Association mapping for phenology and plant architecture in maize shows higher power for developmental traits compared with growth influenced traits.

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.

Maize introduction into Europe: the history reviewed in the light of molecular data.

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.

RFLP diversity and relationships among traditional European maize populations.

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.

Large scale molecular analysis of traditional European maize populations. Relationships with morphological variation.

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.

Evaluation of agronomic traits and analysis of exotic germ plasm polymorphism in adapted x exotic maize crosses.

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.