleles were specific to European populations. Investigation of genetic similarity between populations from both continents made it possible to identify various types of American maize introduced into Europe at different times or in different places and which have given rise to distinctive European races. Beyond confirming the importance of Caribbean germplasm, the first maize type to be introduced into Europe, this research revealed that introductions of Northern American flint populations have played a key role in the adaptation of maize to the European climate. According to a detailed historical investigation, the introduction of these populations must have occurred shortly after the discovery of the New World.

. Given the large extent of hybrid cultivation, the importance of conserving the diversity of crop genetic resources has given birth to numerous collections of old races. In the present paper, we conduct a molecular characterisation of a large collection of 488 European maize populations using the bulk RFLP analysis. The analysis of 23 RFLP loci showed a high allelic richness of 11.5 alleles per locus. Populations from eastern Europe (Poland, Austria, Germany, etc.) showed the lowest genetic diversity, a lower number of unique alleles and a higher percentage of fixed loci than populations from southern Europe. In fact, genetic diversity appeared higher in Southern regions where the first maize populations are thought to have been introduced. Molecular classification based on Rogers' distance (i.e. alleles frequencies) allowed us to distinguish three main clusters which were highly consistent with geographic origins. A Northeastern cluster grouped together early or intermediate populations from Northeastern countries and the Balkans, a southeastern cluster joined late and partially dent populations from Greece and Italy, and, a southwestern cluster was made up of early flint populations from northern Spain, Portugal and the Pyrenees. A correlation between allelic frequencies at some loci and latitude and/or longitude was observed. Such tendencies may reflect the direction of gene flow between different races of maize: for instance, North American (Northern flint) and Caribbean populations were introduced, respectively, to northern and southern Europe, in the past.

A representative sample of 130 European traditional maize populations was analysed for both their morphological and molecular variation. The morphological analysis of 19 variables revealed a significant variability. Correlation analysis allowed us to distinguish between traits affected by earliness (plant and ear height) and structural traits (plant architecture, grain structure). Two main morphological types could be distinguished. Molecular analyses were performed for 29 RFLP loci on DNA bulks. The number of alleles detected was high when compared to previous studies (9.59 alleles per locus). Genetic diversity was also high (0.55), with a strong differentiation between populations (GST value of 35.6%). A clear relationship between the genetic diversity of the populations and their agronomic performances was highlighted. Morphological and molecular distances showed a tendency towards a triangular relationship. We therefore considered a two-phase process to be the most efficient approach for the classification of genetic resources: firstly, a molecular study to define groups of genetically close populations, and secondly a morphological description of populations from each group. In our European collection, this approach allowed us to separate the populations from Northern and Southern Europe and to define six groups of genetically close populations, comparable to European races. This study opens new prospects concerning the molecular analysis of very large collections of genetic resources, hitherto limited by the necessity of individual analyses, and proposes a first molecular classification of European maize germplasm.

A core collection is a subsample of a larger germ plasm collection that contains, with a minimum of repetitiveness, the maximum possible genetic diversity of the species in question (Frankel 1984; Frankel and Brown 1984). Brown (1989a) argued that before creating the core collection, the larger collection should first be hierarchically stratified into groups of accessions that share common characters or that originate from similar ecological and geographic regions. Such a stratification could be based on passport data, knowledge of the structure of the gene pool, or both. Accessions are then drawn from each group. Several sampling strategies are used to determine how to allocate sampling effort across groups (Brown 1989b). In the absence of detailed genetic data about the individuals within the groups, all groups can be represented evenly, or in proportion to their group size, or in proportion to the logarithm of their group size (refereed as the C-, P-, and Lstrategies, respectively) (Brown 1989b). An increasing number of germ plasm collections are being genotyped for marker loci such as allozymes, restriction fragment length polymorphisms (RFLPs), and random amplified polymorphic DNA (RAPD). Schoen and Brown (1993) proposed two strategies that can use marker diversity to allocate sampling effort for the construction of the core collection. The H strategy seeks to maximize the total number of alleles in the core collection by sampling accessions from groups in proportion to their within-group genetic diversity. Such an approach assumes that the sampled alleles follow Ewens (1972) sampling theory for neutral alleles, though the approach is robust to several types of departures from this assumption (Brown and Schoen 1994). Schoen and Brown (1993) formulated an alternative strategy, the so-called M (or maximization) strategy which does not necessarily rely upon stratified sampling. The M strategy examines all possible core collections and singles out those that maximize the number of observed alleles at the marker loci. These can then be chosen as final candidates for the core. The expected superiority of this marker-based method is based on the correlation between observed allelic richness at the marker loci and allelic richness on other loci (hereafter referred as to as ‘‘target loci’’). Such a correlation (or linkage disequilibrium between marker and target alleles) is expected on theoretical grounds either because of (1) shared coancestry of populations, (2) the mating system of the species considered, or (3) episodes of selection whereby selected (target) and neutral (marker) alleles become associated through hitchhiking. Monte Carlo simulations of germ plasm collection and sampling using several marker based sampling strategies have shown that the M strategy performs well when the accessions come from populations with restricted gene flow or when the accessions are predominantly selfing (Bataillon et al. 1996). While it was initially based on variation at marker loci, the M strategy can be extended to the qualitative and quantitative variables. For quantitative variables, the continuous distribution can be broken into a series of discrete classes. Each accession then belongs to one or several classes for this quantitative variable, depending on the value of the individuals comprising the accession. For each qualitative variable, the number of classes is determined by the possible values taken by the variable in question. For example, if the variable of disease resistance is coded as either resistant or susceptible, there would be two classes. Richness of a collection of accessions for such a qualitative variable is defined as the number of classes represented among the accessions. Then when considering several variables corresponding to several traits and/or marker loci, the total richness is defined as the sum richness values across variables. The independent contributions of each variable to the sum may be weighted by the importance of the variable; for instance, a given variable may be an important trait or a locus for which allelic variation is desired. The MSTRAT software implements a generalized version of the M strategy (as discussed above), and helps the user to define the size of the core collection to be sampled, as well as the choice of the type of genetic richness to be maximized in the core collection. The software also allows the user to investigate how much genetic richness has been retained for variables that were not used in the sampling of the core collection.

Exotic germplasm is often used in maize (Zea mays L.) breeding programs. However, the occurrence of genotype × environment interactions (GEIs) may mask the potential utility of exotic material. Our objective was to understand better GEIs for temperate, temperate × highland, and highland tropical maize genotypes cultivated under temperate or highland tropical conditions. We related yield instability to GEIs observed for vegetative or flowering traits, and determined the response of these vegetative or flowering traits to temperature and photoperiod. Forty‐one hybrids grown in four environments were observed for several pre‐flowering, flowering, and yield traits. Grain yield variation was related to two major adaptation factors: disease resistance and planting‐to‐silking duration (PSD). Yield variations were also related to variation in traits measured before flowering, such as emergence duration or early growth. For emergence duration, leaf stage at a given date, or leaf size, one simple temperature covariate accounted for more than half of the interaction sum of squares. Temperate × highland tropical hybrids had an intermediate behavior and were more stable than the pure temperate or pure highland tropical hybrids. For total number of leaves, photoperiod at tassel initiation explained a higher proportion of the interaction sum of squares than temperature. The response of highland tropical hybrids to photoperiod was larger than for the temperate hybrids. We recommend breeders who wish to introduce exotic material into adapted material utilize mass selection and/or advanced backcrossing, with marker assisted selection for specific traits with low heritability, such as cold tolerance.

The heterotic pattern between early European flint and dent inbred lines has been widely used for grain maize in Northern Europe because of its adaptation to temperate climates. Yet, a few lines are used in the breeding programs and the variability of the European material is weak. To preserve the improvement potential, the genetic variability must be increased, and the search for new favorable combinations would be consistent with the management of the heterotic groups. To identify the potential of lines to improve parents of single early cross, we evaluated 15 inbred late lines in cross with 4 early lines for grain yield and grain moisture. The design was conducted in 6 locations in France grouping in 4 areas according to the sum of air temperature during nine weeks after sowing. The 15 late lines were distributed in 5 lines from the Stiff Stalk Synthetic (BSSS) group, 5 lines from the Lancaster Sure Crops (LSC) group, and 5 other lines from different origins. We compared the agronomic values of recently created lines with those of original inbred lines. In order to evaluate and structure the genetic variability of the late lines in cross with early lines, we used two methods. The first one is a model of splitting up the interaction between lines and testers (biadditive model). The second ones are methods for the identification of lines with the greatest number of favorable alleles. Estimators were calculated : the predicted three-way cross, the probabilities of net gain of favorable alleles given complete dominance, the minimum upper bound, the net improvement, and general combining ability effects. Based on results of biadditive model, the genetic variability for yield among late lines in cross with early lines was found to be structured in BSSS and LSC groups in both late and early zones. The BSSS lines exhibited a good combining ability in cross with flint tester, and the LSC lines exhibited a good combining ability in cross with early dent tester. For grain moisture, this structuration had been globally kept in early areas except for the latest LSC line (Mo17) and the earliest BSSS line (F834). Both the biadditive model and genetic models detected the same best early x late crosses. The large part of additive effect in the genetic variability of yield and grain moisture could explain this result. Recently, developed inbred lines were found to be promising: F682 for Lancaster group, F876 for Stiff Stalk group. Moreover, these crosses should not disrupt the flint x dent heterotic pattern.

A multitrait pedigree breeding system including evaluation for European corn borer (Ostrinia nubilalis Hbn.) tolerance and other agronomic traits (yield, earliness, stalk lodging) was used for 16 years to create inbred lines from very different temperate germplasms. The ultimate evaluation of the 63 inbred lines was made in comparison with stable known references. The results allowed us to classify this material into three tolerance classes to the insect and demonstrated the efficacy of the method. High-yielding combining ability might be associated with earliness, lodging tolerance and good insect tolerance. The value of some early flint European materials and of Argentinian sources was discussed to improve European corn borer tolerance. ((C) Inra/Elsevier, Paris.)

Enzymatic polymorphism analysis of West African maize was undertaken to describe their isoenzymatic polymorphism, to analyse the factors of the genetic structuration on some landraces, and to study the occur rence of introgression between an introduced elite variety and landraces. Three different materials were analysed on 18 loci: 100 landraces collected from the two types of fields used in Burkina Faso for maize growing, a West African maize gene pool named Composite Y and constituted by 145 flint landraces of six countries, and an improved elite open-pollinated variety (SR 22) delivered by IITA and released in Burkina Faso. The results indicated that the allelic richness of the West African maize was similar to that of European maize but was quite lon,er than what had been found in American maize. Compared with published data, the results indicated that specific frequency of some alleles are observed in maize sample of West Africa; Europe and South western USA. The enzymatic variability of Composite was lower than that of Burkina's maize landraces. The type of fields (bush field and household field) where maize landraces are cultivated was a main factor of genetic structuration among these materials. The existence of gene flow between landraces and the elite variety have been established by the existence of two alleles which are specific to the elite variety. With these alleles, the maize breeders have a mean to control the introgression of elite variety; and to manage the in situ maize genetic variability. The paper discusses the management of maize landraces of West African savannah zone.

The genetic variability of the whole collection of 262 maize populations originating from metropolitan France was evaluated in two locations for agro-morphological traits, The most important variables in the principal component (PC) axis were related to maturity traits, and ear and grain shapes. On the first plane of the PC analysis, the distribution of populations was continuous, and populations from some particular regions were found grouped together: Pyrenees (early material and conical ears), Alsace (cylindrical ears), Bresse (mostly small kernels), Vallee de la Garonne (late material with long ears or with small kernels). The two distance matrices among populations calculated on the first four PC and on the geographic coordinates were correlated. Based on the first four standardized PC axes, which accounted for 77% of the variability, hierarchical classifications were computed and dendrograms confirmed the magnitude of maturity traits, and ear and kernel shapes in the classification.

The European areas of early maize cultivation have normally one generation of the European corn borer (ECB) per year and sources of tolerance are needed. The evaluation of plant damage under artificial infestation may be considered as a convenient tool of selection for tolerance. Many European inbred lines were tested for ECB tolerance and a 38-line synthetic, designated FS12, was developed. Then two cycles of S1 recurrent multitrait selection were conducted by the independent culling levels method, both for tolerance and agronomic traits. The source (C0) and selected (C1 and C2) populations were evaluated per se and as testcrosses in several locations to determine their yield abilities in the absence of the borer. Furthermore, they were tested under ECB artificial infestation to estimate the tolerance ratings (from 1 = tolerant to 9 = susceptible) and the yield losses due to the borer. In the experimental network without ECB, the grain yield gain averaged 0.23 Mg ha(-1) per cycle on testcrosses. In the most accurate experiment under artificial infestation, the two cycles of selection decreased tolerance ratings from 3.8 to 2.9. The yield reductions decreased by 22.4% to 17.9%. Between C2 and C0 populations on testcrosses, the yield increase averaged 0.49 Mg ha(-1) for uninfested plots and 0.74 Mg ha(-1) for infested plots. It can be assumed that the difference corresponds to the gain in tolerance. Thus, the recurrent selection in FS12 led to valuable results in breeding for both agronomic performance and tolerance to ECB.

The utilization of exotic germ plasm is difficult due to its non-adaptability. This study investigates the possibility of exotic germ plasm loss during adaptation, and the effect of an additional cross with elite material on the breeding value of exotic x adapted material. The study was conducted on a temperate x highland tropical composite (or pool) developed in order to broaden the genetic variability of maize in north western Europe. The frequency of unique exotic alleles and the isoenzymatic polymorphism at four loci were analysed in the pool itself, in the pool after mild selection, and in the selected pool crossed with elite material. Based on these data, no significant deviation seemed to occur during the mild selection and the cross. The pool and the pool x elite germ plasm cross were evaluated in testcrosses with two complementary testers for both grain and forage production. The pool was later in maturity, more susceptible to lodging, and yielded less than the pool x elite germ plasm crosses for all evaluations. The highest estimates of genetic variance were obtained in the pool for earliness and height traits, and for yield. However, based on the predicted genotypic mean of the selected population, the pool had a lower breeding value than the pool x elite germ plasm cross. The pool x elite germ plasm cross is thus preferred to initiate selection.

Trois composites de mais, FC(TC)CO, FC(TD)CO et FC(TI)CO, ont ete constitues a partir des familles S1 ayant une bonne aptitude a la combinaison sur trois testeurs complementaires, et issues de synthetiques a base large. Ces synthetiques ont ete constitues par le laboratoire mais de l'lnstitut national de la recherche agronomique (Inra) de Montpellier (France) a partir de lignees appartenant a divers groupes heterotiques. L'idee etait de developper de nouveaux groupes heterotiques. La constitution des composites est la phase initiale d'un programme de selection recurrente reciproque. Le comportement agronomique des composites pour la production de grain (rendement, humidite a maturite, resistance a la verse) et leur divergence genetique ont ete evalues dans un reseau multilocal dans un plan de croisement diallele associe a un plan de croisement avec les testeurs utilises pour la selection des constituants des composites. En croisement avec les testeurs, les composites ont un rendement moyen de 11 a 14 q/ha moins eleve que les temoins commerciaux actuels a precocite egale et une plus grande sensibilite a la verse. L'heterosis pour le rendement entre les composites est faible. Cette faible divergence est en accord avec la procedure de constitution des composites privilegiant l'aptitude generale a la combinaison.

Les marqueurs génétiques neutres, tels que les isoenzymes ou les polymorphismes de longueur des fragments de restriction (RFLP) ont désormais des applications dans des domaines très divers.Trois exemples en sont présentés.i) Près de 130 lignées de maïs ont été comparées en utilisant 46 sondes RFLP réparties sur les 10 chromosomes.Des analyses de données et des calculs de distances ont révélé une structuration très cohérente avec les classifications en groupes hétérotiques et avec les généalogies, ce qui souligne l'intérêt des marqueurs en sélection pour caractériser un matériel d'origine inconnue.Pour la prédiction de l'hétérosis, il apparaît que les indices de distances ne seront guère effi- caces pour les hybrides entre lignées appartenant à des groupes génétiques différents.Il y a donc nécessité de rechercher des polymorphismes de gènes directement impliqués dans les caractères.ii) Des marqueurs enzymatiques ont été utilisés pour suivre l'introgression de matériel exotique dans du matériel adapté, dans le cadre d'une expérience d'amélioration de la tolérance du maïs aux basses températures.Il est apparu que la structuration ob- servée était celle que l'on pouvait attendre en l'absence de dérive et de sélection.iii) Grâce à une carte génétique saturée construite à partir d'une descendance F2, nous avons localisé des QTL (quantitative trait loci) de caractères agromorphologiques, et de protéines quan- tifiées sur gels d'électrophorèse bidimensionnels à l'aide d'un logiciel d'analyse d'images.Nous avons ainsi pu comparer, à 2 niveaux phénotypiques très différents, la complexité des déterminismes et les valeurs des paramètres génétiques des locus impliqués (effet de substitution, dominance, épistasie), et avons montré qu'il y avait bien moins d'interactions intraou interlocus au niveau macroscopique qu'au niveau des produits de gènes.Zea mays L / marquage moléculaire / ressources génétiques / QTL / expression génétique Summary -Molecular markers as a tool for analyzing genetic diversity and genome expression in maize.Neutral markers, such as isoenzymes and RFLPs (restriction fragment length polymorphism), have applications in various fields of genetics and breeding.Three examples are documented.Firstly, about 130 maize lines were compared using 46 RFLP probes distributed over the 10 chromosomes.Data analyses and distance indices gave a classification consistent with heterotic groups and pedigrees, showing the value of molecular markers for characterizing unknown material.Distance indices were not very useful for predicting heterosis when parental lines belong to different genetic groups: the identification of polymorphisms of genes involved in the traits analysed is required.Second, enzyme markers were used to follow introgression of exotic into adapted material, in an experiment on cold tolerance.The classification observed revealed neither genetic drift nor selection.Finally, using a saturated genetic map constructed from an F2 progeny, we located QTLs (quantitative trait loci) for agromorphological traits and QTLs for amounts of proteins as revealed by 2-dimensional electrophoresis and quantified with a computer- assisted system.The comparison of genetic parameters (dominance, epistasis) between these 2 contrasted phenotypic levels suggested that interactions are much more common at the gene product level than at the macroscopic level.Zea mays L / molecular markers / genetic resources / QTL / genetic expression

Neutral markers, such as isoenzymes and RFLPs (restriction fragment length polymorphism), have applications in various fields of genetics and breeding. Three examples are documented. Firstly, about 130 maize lines were compared using 46 RFLP probes distributed over the 10 chromosomes. Data analyses and distance indices gave a classification consistent with heterotic groups and pedigrees, showing the value of molecular markers for characterizing unknown material. Distance indices were not very useful for predicting heterosis when parental lines belong to different genetic groups: the identification of polymorphisms of genes involved in the traits analysed is required. Second, enzyme markers were used to follow introgression of exotic into adapted material, in an experiment on cold tolerance. The classification observed revealed neither genetic drift nor selection. Finally, using a saturated genetic map constructed from an F2 progeny, we located QTLs (quantitative trait loci) for agromorphological traits and QTLs for amounts of proteins as revealed by 2-dimensional electrophoresis and quantified with a computer-assisted system. The comparison of genetic parameters (dominance, epistasis) between these 2 contrasted phenotypic levels suggested that interactions are much more common at the gene product level than at the macroscopic level.