Disease lesion mimics in maize: I. Effect of genetic background,temperature, developmental age,and wounding on necrotic spot formation with Les1

Les1, a dominant gene of maize (Zea mays L.), results in the production of necrotic leaf spots. Expression of this trait is temperature sensitive, and the nonpermissive temperature for expression is determined by the genetic background in which the gene is placed. Exposure to nonpermissive conditions for 24–48 hr will induce lesion production in the most sensitive genotype. Lesions form first at the leaf tip, the oldest part of the leaf, and progress basipetally through fully expanded tissue. Leaves excised from plants grown at either permissive or nonpermissive temperatures and placed in water, gibberellic acid, or abscisic acid solutions form no new lesions at either 20 or 30°C, and the leaves senesce rapidly. However, when leaves excised from normal and Les1 plants are placed in kinetin, senescence is delayed and numerous lesions develop at 20°C on Les1 plants. Our results suggest that there is a developmental time window during which maize leaf cells can be induced to form lesions: cells must be fully elongated but not yet senescent. This hypothesis is strengthened by the observation that pinprick wounding of leaves induces lesions only in a band of tissue approximately 2 days younger than the area of the leaf currently producing lesions. Various models for the action of Les1 in causing discrete lesion formation are discussed.     

Comparative Map and Trait Viewer (CMTV): an integrated bioinformatic tool to construct consensus maps and compare QTL and functional genomics data across genomes and experiments

In the past few decades, a wealth of genomic data has been produced in a wide variety of species using a diverse array of functional and molecular marker approaches. In order to unlock the full potential of the information contained in these independent experiments, researchers need efficient and intuitive means to identify common genomic regions and genes involved in the expression of target phenotypic traits across diverse conditions. To address this need, we have developed a Comparative Map and Trait Viewer (CMTV) tool that can be used to construct dynamic aggregations of a variety of types of genomic datasets. By algorithmically determining correspondences between sets of objects on multiple genomic maps, the CMTV can display syntenic regions across taxa, combine maps from separate experiments into a consensus map, or project data from different maps into a common coordinate framework using dynamic coordinate translations between source and target maps. We present a case study that illustrates the utility of the tool for managing large and varied datasets by integrating data collected by CIMMYT in maize drought tolerance research with data from public sources. This example will focus on one of the visualization features for Quantitative Trait Locus (QTL) data, using likelihood ratio (LR) files produced by generic QTL analysis software and displaying the data in a unique visual manner across different combinations of traits, environments and crosses. Once a genomic region of interest has been identified, the CMTV can search and display additional QTLs meeting a particular threshold for that region, or other functional data such as sets of differentially expressed genes located in the region; it thus provides an easily used means for organizing and manipulating data sets that have been dynamically integrated under the focus of the researchers specific hypothesis.     

Wheat genetic diversity trends during domestication and breeding

It has been claimed that plant breeding reduces genetic diversity in elite germplasm which could seriously jeopardize the continued ability to improve crops. The main objective of this study was to examine the loss of genetic diversity in spring bread wheat during (1) its domestication, (2) the change from traditional landrace cultivars (LCs) to modern breeding varieties, and (3) 50 years of international breeding. We studied 253 CIMMYT or CIMMYT-related modern wheat cultivars, LCs, and Triticum tauschii accessions, the D-genome donor of wheat, with 90 simple sequence repeat (SSR) markers dispersed across the wheat genome. A loss of genetic diversity was observed from T. tauschii to the LCs, and from the LCs to the elite breeding germplasm. Wheats genetic diversity was narrowed from 1950 to 1989, but was enhanced from 1990 to 1997. Our results indicate that breeders averted the narrowing of the wheat germplasm base and subsequently increased the genetic diversity through the introgression of novel materials. The LCs and T. tauschii contain numerous unique alleles that were absent in modern spring bread wheat cultivars. Consequently, both the LCs and T. tauschii represent useful sources for broadening the genetic base of elite wheat breeding germplasm.     

An international reference consensus genetic map with 897 marker loci based on 11 mapping populations for tetraploid groundnut (Arachis hypogaea L.)

Only a few genetic maps based on recombinant inbred line (RIL) and backcross (BC) populations have been developed for tetraploid groundnut. The marker density, however, is not very satisfactory especially in the context of large genome size (2800 Mb/1C) and 20 linkage groups (LGs). Therefore, using marker segregation data for 10 RILs and one BC population from the international groundnut community, with the help of common markers across different populations, a reference consensus genetic map has been developed. This map is comprised of 897 marker loci including 895 simple sequence repeat (SSR) and 2 cleaved amplified polymorphic sequence (CAPS) loci distributed on 20 LGs (a01-a10 and b01-b10) spanning a map distance of 3, 863.6 cM with an average map density of 4.4 cM. The highest numbers of markers (70) were integrated on a01 and the least number of markers (21) on b09. The marker density, however, was lowest (6.4 cM) on a08 and highest (2.5 cM) on a01. The reference consensus map has been divided into 20 cM long 203 BINs. These BINs carry 1 (a10_02, a10_08 and a10_09) to 20 (a10_04) loci with an average of 4 marker loci per BIN. Although the polymorphism information content (PIC) value was available for 526 markers in 190 BINs, 36 and 111 BINs have at least one marker with >0.70 and >0.50 PIC values, respectively. This information will be useful for selecting highly informative and uniformly distributed markers for developing new genetic maps, background selection and diversity analysis. Most importantly, this reference consensus map will serve as a reliable reference for aligning new genetic and physical maps, performing QTL analysis in a multi-populations design, evaluating the genetic background effect on QTL expression, and serving other genetic and molecular breeding activities in groundnut.     

Re-evaluation of the prospects of marker-assisted selection for improving insect resistance against Diatraea spp. in tropical maize by cross validation and independent validation

Cross validation (CV) and validation with an independent sample (IV) are new biometric approaches in QTL analysis to obtain unbiased estimates of QTL effects and the proportion of the genetic variance explained by the detected marker-QTL association (p). Our objective with these methods was to obtain a realistic picture on the prospects of marker-assisted selection (MAS) for improving the resistance of maize against the tropical stem borer species Diatraea grandiosella (SWCB) and Diatraea saccharalis (SCB). Published QTL mapping studies on leaf-damage ratings (LDR) with populations of F_2:3 lines and recombinant inbred lines (RIL) from crosses CML131×CML67 and Ki3× CML139 of tropical maize inbreds were re-analyzed with CV and IV. With CV, the reduction in p for LDR compared to p obtained with the whole data set varied between 41.0 and 79.6% in the populations of F_2:3 lines and between 30.1 and 65.2% in the two populations of RIL. Estimates of p for SCB LDR were similar for CV and IV. For SWCB LDR, p estimates obtained with IV were larger than those obtained with CV in CML131× CML67. The reverse was observed for Ki3×CML139. Under the assumption of identical selection intensities, and based on the re-estimates of p, MAS using only molecular marker information is less-efficient than conventional phenotypic selection (CPS). MAS combining marker and phenotypic data increases the relative efficiency by only 4% in comparison to CPS. In conclusion, MAS for improving SWCB and SCB LDR seems not-promising unless additional QTLs with proven large effects are available or the costs of marker assays are considerably reduced. Received: 7 December 2000 / Accepted: 5 February 2001     

Selection and characterization of mexican strains ofBacillus thuringiensis active against four major lepidopteran maize pests

In order to isolate novel delta-endotoxins fromBacillus thuringiensis Berliner, a total of 426 native isolates (in varying numbers for each pest) were screened against four major maize pests: corn earworm,Helicoverpa zea; fall armyworm,Spodoptera frugiperda; southwestern corn borer,Diatraea graridiosella, and sugarcane borer,Diatraea saccharalis. Spore-crystal complexes from the isolates were integrated into semi-artificial diets of each pest and mortality was assessed 7 days after treatment. A total of 25 isolates were selected on the basis of highest larval mortality against at least one insect species. There was no correspondence of the most toxic isolates when tested against the four different insect species. Most of the 25 selected isolates caused higher toxicities against all four pests than the standard strain HD-1, regardless of not achieving 100% mortality in any bioassay.H. zea demonstrated the highest level of mortality (96%) and was susceptible to the largest number of isolates (98). None of the other insect species were found susceptible at levels greater than 60%. All the selected active strains were isolated from stored grain dusts (except for LBIT-167), and had bipyramidal crystals with Cry I-like proteins. Most isolates also formed an associated square (cubic) inclusion, with Cry Il-like proteins according to SDS-PAGE analysis of their parasporal bodies. The most active isolates will be subjected to further studies, in order to identify putative novel genes to be expressed in transgenic maize     

Use of STSs and SSRs as rapid and reliable preselection tools in a marker-assisted selection-backcross scheme

We describe a new approach for using suitable STS and SSR markers as a powerful molecular tool for screening segregating populations involved in backcross schemes for marker-assisted selection, as a preselection step. Since it can be applied to very large populations, this preselection strategy allows one to increase substantially the pressure of selection at each backcross generation. The technique is fast and reproducible, and can be made even more efficient and costeffective by simultaneous DNA amplification from different primer pairs. In the example illustrated here, three suitable PCR-based markers were used to complete the selection of 300 individuals out of 2300 in less than one month with two people working on the project.     

Genetic analysis of adaptation differences between highland and lowland tropical maize using molecular markers

Molecular-marker loci were used to investigate the adaptation differences between highland and lowland tropical maize. An F₂ population from the cross of two inbred lines independently derived from highland and lowland maize germplasm was developed, and extracted F_3:4 lines were phenotype in replicated field trials at four thermally diverse tropical testing sites, ranging from lowland to extreme highland (mean growing season temperature range 13.2–24.6°C). Traits closely related with adaptation, such as biomass and grain yield, yield components, days from sowing to male and female flowering, total leaf number, plant height and number of primary tassel branches (TBN), were analyzed. A large line × environment interaction was observed for most traits. The genetic basis of this interaction was reflected by significant, but systematic, changes from lowland to highland sites in the correlation between the trait value and genomic composition (designated by the proportion of marker alleles with the same origin). Joint analysis of quantitative trait loci (QTLs) over sites detected 5–8 QTLs for each trait (except disease scores, with data only from one site). With the exception of one QTL for TBN, none of these accounted for more than 15% of the total phenotypic variation. In total, detected QTLs accounted for 24–61% of the variation at each site on average. For yield, yield components and disease scores, alleles generally favored the site of origin. Highland-derived alleles had little effect at lowland sites, while lowland-derived alleles showed relatively broader adaptation. Gradual changes in the estimated QTL effects with increasing mean site temperature were observed, and paralleled the observed patterns of adaptation in highland and lowland germplasm. Several clusters of QTLs for different traits reflected the relative importance in the adaptation differences between the two germplasm types, and pleiotropy is suggested as the main cause for the clustering. Breeding for broad thermal adaptation should be possible by pooling genes showing adaptation to specific thermal regimes, though perhaps at the expense of reduced progress for adaptation to a specific site. Molecular marker-assisted selection would be an ideal tool for this task, since it could greatly reduce the linkage drag caused by the unintentional transfer of undesirable traits. Received: 10 October 1998 / Accepted: 9 April 1999     

Production of transgenic tropical maize with cryIAb and cryIAc genes via microprojectile bombardment of immature embryos

To enhance the level of resistance to insects in tropical maize germplasm we have developed techniques to successfully transform elite tropical maize inbred based on the activity of specific cryI proteins against four major maize pests – corn earworm, fall armyworm, southwestern corn borer and sugarcane borer. Constructs containing cryIAb or cryIAc synthetic genes were used. To generate transgenic plants we have established methods for biolistic bombardment and the selection and regeneration of immature embryos and calli from the elite tropical lines CML72, CML216, CML323, CML327 and hybrids. Transgenic plants resistant to the herbicide Basta^TM contained the bands for the cry, bar and gus genes as detected by Southern blot analyses. A simple leaf bioassay presented varying levels of resistance to Southwestern corn borer of transgenic tropical maize carrying the cryIAc gene. Analyses of the progenies confirmed the sexual transmission of the introduced genes and their stable expression. Received: 25 September 1998 / Accepted: 27 October 1998