This study was aimed to identify quantitative trait loci (QTL) for nitrogen use efficiency (NUE) and related traits in a maize population derived from a cross between two lines with different genetic background (B100 and LP2). Recombinant inbred lines (181) from this population were evaluated under field conditions during two growing seasons, and significant (P < 0.01) phenotypic and genotypic variability was detected for most evaluated traits. Two different mapping methods were applied for detecting QTLs. Firstly, a trait by trait approach was performed on across environments, and 19 QTLs were identified. Secondly, a multi-trait multi-environment analysis detected seven joint QTLs. Almost all joint QTLs had inconsistent additive effects from one environment to another, which would reflect presence of QTL x Environment interaction. Most joint QTLs co-localized with QTLs detected by individual mapping. We detected consistent additive effects for grain yield per plant and NUE, as well as for biomass and nitrogen harvest index in some joint QTLs, especially QTL-1 and QTL-6. These QTLs had positive and stable effects across environments, and presence of some genes within these QTL intervals could be relevant for selecting for both NUE and grain yield simultaneously. Up today, this is a first report on the co-localization of QTLs for enhanced allocation of biomass allocation to grains with NUE, and NUE candidate gene identification. Fine mapping of these regions could allow to detect additional markers more closely linked to these QTLs that could be used for marker assisted selection for NUE.

Previous studies in maize have identified three quantitative trait loci (QTLs) coding for high oleic acid content (HOAC) (QTLs oleic6-1, oleic6-2, and oleic6-3) at bins 6.04–6.05, proximal to the DGAT1-2 gene. The aims of this work were (i) to discover new markers for linkage disequilibrium (LD) and haplotype analysis distal to DGAT1-2, (ii) to develop a new DGAT1-2 PCR probe to detect the allele determining HOAC (F469 insertion), (iii) to conduct cluster analysis for kernel traits, and (iv) to assess genetic diversity, LD, and association analysis for kernel traits with the DGAT1-2 PCR probe and 13 markers previously mapped near DGAT1-2 in the Argentine temperate maize collection of 111 inbred lines. The results showed high haplotype diversity distal to DGAT1-2 and relatedness between the inbred line LP199 (with HOAC) and the Non-Stiff Stalk line W22 (reference genome). The frequency of F469 was low (20%). F469 was clustered with flint-grain type characteristics, whereas HOAC was associated with F469 across linear models. Genetic diversity at bins 6.04–6.05 was high (0.62), whereas LD extent was low (r2 ≤ 0.45). This low extent of LD indicates a high level of recombination and no LD between DGAT1-2 and markers flanking QTLs oleic6-1 to 3. Nevertheless, the significant LD between markers flanking those QTLs and the cosegregation of F469 with nc009 (markers flanking QTLs oleic6-1 and 2) during inbred line conversion suggest that these QTLs might contribute to HOAC in the breeding collection. However, further studies are needed to precise mapping at bins 6.04–6.05 for breeding purposes.

NAC proteins are plant transcription factors involved in biotic and abiotic stress responses, with conserved sub-domain motifs. To our knowledge, no diversity studies have been carried out at the nucleotide level of candidate NAC genes of crops. Here, we investigated allele diversity and putative protein-coding sequences in two paralogous maize NAC genes, GRMZM2G179885 and GRMZM2G347043. The GRMZM2G179885 gene is the closest ortholog of WNAC-B1, responsible for enhancing nitrogen remobilization and senescence in wheat. The GRMZM2G347043 gene is the homolog to the characterized ZmSNAC1 allele, which promotes the dehydration tolerance response in Arabidopsis. Nucleotide diversity for both genes was determined in a core set of the Argentine public maize collection of temperate inbred lines structured in four diverse subpopulations. The frequency of nucleotide changes at both exons and introns of GRMZM2G179885 and GRMZM2G347043 was lower (1/137 bp-1/83 bp and 1/472 bp-1/51 bp, respectively) than that of other maize genes. Tajima's D values were non-significant, indicating the absence of selection at these genes. The five sub-domains of NAC proteins were predicted at both genes, thus implying: (i) high conservation of these two genes at the genomic level, and (ii) scarce divergence between GRMZM2G179885 and GRMZM2G347043 protein-coding sequences, which confirms their relationships as ancestral paralogs. Although the functional activity of both genes in maize metabolism is still unclear, our findings contribute to demonstrating that their NAC proteins are common alleles and would serve as a source for abiotic stress improvement in the Argentine public maize inbred line collection.

Knowledge of linkage disequilibrium (LD) patterns is considered a prerequisite for effective association mapping studies. However, no LD analysis in the Argentine public temperate maize collection has been reported to date. In this study, a panel of 111 temperate maize inbreds genotyped at 74 single sequence repeats (SSRs) loci was used to assess LD, genetic diversity and population structure to evaluate the suitability of the panel for association mapping. Mini-core sets were also designed for in-depth phenotyping and allele mining purposes. The panel consisted of: (1) locally developed orange flint germplasm; (2) temperate inbred lines with Iowa Stiff Stalk Synthetic background; and (3) eight historic flint lines, some of them from the Cuarentin race. As a result, four subpopulations were defined. Joint analysis of population structure and combining ability allowed identifying two main heterotic patterns. High molecular diversity, a low extent of LD and a high ratio of linked to unlinked SSR loci pairs in significant LD were detected indicating the suitability of the entire collection for association mapping. The fact that the LD extent in the mini-core sets was similar to that observed in the entire collection and that only a small percentage of allelic richness was reduced suggests that these mini-core sets are suitable to capture diversity, exploit phenotypic variance and discover useful variants representative of the entire collection.

The genetic control of maize kernel weight (KW) determination could be studied through its physiological and/ or compositional determinants. Our objective was to dissect the genetic control of maize KW by analyzing its physiological (KGR: kernel growth rate; KFD: kernel filling duration) and compositional (protein, oil, starch) determinants in a dentxflint Caribbean RIL population, which combines a broad genetic background with grains of high added value for industry. An additional objective was to determine the stability of the genetic control under contrasting growing conditions, for which soil nitrogen offer was modified across experiments. Heritability (H-2) values were high for KW (H-2 = 0.74) and intermediate for the other traits (from 0.62 to 0.42). Kernel weight had a strong correlation with KFD (r = 0.69), KGR (r = 0.60) and protein concentration (r = 0.56). Ten joint QTL with inconsistent effects across years and seven epistatic interactions were detected. Despite changes in effect size, most QTL were significant under both environments. Nine QTL were associated with variations in potential KW (KWP), mean KW, KGR and oil concentration, eight with variations in protein and starch concentration and seven with KFD. Epistatic interactions were related to regions with significant main effects. The most important finding was the existence of a common QTL for KWP, KGR and KFD on chromosome 5, for which there was no previous report. Results increased our knowledge on the genetic control of KW through its phenotypic and genetic correlation with KFD, confirming the need to explore different physiological strategies in different genetic backgrounds.

The maize inbred line B73 has been extensively studied at the molecular level. Researchers now have access to the genome sequence of B73 as well as to databases of biallelic and multiallelic markers where functional polymorphisms between B73 and a public inbred line population can be compared to relate genotypes to phenotypes. This indicates the importance to determine the genetic fidelity of the germplasm during preservation and propagation processes, in particular, when seeds of reference inbred lines such as B73 are maintained. The aim of this study was to assess the genetic uniformity among three different sources of the B73 inbred line by means of 75 Simple Sequence Repeats (SSRs). The three B73 sources showed homozygosis; however, some accessions differed greatly from the expected locus size predicted at the reference B73 genomic sequence. A particular haplotype was prevalent in the USDA accession PI550473. The error rate of the allele size determination was estimated. The genotyping technique used in this work allowed the separation of alleles of +/- 2 bp range difference within the same electrophoresis run, whereas allele size estimations between experiments, within the laboratory, differed in +/- 4 bp range difference. Besides experimental errors in genotyping, the putative cause of differences among accessions could be attributed to seed contamination and genetic drift. The B73 accessions evaluated in our work can be shared among laboratories to precise genotyping and phenotyping of maize inbred lines.

Genetic diversity of an Argentinean public temperate inbred maize collection has not been previously assessed. This collection includes mainly locally developed orange flint germplasm and a group of temperate inbred lines introduced from the US or derived from selection of crosses to genetic stocks from other countries, providing representativeness of exotic gene pools. To establish heterotic groups and patterns for breeding purposes and to assess genetic structure and relatedness for association-mapping studies, a public panel of a 103 maize inbreds was characterized using 50 microsatellite markers and pedigree information. By means of clustering-based and model-based methods the flint germplasm collection was split into two subpopulations that were separated from the BSSS-BS13-related lines. Relatedness estimates with coancestry and kinship coefficients provided additional information in the case of structured mixed membership of some germplasm. These three main subpopulations were in agreement with prior pedigree records. Allele diversity was high and sufficient to give major, minor and specific allele profiles to characterize inbred lines. Convenience of the use of minor allele frequency for structure and relatedness assessment is also discussed. In addition, molecular characterization provided useful information to elucidate inbred ancestry origins of germplasm with unknown pedigree records and to group them into known heterotic groups to define heterotic patterns.

Tetraploid wheat (Triticum turgidum L. var. durum) cv. 'Langdon' (LDN) and its near-isogenic recombinant substitution line no. 68 (RSL no. 68) carrying the high grain protein gene Gpc-B1 from emmer wheat, were compared in three greenhouse experiments to establish in which way Gpc-B1 increases grain protein concentration (GPC). At anthesis, RSL no. 68 had higher soluble protein and amino acids concentrations in the flag leaf than LDN. At maturity, both lines presented a similar above ground biomass and grain yield. However, RSL no. 68 showed a higher total N content in ears, grain and chaff than LDN; N harvest index (NHI) was also higher because of a lower straw N concentration and higher grain N concentration. When both lines were grown with a low N supply, and when N supply was interrupted before anthesis, similar trends were observed but the differences in GPC were smaller. It is concluded that RSL no. 68 accumulates a higher GPC than LDN mainly because of a more efficient N remobilization from the leaves to the ears during grain filling.

El proceso tradicional de mejoramiento requerido para la liberación de variedades en los trigos insume aproximadamente de 10 a 15 años (Poehlman, 1979). A través de los años, los fitomejoradores han seleccionado las mejores plantas basados en su fenotipo. Este es un proceso lento, y por lo tanto costoso, al no disponer de variedades que respondan a las demandas dinámicas de calidad y de resistencia a enfermedades.

The conservation of the linear order (colinearity) of genetic markers along large chromosome segments in wheat and rice is well established, but less is known about the microcolinearity between both genomes at subcentimorgan distances. In this study we focused on the microcolinearity between a 2.6-cM interval flanked by markers Xcdo365 and Xucw65 on wheat chromosome 6B and rice chromosome 2. A previous study has shown that this wheat segment includes the Gpc-6B1 locus, which is responsible for large differences in grain protein content (GPC) and is the target of a positional cloning effort in our laboratories. Twenty-one recombination events between Xcdo365 and Xucw65 were found in a large segregating population (935 gametes) and used to map 17 genes selected from rice chromosome 2 in the wheat genetic map. We found a high level of colinearity between a 2.1-cM region flanked by loci Xucw75 and Xucw67 on wheat chromosome 6B and a 350-kb uninterrupted sequenced region in rice chromosome arm 2S. Colinearity between these two genomes was extended to the region proximal to Xucw67 (eight colinear RFLP markers), but was interrupted distal to Xucw75 (six non-colinear RFLP markers). Analysis of different comparative studies between rice and wheat suggests that microcolinearity is more frequently disrupted in the distal region of the wheat chromosomes. Fortunately, the region encompassing the Gpc-6B1 locus showed an excellent conservation between the two genomes, facilitating the saturation of the target region of the wheat genetic map with molecular markers. These markers were used to map the Gpc-6B1 locus into a 0.3-cM interval flanked by PCR markers Xucw79 and Xucw71, and to identify five candidate genes within the colinear 64-kb region in rice.

Grain protein content (GPC) is an important factor in pasta and breadmaking quality, and in human nutrition. It is also an important trait for wheat growers because premium prices are frequently paid for wheat with high GPC. A promising source for alleles to increase GPC was detected on chromosome 6B of Triticum turgidum var. dicoccoides accession FA-15-3 (DIC). Two previous quantitative trait locus (QTL) studies found that the positive effect of DIC-6B was associated to a single locus located between the centromere and the Nor-B2 locus on the short arm of chromosome 6B. Microsatellite markers Xgwm508 and Xgwm193 flanking the QTL region were used in this study to develop 20 new homozygous recombinant substitution lines (RSLs) with crossovers between these markers. These 20 RSLs, plus nine RSLs developed in previous studies were characterized with four new RFLP markers located within this chromosome segment. Grain protein content was determined in three field experiments organized as randomized complete block designs with ten replications each. The QTL peaks for protein content were located in the central region of a 2.7-cM interval between RFLP markers Xcdo365 and Xucw67 in the three experiments. Statistical analyses showed that almost all lines could be classified unequivocally within low- and high- protein groups, facilitating the mapping of this trait as a single Mendelian locus designated Gpc-6B1. The Gpc-6B1 locus was mapped 1.5-cM proximal to Xcdo365 and 1.2-cM distal to Xucw67. These new markers can be used to reduce the size of the DIC chromosome segment selected in marker-assisted selection programs. Markers Nor-B2 and Xucw66 flanking the previous two markers can be used to select against the DIC segment and reduce the linkage drag during the transfer of Gpc-6B1 into commercial bread and pasta wheat varieties. The precise mapping of the high GPC gene, the high frequency of recombinants recovered in the targeted region, and the recent development of a tetraploid BAC library including the Gpc-6B1 DIC allele are the first steps towards the map-based cloning of this gene.

The conservation of the linear order (colinearity) of genetic markers along large chromosome segments in wheat and rice is well established, but less is known at the molecular level (microcolinearity). Previous comparative maps between these two species revealed conserved linkage between rice chromosome 2 and wheat chromosome 6. As part of our efforts to identify a locus affecting Grain Protein Content (GPC) using a positional cloning approach, we established the microcolinearity between a region on wheat chromosome 6BS, including the GPC locus, and 20.4 cM on rice chromosome 2. Blast analysis of the rice BAC sequences covering the colinear GPC region against the Triticeae EST database revealed several sequences that were used as markers in the wheat genetic map. Using this approach we mapped eleven colinear genes between wheat and rice in this region. The closest markers flanking the GPC locus were mapped 0.9 cM apart in wheat and 100-kb apart in rice. Further work is underway to complete the mapping of additional wheat ESTs identified within the 100-kb rice sequence and to construct a physical map using a tetraploid wheat BAC library that contains a T. dicoccoides 6BS chromosome segment carrying the high GPC locus (http://agronomy.ucdavis.edu/ Dubcovsky/BAC-library/BAC_Langdon.htm). These results demonstrated a high level of microcolinearity between rice and wheat in the GPC region.

Plants were regenerated by shoot multiplication from four clones of Melia azedarach L. during 12 mo. of subculturing. One hundred and one of these plants were examined by randomly amplified polymorphic DNA analysis. All regenerated plants showed at least one polymorphism. However, no chromosome number alterations were observed. The pattern of variation obtained by principal coordinated analysis showed a random distribution of variation among regenerated plants and their controls, indicating that genetic alterations were not cumulative during in vitro culture. Similar results were found using Shannon's index, which revealed that 50% of the observed diversity resided among plants coming from the same subculture generation. This high intraclonal variation does not provide a clear scenario for predicting the amount of culture time required to preserve genetic fidelity in commercially micropropagated M. azedarach plants. Our work suggests that other mechanisms, such as chimerism, contribute to intraclonal heterogeneity in vitro.