Construction of BAC and BIBAC libraries and their applications for generation of SSR markers for genome analysis of chickpea, Cicer arietinum L

Advancing Chickpea Breeding: Omics Insights for Targeted Abiotic Stress Mitigation and Genetic Enhancement

Chickpea is a major source of proteins and is considered the most economically vital food legume. Chickpea production is threatened by several abiotic and biotic factors worldwide. The main constraints limiting worldwide chickpea production are abiotic conditions such as drought, heat, salinity, and cold. It is clear that chickpea is treasured for its nutritive value, in particular its high protein content, and hence study of problems like drought, cold and salinity stresses are very important concerning chickpeas. In this regard, several physiological, biochemical, and molecular mechanisms are reviewed to confer tolerance to abiotic stress. The most crippling economic losses in agriculture occur due to these abiotic stressors, which affect plants in many ways. All these abiotic stresses affect the water relations of the plant, both at the cellular level as well as the whole-plant level, causing both specific and non-specific reactions, damage and adaptation reactions. These stresses share common features. Breeding programs use a huge collection of over 100,000 chickpea accessions as their foundation. Significant advancements in conventional breeding, including mutagenesis, gene/allele introgression, and germplasm introduction, have been made through this method. Abiotic tolerance and yield component selection are made easier by creating unique DNA markers for the genus Cicer, which has been made possible by developments in high-throughput sequencing and molecular biology. Transcriptomics, proteomics, and metabolomics have also made it possible to identify particular genes, proteins, and metabolites linked to chickpea tolerance to abiotic stress. Chickpea abiotic stress tolerance has been directly and potentially improved by biotechnological applications, which are covered by all 'Omics' approaches. It requires information on the abiotic stress response at the different molecular levels, which comprises gene expression analysis for metabolites or proteins and its impact on phenotype. Studies on chickpea genome-wide expression profiling have been conducted to determine important candidate genes and their regulatory networks for abiotic stress response. This study aimed to offer a detailed overview of the diverse 'Omics' approaches for resilience's to abiotic stresses on chickpea plants.

Chickpea (Cicer arietinum L.) Biology and Biotechnology: From Domestication to Biofortification and Biopharming

Chickpea (Cicer arietinum L.), the world's second most consumed legume crop, is cultivated in more than 50 countries around the world. It is a boon for diabetics and is an excellent source of important nutrients such as vitamins A, C, E, K, B1-B3, B5, B6, B9 and minerals (Fe, Zn, Mg and Ca) which all have beneficial effects on human health. By 2050, the world population can cross 9 billion, and in order to feed the teaming millions, chickpea production should also be increased, as it is a healthy alternative to wheat flour and a boon for diabetics. Moreover, it is an important legume that is crucial for food, nutrition, and health security and the livelihood of the small-scale farmers with poor resources, in developing countries. Although marvelous improvement has been made in the development of biotic and abiotic stress-resistant varieties, still there are many lacunae, and to fulfill that, the incorporation of genomic technologies in chickpea breeding (genomics-assisted breeding, high-throughput and precise-phenotyping and implementation of novel breeding strategies) will facilitate the researchers in developing high yielding, climate resilient, water use efficient, salt-tolerant, insect/pathogen resistant varieties, acceptable to farmers, consumers, and industries. This review focuses on the origin and distribution, nutritional profile, genomic studies, and recent updates on crop improvement strategies for combating abiotic and biotic stresses in chickpea.

Unraveling genetics of semi-determinacy and identification of markers for indeterminate stem growth habit in chickpea (Cicer arietinum L.)

Chickpea (Cicer arietinum L.) is predominantly an indeterminate plant and tends to generate vegetative growth when the ambient is conducive for soil moisture, temperature and certain other environmental conditions. The semi-determinate (SDT) types are comparatively early, resistant to lodging and found to be similar in their yield potential to indeterminate (IDT) lines. Indeterminate and semi-determinate genotypes are found to be similar during early stage, which makes it difficult to distinguish between them. Thus, there is a need to identify molecular markers linked either to indeterminate or semi-determinate plant types. The present study was carried out to study the genetics of semi-determinacy and identify molecular markers linked to stem growth habit. The study was undertaken in the cross involving BG 362(IDT) × BG 3078-1(SDT). All F₁ plants were indeterminate, which indicates that indeterminate stem type is dominant over semi-determinate. In further advancement to F₂ generation, F₂ plants are segregated in the ratio of 3(Indeterminate): 1(Semi-determinate) that indicates that the IDT and SDT parents which are involved in the cross differed for a single gene. The segregation pattern observed in F₂ is confirmed in F₃ generation. The parental polymorphic survey was undertaken for molecular analysis using total of 245 SSR markers, out of which 41 polymorphic markers were found to distinguish the parents and were utilized for bulked segregant analysis (BSA). The segregation pattern in F₂ indicates that the IDT (Indeterminate) and SDT (Semi-determinate) parents which are involved in the cross differed for single gene. The segregation pattern of F₂ and F₃ derived from the cross BG 362 (IDT) × BG 3078-1 (SDT) confirmed the genotypic structure of the newly found SDT genotype BG 3078-1 as dt1dt1Dt2Dt2. Three SSR markers TA42, Ca_GPSSR00560 and H3DO5 were found to be putatively linked to Dt1 locus regulating IDT stem growth habit. Our results indicate that the SSR markers identified for Dt1 locus helps to differentiate stem growth habit of chickpea in its early growth stage itself and can be efficiently utilized in Marker Assisted Selection (MAS) for changed plant type in chickpea.

Development of genomic simple sequence repeat markers for Glycyrrhiza lepidota and cross-amplification of other Glycyrrhiza species

BACKGROUND

Licorice (Glycyrrhiza spp. L.) is used as a natural sweetener and medicinal herb in European and Asian countries. Molecular studies have been conducted to find differences between wild and cultivated species because most wild species are highly resistant to abiotic and biotic stresses compared with their cultivated species. However, few molecular markers have been developed for studying the genetic diversity and population structure of licorice species and to identify differences between cultivars. Thus, the present study aimed to develop a set of genomic simple sequence repeat (SSR) markers for molecular studies of these species.

METHODS

In the present study, we developed polymorphic SSR markers based on whole-genomesequence data of Glycyrrhiza lepidota. Then, based on the sequence information, the polymorphic SSR markers were developed. The SSR markers were applied to 23 Glycyrrhiza individual plants. We also evaluated the phylogenetic relationships and interspecies transferability among samples.

RESULTS

The genetic diversity analysis using these markers identified 2-23 alleles, and the major allele frequency, observed heterozygosity, genetic diversity, and polymorphism information content were 0.11-0.91, 0-0.90, 0.17-0.94, and 0.15-0.93, respectively. Interspecies transferability values were 93.5%, 91.6%, and 91.1% for G. echinata, G. glabra, and G. uralensis, respectively. Phylogenetic analysis clustered cultivated (group 1) and wild (group 2) species into three and two subgroups, respectively. The reported markers represent a valuable resource for the genetic characteri z ation of Glycyrrhiza spp. for theanalysis of its genetic variability, and as a tool for licorice transferability. This is the first intraspecific study in a collection of Glycyrrhiza spp. germplasm using SSR markers.

An assessment of transgenomics as a tool for gene discovery in Populus euphratica Oliv

Transgenomics for gene discovery in Populus euphratica. Transgenomics, a member of the omics family of methodologies, is characterized as the introduction of DNA from one organism into another on a genome-wide scale followed by the identification of recipients with altered phenotypes. This strategy allows investigators to identify the gene(s) involved in these phenotypic changes. It is particularly promising for woody plants that have a long life cycle and for which molecular tools are limited. In this study, we constructed a large-insert binary bacterial artificial chromosome library of Populus euphratica, a stress-tolerant poplar species, which included 55,296 clones with average insert sizes of about 127 kb. To date, 1077 of the clones have been transformed into Arabidopsis thaliana via Agrobacterium by the floral dip method. Of these, 69 transgenic lines showed phenotypic changes represented by diverse aspects of plant form and development, 22 of which were reproducibly associated with the same phenotypic change. One of the clones conferring transgenic plants with increased salt tolerance, 002A1F06, was further analyzed and the 127,284 bp insert in this clone harbored eight genes that have been previously reported to be involved in stress resistance. This study demonstrates that transgenomics is useful in the study of functional genomics of woody plants and in the identification of novel gene(s) responsible for economically important traits. Thus, transgenomics can also be used for validation of quantitative trait loci mapped by molecular markers.

Marker-assisted introgression of resistance to fusarium wilt race 2 in Pusa 256, an elite cultivar of desi chickpea

Fusarium wilt caused by F. oxysporum f. sp. ciceris causes extensive damage to chickpea (Cicer arietinum L.) in many parts of the world. In the central part of India, pathogen race 2 (Foc 2) causes severe yield losses. We initiated molecular marker-assisted backcrossing (MABC) using desi cultivar, Vijay, as a donor to introgress resistance to this race (Foc2) in Pusa 256, another elite desi cultivar of chickpea. To confirm introgression of resistance for this race, foreground selection was undertaken using two SSR markers (TA 37 and TA110), with background selection to observe the recovery of recurrent parent genome using 45 SSRs accommodated in 8 multiplexes. F₁ plants were confirmed with molecular markers and backcrossed with Pusa 256, followed by cycles of foreground and background selection at each stage to generate 161 plants in BC₃F₂ during the period 2009-2013. Similarly, 46 BC₃F₁ plants were also generated in another set during the same period. On the basis of foreground selection, 46 plants were found homozygotes in BC₃F₂. Among them, 17 plants recorded >91% background recovery with the highest recovery percentage of 96%. In BC₃F₁ also, 14 hybrid plants recorded a background recovery of >85% with the highest background recovery percentage of >94%. The identified plants were selfed to obtain 1341 BC₃F₃ and 2198 BC₃F₂ seeds which were screened phenotypically for resistance to fusarium wilt (race 2) besides doing marker analysis. Finally, 17 BC₃F₄ and 11 BC₃F₃ lines were obtained which led to identification of 5 highly resistant lines of Pusa 256 with Foc 2 gene introgressed in them. Development of these lines will help in horizontal as well as vertical expansion of chickpea in central part of India.

Genetic variation of a global germplasm collection of chickpea (Cicer arietinum L.) including Italian accessions at risk of genetic erosion

Chickpea (Cicer arietinum L.) is one of the most important legumes worldwide. We addressed this study to the genetic characterization of a germplasm collection from main chickpea growing countries. Several Italian traditional landraces at risk of genetic erosion were included in the analysis. Twenty-two simple sequence repeat (SSR) markers, widely used to explore genetic variation in plants, were selected and yielded 218 different alleles. Structure analysis and hierarchical clustering indicated that a model with three distinct subpopulations best fits the data. The composition of two subpopulations, named K1 and K2, broadly reflects the commercial classification of chickpea in the two types desi and kabuli, respectively. The third subpopulation (K3) is composed by both desi and kabuli genotypes. Italian accessions group both in K2 and K3. Interestingly, this study highlights genetic distance between desi genotypes cultivated in Asia and Ethiopia, which respectively represent the chickpea primary and the secondary centres of diversity. Moreover, European desi are closer to the Ethiopian gene pool. Overall, this study will be of importance for chickpea conservation genetics and breeding, which is limited by the poor characterization of germplasm collection.

Molecular Mapping of Flowering Time Major Genes and QTLs in Chickpea (Cicer arietinum L.)

Flowering time is an important trait for adaptation and productivity of chickpea in the arid and the semi-arid environments. This study was conducted for molecular mapping of genes/quantitative trait loci (QTLs) controlling flowering time in chickpea using F₂ populations derived from four crosses (ICCV 96029 × CDC Frontier, ICC 5810 × CDC Frontier, BGD 132 × CDC Frontier and ICC 16641 × CDC Frontier). Genetic studies revealed monogenic control of flowering time in the crosses ICCV 96029 × CDC Frontier, BGD 132 × CDC Frontier and ICC 16641 × CDC Frontier, while digenic control with complementary gene action in ICC 5810 × CDC Frontier. The intraspecific genetic maps developed from these crosses consisted 75, 75, 68 and 67 markers spanning 248.8 cM, 331.4 cM, 311.1 cM and 385.1 cM, respectively. A consensus map spanning 363.8 cM with 109 loci was constructed by integrating four genetic maps. Major QTLs corresponding to flowering time genes efl-1 from ICCV 96029, efl-3 from BGD 132 and efl-4 from ICC 16641 were mapped on CaLG04, CaLG08 and CaLG06, respectively. The QTLs and linked markers identified in this study can be used in marker-assisted breeding for developing early maturing chickpea.

Development of gene-based markers for use in construction of the chickpea (Cicer arietinum L.) genetic linkage map and identification of QTLs associated with seed weight and plant height

Seed weight and plant height are important agronomic traits and contribute to seed yield. The objective of this study was to identify QTLs underlying these traits using an intra-specific mapping population of chickpea. A F11 population of 177 recombinant inbred lines derived from a cross between SBD377 (100-seed weight--48 g and plant height--53 cm) and BGD112 (100-seed weight--15 g and plant height--65 cm) was used. A total of 367 novel EST-derived functional markers were developed which included 187 EST-SSRs, 130 potential intron polymorphisms (PIPs) and 50 expressed sequence tag polymorphisms (ESTPs). Along with these, 590 previously published markers including 385 EST-based markers and 205 genomic SSRs were utilized. Of the 957 markers tested for analysis of parental polymorphism between the two parents of the mapping population, 135 (14.64%) were found to be polymorphic. Of these, 131 polymorphic markers could be mapped to the 8 linkage groups. The linkage map had a total length of 1140.54 cM with an average marker density of 8.7 cM. The map was further used for QTL identification using composite interval mapping method (CIM). Two QTLs each for seed weight, qSW-1 and qSW-2 (explaining 11.54 and 19.24% of phenotypic variance, respectively) and plant height, qPH-1 and qPH-2 (explaining 13.98 and 12.17% of phenotypic variance, respectively) were detected. The novel set of genic markers, the intra-specific linkage map and the QTLs identified in the present study will serve as valuable genomic resources in improving the chickpea seed yield using marker-assisted selection (MAS) strategies.

Characterization of genetic diversity in chickpea using SSR markers, Start Codon Targeted Polymorphism (SCoT) and Conserved DNA-Derived Polymorphism (CDDP)

To evaluate the genetic diversity among 48 genotypes of chickpea comprising cultivars, landraces and internationally developed improved lines genetic distances were evaluated using three different molecular marker techniques: Simple Sequence Repeat (SSR); Start Codon Targeted (SCoT) and Conserved DNA-derived Polymorphism (CDDP). Average polymorphism information content (PIC) for SSR, SCoT and CDDP markers was 0.47, 0.45 and 0.45, respectively, and this revealed that three different marker types were equal for the assessment of diversity amongst genotypes. Cluster analysis for SSR and SCoT divided the genotypes in to three distinct clusters and using CDDP markers data, genotypes grouped in to five clusters. There were positive significant correlation (r = 0.43, P < 0.01) between similarity matrix obtained by SCoT and CDDP. Three different marker techniques showed relatively same pattern of diversity across genotypes and using each marker technique it's obvious that diversity pattern and polymorphism for varieties were higher than that of genotypes, and CDDP had superiority over SCoT and SSR markers. These results suggest that efficiency of SSR, SCOT and CDDP markers was relatively the same in fingerprinting of chickpea genotypes. To our knowledge, this is the first detailed report of using targeted DNA region molecular marker (CDDP) for genetic diversity analysis in chickpea in comparison with SCoT and SSR markers. Overall, our results are able to prove the suitability of SCoT and CDDP markers for genetic diversity analysis in chickpea for their high rates of polymorphism and their potential for genome diversity and germplasm conservation.

Transformation of rice with large maize genomic DNA fragments containing high content repetitive sequences

Large and complex maize BIBAC inserts, even with a length of about 164 kb and repeat sequences of 88.1%, were transferred into rice. The BIBAC vector has been established to clone large DNA fragments and directly transfer them into plants. Previously, we have constructed a maize B73 BIBAC library and demonstrated that the BIBAC clones were stable in Agrobacterium. In this study, we demonstrated that the maize BIBAC clones could be used for rice genetic transformation through Agrobacterium-mediated method, although the average transformation efficiency for the BIBAC clones (0.86%) is much lower than that for generally used binary vectors containing small DNA fragments (15.24%). The 164-kb B73 genomic DNA insert of the BIBAC clone B2-6 containing five maize gene models and 88.1% of repetitive sequences was transferred into rice. In 18.75% (3/16) of the T1, 13.79% (4/29) of the T2, and 5.26% (1/19) of the T3 generation transgenic rice plants positive for the GUS and HYG marker genes, all the five maize genes can be detected. To our knowledge, this is the largest and highest content of repeat sequence-containing DNA fragment that was successfully transferred into plants. Gene expression analysis (RT-PCR) showed that the expression of three out of five genes could be detected in the leaves of the transgenic rice plants. Our study showed a potential to massively use maize genome resource for rice breeding by mass transformation of rice with large maize genomic DNA fragment BIBAC clones.

Genomics-assisted breeding for drought tolerance in chickpea

Terminal drought is one of the major constraints in chickpea (Cicer arietinum L.), causing more than 50% production losses. With the objective of accelerating genetic understanding and crop improvement through genomics-assisted breeding, a draft genome sequence has been assembled for the CDC Frontier variety. In this context, 544.73Mb of sequence data were assembled, capturing of 73.8% of the genome in scaffolds. In addition, large-scale genomic resources including several thousand simple sequence repeats and several million single nucleotide polymorphisms, high-density diversity array technology (15360 clones) and Illumina GoldenGate assay genotyping platforms, high-density genetic maps and transcriptome assemblies have been developed. In parallel, by using linkage mapping approach, one genomic region harbouring quantitative trait loci for several drought tolerance traits has been identified and successfully introgressed in three leading chickpea varieties (e.g. JG 11, Chefe, KAK 2) by using a marker-assisted backcrossing approach. A multilocation evaluation of these marker-assisted backcrossing lines provided several lines with 10-24% higher yield than the respective recurrent parents.Modern breeding approaches like marker-assisted recurrent selection and genomic selection are being deployed for enhancing drought tolerance in chickpea. Some novel mapping populations such as multiparent advanced generation intercross and nested association mapping populations are also being developed for trait mapping at higher resolution, as well as for enhancing the genetic base of chickpea. Such advances in genomics and genomics-assisted breeding will accelerate precision and efficiency in breeding for stress tolerance in chickpea.

CicArMiSatDB: the chickpea microsatellite database

Background

Chickpea (Cicer arietinum) is a widely grown legume crop in tropical, sub-tropical and temperate regions. Molecular breeding approaches seem to be essential for enhancing crop productivity in chickpea. Until recently, limited numbers of molecular markers were available in the case of chickpea for use in molecular breeding. However, the recent advances in genomics facilitated the development of large scale markers especially SSRs (simple sequence repeats), the markers of choice in any breeding program. Availability of genome sequence very recently opens new avenues for accelerating molecular breeding approaches for chickpea improvement.

Description

In order to assist genetic studies and breeding applications, we have developed a user friendly relational database named the Chickpea Microsatellite Database (CicArMiSatDB http://cicarmisatdb.icrisat.org). This database provides detailed information on SSRs along with their features in the genome. SSRs have been classified and made accessible through an easy-to-use web interface.

Conclusions

This database is expected to help chickpea community in particular and legume community in general, to select SSRs of particular type or from a specific region in the genome to advance both basic genomics research as well as applied aspects of crop improvement.

Integrated physical, genetic and genome map of chickpea (Cicer arietinum L.)

Physical map of chickpea was developed for the reference chickpea genotype (ICC 4958) using bacterial artificial chromosome (BAC) libraries targeting 71,094 clones (~12× coverage). High information content fingerprinting (HICF) of these clones gave high-quality fingerprinting data for 67,483 clones, and 1,174 contigs comprising 46,112 clones and 3,256 singletons were defined. In brief, 574 Mb genome size was assembled in 1,174 contigs with an average of 0.49 Mb per contig and 3,256 singletons represent 407 Mb genome. The physical map was linked with two genetic maps with the help of 245 BAC-end sequence (BES)-derived simple sequence repeat (SSR) markers. This allowed locating some of the BACs in the vicinity of some important quantitative trait loci (QTLs) for drought tolerance and reistance to Fusarium wilt and Ascochyta blight. In addition, fingerprinted contig (FPC) assembly was also integrated with the draft genome sequence of chickpea. As a result, ~965 BACs including 163 minimum tilling path (MTP) clones could be mapped on eight pseudo-molecules of chickpea forming 491 hypothetical contigs representing 54,013,992 bp (~54 Mb) of the draft genome. Comprehensive analysis of markers in abiotic and biotic stress tolerance QTL regions led to identification of 654, 306 and 23 genes in drought tolerance "QTL-hotspot" region, Ascochyta blight resistance QTL region and Fusarium wilt resistance QTL region, respectively. Integrated physical, genetic and genome map should provide a foundation for cloning and isolation of QTLs/genes for molecular dissection of traits as well as markers for molecular breeding for chickpea improvement.

Mapping and identification of a Cicer arietinum NSP2 gene involved in nodulation pathway

For the first time the putative NSP2 gene in chickpea has been identified using pairs of NILs differing for the Rn1 / rn1 nodulation gene that was located in LG5 of chickpea genetic map. An intraspecific cross between the mutant non-nodulating genotype PM233, carrying the recessive gene rn1, and the wild-type CA2139 was used to develop two pairs of near-isogenic lines (NILs) for nodulation in chickpea. These pairs of NILs were characterized using sequence tagged microsatellite site (STMS) markers distributed across different linkage groups (LGs) of the chickpea genetic map leading to the detection of polymorphic markers located in LG5. Using this information, together with the genome annotation in Medicago truncatula, a candidate gene (NSP2) known to be involved in nodulation pathway was selected for mapping in chickpea. The full length sequence obtained in chickpea wild-type (CaNSP2) was 1,503 bp. Linkage analysis in an F3 population of 118 plants derived from the cross between the pair of NILS NIL7-2A (nod) × NIL7-2B (non-nod) revealed a co-localization between CaNSP2 and Rn1 gene. These data implicate the CaNSP2 gene as a candidate for identity to Rn1, and suggest that it could act in the nodulation signaling transduction pathway similarly to that in other legumes species.

Genetic dissection of drought tolerance in chickpea (Cicer arietinum L.)

Analysis of phenotypic data for 20 drought tolerance traits in 1-7 seasons at 1-5 locations together with genetic mapping data for two mapping populations provided 9 QTL clusters of which one present on CaLG04 has a high potential to enhance drought tolerance in chickpea improvement. Chickpea (Cicer arietinum L.) is the second most important grain legume cultivated by resource poor farmers in the arid and semi-arid regions of the world. Drought is one of the major constraints leading up to 50% production losses in chickpea. In order to dissect the complex nature of drought tolerance and to use genomics tools for enhancing yield of chickpea under drought conditions, two mapping populations-ICCRIL03 (ICC 4958 × ICC 1882) and ICCRIL04 (ICC 283 × ICC 8261) segregating for drought tolerance-related root traits were phenotyped for a total of 20 drought component traits in 1-7 seasons at 1-5 locations in India. Individual genetic maps comprising 241 loci and 168 loci for ICCRIL03 and ICCRIL04, respectively, and a consensus genetic map comprising 352 loci were constructed ( http://cmap.icrisat.ac.in/cmap/sm/cp/varshney/). Analysis of extensive genotypic and precise phenotypic data revealed 45 robust main-effect QTLs (M-QTLs) explaining up to 58.20% phenotypic variation and 973 epistatic QTLs (E-QTLs) explaining up to 92.19% phenotypic variation for several target traits. Nine QTL clusters containing QTLs for several drought tolerance traits have been identified that can be targeted for molecular breeding. Among these clusters, one cluster harboring 48% robust M-QTLs for 12 traits and explaining about 58.20% phenotypic variation present on CaLG04 has been referred as "QTL-hotspot". This genomic region contains seven SSR markers (ICCM0249, NCPGR127, TAA170, NCPGR21, TR11, GA24 and STMS11). Introgression of this region into elite cultivars is expected to enhance drought tolerance in chickpea.

Multi-environment QTL analyses for drought-related traits in a recombinant inbred population of chickpea (Cicer arietinum L.)

A recombinant inbred line (RIL) population, comprising 181 lines derived from ILC588 × ILC3279, was evaluated in 10 environments across three locations with different moisture gradients. A drought resistance score (DRS) and three phenology traits-plant height (PLHT), days to flowering (DFLR), and days to maturity (MAT)-were recorded along with seven yield-related traits-grain yield (GY), biological yield (BY), harvest index (HI), the number of pods/3 plants (Pod), percentage of empty pods (%Epod), 100 seed weight (100 sw), and seed number/3 plants (SN). Two RILs (152, 162) showed the best GYs and DRSs under stressed and non-stressed environments. The quantitative trait loci (QTLs) analyses detected 93 significant QTLs (LOD ≥ 2.0) across the genome × environment interactions. The highest phenotypic variation (>24 %) was explained by the QTLDFLR in Terbol-11. Four common possible pleiotropic QTLs on LG3 and LG4 were identified as associated with DFLR, DRS, GY, MAT, HI, SN, and Pod. No significant epistatic interactions were found between these QTLs and the other markers. However, the QTL for DRS was detected as a conserved QTL in three late planting environments. The markers H6C-07 (on LG3) and H5G01 (on LG4) were associated with QTLs for many traits in all environments studied except two. The allele 'A' of marker H6C07 (from the tolerant parent ILC588) explained 80 % of the yield increase under late planting and 29.8 % of that under dry environments. Concentrating on LG3 and LG4 in molecular breeding programs for drought could speed up improvement for these traits.

Molecular mapping and characterization of genes governing time to flowering, seed weight, and plant height in an intraspecific genetic linkage map of chickpea (Cicer arietinum)

Drought is the major constraint to chickpea productivity worldwide. Utilizing early flowering genotypes and larger seed size have been suggested as strategies for breeding in drought zones. Therefore, this study aimed to identify potential markers linked to days-to-flowering, 100-seed weight, and plant height in a chickpea intraspecific F(2:3) population derived from the cross ILC3279 × ICCV2. A closely linked marker (TA117) on linkage group LG3 was identified for the days-to-flowering trait, explaining 33% of the variation. In relation to plant height, a quantitative trait loci (QTL) was located in LG3, close to the Ts5 marker, that explained 29% of phenotypic variation. A QTL for 100-seed weight located in LG4, close to TA176, explained 51% of variation. The identification of a locus linked both to high 100-seed weight and days-to-flowering may account for the correlation observed between these traits in this and other breeding attempts.

High-throughput SNP discovery and genotyping for constructing a saturated linkage map of chickpea (Cicer arietinum L.)

The present study reports the large-scale discovery of genome-wide single-nucleotide polymorphisms (SNPs) in chickpea, identified mainly through the next generation sequencing of two genotypes, i.e. Cicer arietinum ICC4958 and its wild progenitor C. reticulatum PI489777, parents of an inter-specific reference mapping population of chickpea. Development and validation of a high-throughput SNP genotyping assay based on Illumina's GoldenGate Genotyping Technology and its application in building a high-resolution genetic linkage map of chickpea is described for the first time. In this study, 1022 SNPs were identified, of which 768 high-confidence SNPs were selected for designing the custom Oligo Pool All (CpOPA-I) for genotyping. Of these, 697 SNPs could be successfully used for genotyping, demonstrating a high success rate of 90.75%. Genotyping data of the 697 SNPs were compiled along with those of 368 co-dominant markers mapped in an earlier study, and a saturated genetic linkage map of chickpea was constructed. One thousand and sixty-three markers were mapped onto eight linkage groups spanning 1808.7 cM (centiMorgans) with an average inter-marker distance of 1.70 cM, thereby representing one of the most advanced maps of chickpea. The map was used for the synteny analysis of chickpea, which revealed a higher degree of synteny with the phylogenetically close Medicago than with soybean. The first set of validated SNPs and map resources developed in this study will not only facilitate QTL mapping, genome-wide association analysis and comparative mapping in legumes but also help anchor scaffolds arising out of the whole-genome sequencing of chickpea.

Transcriptome sequencing of wild chickpea as a rich resource for marker development

The transcriptome of cultivated chickpea (Cicer arietinum L.), an important crop legume, has recently been sequenced. Here, we report sequencing of the transcriptome of wild chickpea, C. reticulatum (PI489777), the progenitor of cultivated chickpea, by GS-FLX 454 technology. The optimized assembly of C. reticulatum transcriptome generated 37 265 transcripts in total with an average length of 946 bp. A total of 4072 simple sequence repeats (SSRs) could be identified in these transcript sequences, of which at least 561 SSRs were polymorphic between C. arietinum and C. reticulatum. In addition, a total of 36 446 single-nucleotide polymorphisms (SNPs) were identified after optimization of probability score, quality score, read depth and consensus base ratio. Several of these SSRs and SNPs could be associated with tissue-specific and transcription factor encoding transcripts. A high proportion (92-94%) of polymorphic SSRs and SNPs identified between the two chickpea species were validated successfully. Further, the estimation of synonymous substitution rates of orthologous transcript pairs suggested that the speciation event for divergence of C. arietinum and C. reticulatum may have happened approximately 0.53 million years ago. The results of our study provide a rich resource for exploiting genetic variations in chickpea for breeding programmes.

Characterization and genetic analysis of an EIN4-like sequence (CaETR-1) located in QTL(AR1) implicated in ascochyta blight resistance in chickpea

Two alleles of a chickpea (Cicer arietinum L.) ethylene receptor-like sequence (CaETR-1) were sequence-characterized using synteny analysis with genome sequences of Medicago truncatula L. The full length of the sequence obtained in the accession FLIP84-92C resistant to ascochyta blight (CaETR-1a) span 4,428 bp, including the polyadenylation signal in the 3'-untranslated region (UTR), whereas it has a 730 bp deletion in the 3'-UTR region in the susceptible accession PI359075 (CaETR-1b). The deduced protein belongs to subfamily II of the ethylene receptors and contains all the domains that define EIN4 homologs in Arabidopsis. The EIN4-like sequence (CaETR-1) has been mapped using a recombinant inbred line (RIL) population derived from an intraspecific cross between ILC3279 and WR315, resistant and susceptible to blight, respectively. The locus was located in LGIVa of the genetic map, flanked by markers NCPGR91 and GAA47 (at distances of 11.3 and 17.9 cM, respectively). This is the first potentially functional sequence identified under a QTL peak for ascochyta blight resistance in chickpea (QTL(AR1)). This EIN4-like (CaETR-1) sequence explained up to 33.8% of the total phenotypic variation. This sequence could be directly related to blight resistance, together with other QTLs that have been found to be involved in resistance to this major chickpea disease.

EST-derived genic molecular markers: development and utilization for generating an advanced transcript map of chickpea

Well-saturated linkage maps especially those based on expressed sequence tag (EST)-derived genic molecular markers (GMMs) are a pre-requisite for molecular breeding. This is especially true in important legumes such as chickpea where few simple sequence repeats (SSR) and even fewer GMM-based maps have been developed. Therefore, in this study, 2,496 ESTs were generated from chickpea seeds and utilized for the development of 487 novel EST-derived functional markers which included 125 EST-SSRs, 151 intron targeted primers (ITPs), 109 expressed sequence tag polymorphisms (ESTPs), and 102 single nucleotide polymorphisms (SNPs). Whereas ESTSSRs, ITPs, and ESTPs were developed by in silico analysis of the developed EST sequences, SNPs were identified by allele resequencing and their genotyping was performedusing the Illumina GoldenGate Assay. Parental polymorphism was analyzed between C. arietinum ICC4958 and C. reticulatum PI489777, parents of the reference chickpea mapping population, using a total of 872 markers: 487 new gene-based markers developed in this study along with 385 previously published markers, of which 318 (36.5%) were found to be polymorphic and were used for genotyping. The genotypic data were integrated with the previously published data of 108 markers and an advanced linkage map was generated that contained 406 loci distributed on eight linkage groups that spanned 1,497.7 cM. The average marker density was 3.68 cM and the average number of markers per LG was 50.8. Among the mapped markers, 303 new genomic locations were defined that included 177 gene-based and 126 gSSRs (genomic SSRs) thereby producing the most advanced gene-rich map of chickpea solely based on co-dominant markers.

Sequence-based novel genomic microsatellite markers for robust genotyping purposes in foxtail millet [Setaria italica (L.) P. Beauv]

The unavailability of microsatellite markers and saturated genetic linkage map has restricted the genetic improvement of foxtail millet [Setaria italica (L.) P. Beauv.], despite the fact that in recent times it has been documented as a new model species for biofuel grasses. With the objective to generate a good number of microsatellite markers in foxtail millet cultivar 'Prasad', 690 clones were sequenced which generated 112.95 kb high quality sequences obtained from three genomic libraries each enriched with different microsatellite repeat motifs. Microsatellites were identified in 512 (74.2%) of the 690 positive clones and 172 primer pairs (pp) were successfully designed from 249 (48.6%) unique SSR-containing clones. The efficacies of the microsatellite containing genomic sequences were established by superior primer designing ability (69%), PCR amplification efficiency (85.5%) and polymorphic potential (52%) in the parents of F(2) mapping population. Out of 172 pp, functional 147 markers showed high level of cross-species amplification (~74%) in six grass species. Higher polymorphism rate and broad range of genetic diversity (0.30-0.69 averaging 0.58) obtained in constructed phylogenetic tree using 52 microsatellite markers, demonstrated the utility of markers in germplasm characterizations. In silico comparative mapping of 147 foxtail millet microsatellite containing sequences against the mapping data of sorghum (~18%), maize (~16%) and rice (~5%) indicated the presence of orthologous sequences of the foxtail millet in the respective species. The result thus demonstrates the applicability of microsatellite markers in various genotyping applications, determining phylogenetic relationships and comparative mapping in several important grass species.

Novel SSR markers from BAC-end sequences, DArT arrays and a comprehensive genetic map with 1,291 marker loci for chickpea (Cicer arietinum L.)

Chickpea (Cicer arietinum L.) is the third most important cool season food legume, cultivated in arid and semi-arid regions of the world. The goal of this study was to develop novel molecular markers such as microsatellite or simple sequence repeat (SSR) markers from bacterial artificial chromosome (BAC)-end sequences (BESs) and diversity arrays technology (DArT) markers, and to construct a high-density genetic map based on recombinant inbred line (RIL) population ICC 4958 (C. arietinum)×PI 489777 (C. reticulatum). A BAC-library comprising 55,680 clones was constructed and 46,270 BESs were generated. Mining of these BESs provided 6,845 SSRs, and primer pairs were designed for 1,344 SSRs. In parallel, DArT arrays with ca. 15,000 clones were developed, and 5,397 clones were found polymorphic among 94 genotypes tested. Screening of newly developed BES-SSR markers and DArT arrays on the parental genotypes of the RIL mapping population showed polymorphism with 253 BES-SSR markers and 675 DArT markers. Segregation data obtained for these polymorphic markers and 494 markers data compiled from published reports or collaborators were used for constructing the genetic map. As a result, a comprehensive genetic map comprising 1,291 markers on eight linkage groups (LGs) spanning a total of 845.56 cM distance was developed (http://cmap.icrisat.ac.in/cmap/sm/cp/thudi/). The number of markers per linkage group ranged from 68 (LG 8) to 218 (LG 3) with an average inter-marker distance of 0.65 cM. While the developed resource of molecular markers will be useful for genetic diversity, genetic mapping and molecular breeding applications, the comprehensive genetic map with integrated BES-SSR markers will facilitate its anchoring to the physical map (under construction) to accelerate map-based cloning of genes in chickpea and comparative genome evolution studies in legumes.

Development of genomic resources for the narrow-leafed lupin (Lupinus angustifolius): construction of a bacterial artificial chromosome (BAC) library and BAC-end sequencing

Background

Lupinus angustifolius L, also known as narrow-leafed lupin (NLL), is becoming an important grain legume crop that is valuable for sustainable farming and is becoming recognised as a potential human health food. Recent interest is being directed at NLL to improve grain production, disease and pest management and health benefits of the grain. However, studies have been hindered by a lack of extensive genomic resources for the species.

Results

A NLL BAC library was constructed consisting of 111,360 clones with an average insert size of 99.7 Kbp from cv Tanjil. The library has approximately 12 × genome coverage. Both ends of 9600 randomly selected BAC clones were sequenced to generate 13985 BAC end-sequences (BESs), covering approximately 1% of the NLL genome. These BESs permitted a preliminary characterisation of the NLL genome such as organisation and composition, with the BESs having approximately 39% G:C content, 16.6% repetitive DNA and 5.4% putative gene-encoding regions. From the BESs 9966 simple sequence repeat (SSR) motifs were identified and some of these are shown to be potential markers.

Conclusions

The NLL BAC library and BAC-end sequences are powerful resources for genetic and genomic research on lupin. These resources will provide a robust platform for future high-resolution mapping, map-based cloning, comparative genomics and assembly of whole-genome sequencing data for the species.

A pair of new BAC and BIBAC vectors that facilitate BAC/BIBAC library construction and intact large genomic DNA insert exchange

BACKGROUND

Large-insert BAC and BIBAC libraries are important tools for structural and functional genomics studies of eukaryotic genomes. To facilitate the construction of BAC and BIBAC libraries and the transfer of complete large BAC inserts into BIBAC vectors, which is desired in positional cloning, we developed a pair of new BAC and BIBAC vectors.

RESULTS

The new BAC vector pIndigoBAC536-S and the new BIBAC vector BIBAC-S have the following features: 1) both contain two 18-bp non-palindromic I-SceI sites in an inverted orientation at positions that flank an identical DNA fragment containing the lacZ selection marker and the cloning site. Large DNA inserts can be excised from the vectors as single fragments by cutting with I-SceI, allowing the inserts to be easily sized. More importantly, because the two vectors contain different antibiotic resistance genes for transformant selection and produce the same non-complementary 3' protruding ATAA ends by I-SceI that suppress self- and inter-ligations, the exchange of intact large genomic DNA inserts between the BAC and BIBAC vectors is straightforward; 2) both were constructed as high-copy composite vectors. Reliable linearized and dephosphorylated original low-copy pIndigoBAC536-S and BIBAC-S vectors that are ready for library construction can be prepared from the high-copy composite vectors pHZAUBAC1 and pHZAUBIBAC1, respectively, without the need for additional preparation steps or special reagents, thus simplifying the construction of BAC and BIBAC libraries. BIBAC clones constructed with the new BIBAC-S vector are stable in both E. coli and Agrobacterium. The vectors can be accessed through our website http://GResource.hzau.edu.cn.

CONCLUSIONS

The two new vectors and their respective high-copy composite vectors can largely facilitate the construction and characterization of BAC and BIBAC libraries. The transfer of complete large genomic DNA inserts from one vector to the other is made straightforward.

Large-scale transcriptome analysis in chickpea (Cicer arietinum L.), an orphan legume crop of the semi-arid tropics of Asia and Africa

Chickpea (Cicer arietinum L.) is an important legume crop in the semi-arid regions of Asia and Africa. Gains in crop productivity have been low however, particularly because of biotic and abiotic stresses. To help enhance crop productivity using molecular breeding techniques, next generation sequencing technologies such as Roche/454 and Illumina/Solexa were used to determine the sequence of most gene transcripts and to identify drought-responsive genes and gene-based molecular markers. A total of 103,215 tentative unique sequences (TUSs) have been produced from 435,018 Roche/454 reads and 21,491 Sanger expressed sequence tags (ESTs). Putative functions were determined for 49,437 (47.8%) of the TUSs, and gene ontology assignments were determined for 20,634 (41.7%) of the TUSs. Comparison of the chickpea TUSs with the Medicago truncatula genome assembly (Mt 3.5.1 build) resulted in 42,141 aligned TUSs with putative gene structures (including 39,281 predicted intron/splice junctions). Alignment of ∼37 million Illumina/Solexa tags generated from drought-challenged root tissues of two chickpea genotypes against the TUSs identified 44,639 differentially expressed TUSs. The TUSs were also used to identify a diverse set of markers, including 728 simple sequence repeats (SSRs), 495 single nucleotide polymorphisms (SNPs), 387 conserved orthologous sequence (COS) markers, and 2088 intron-spanning region (ISR) markers. This resource will be useful for basic and applied research for genome analysis and crop improvement in chickpea.

Gene discovery and tissue-specific transcriptome analysis in chickpea with massively parallel pyrosequencing and web resource development

Chickpea (Cicer arietinum) is an important food legume crop but lags in the availability of genomic resources. In this study, we have generated about 2 million high-quality sequences of average length of 372 bp using pyrosequencing technology. The optimization of de novo assembly clearly indicated that hybrid assembly of long-read and short-read primary assemblies gave better results. The hybrid assembly generated a set of 34,760 transcripts with an average length of 1,020 bp representing about 4.8% (35.5 Mb) of the total chickpea genome. We identified more than 4,000 simple sequence repeats, which can be developed as functional molecular markers in chickpea. Putative function and Gene Ontology terms were assigned to at least 73.2% and 71.0% of chickpea transcripts, respectively. We have also identified several chickpea transcripts that showed tissue-specific expression and validated the results using real-time polymerase chain reaction analysis. Based on sequence comparison with other species within the plant kingdom, we identified two sets of lineage-specific genes, including those conserved in the Fabaceae family (legume specific) and those lacking significant similarity with any non chickpea species (chickpea specific). Finally, we have developed a Web resource, Chickpea Transcriptome Database, which provides public access to the data and results reported in this study. The strategy for optimization of de novo assembly presented here may further facilitate the transcriptome sequencing and characterization in other organisms. Most importantly, the data and results reported in this study will help to accelerate research in various areas of genomics and implementing breeding programs in chickpea.

Bacterial artificial chromosome libraries of pulse crops: characteristics and applications

Pulse crops are considered minor on a global scale despite their nutritional value for human consumption. Therefore, they are relatively less extensively studied in comparison with the major crops. The need to improve pulse crop production and quality will increase with the increasing global demand for food security and people's awareness of nutritious food. The improvement of pulse crops will require fully utilizing all their genetic resources. Bacterial artificial chromosome (BAC) libraries of pulse crops are essential genomic resources that have the potential to accelerate gene discovery and enhance molecular breeding in these crops. Here, we review the availability, characteristics, applications, and potential applications of the BAC libraries of pulse crops.

Characterization of a plant-transformation-ready large-insert BIBAC library of Arabidopsis and bombardment transformation of a large-insert BIBAC of the library into tobacco

Plant-transformation-ready, large-insert binary bacterial artificial chromosome (BIBAC) libraries are of significance for functional and network analysis of large genomic regions, gene clusters, large-spanning genes, and complex loci in the post-genome era. Here, we report the characterization of a plant-transformation-ready BIBAC library of the sequenced Arabidopsis genome for which such a library is not available to the public, the transformation of a large-insert BIBAC of the library into tobacco by biolistic bombardment, and the expression analysis of its containing genes in transgenic plants. The BIBAC library was constructed from nuclear DNA partially digested with BamHI in the BIBAC vector pCLD04541. It contains 6144 clones and has a mean insert size of 108 kb, representing 5.2× equivalents of the Arabidopsis genome or a probability of greater than 99% of obtaining at least one positive clone from the library using a single-copy sequence as a probe. The transformation of the large-insert BIBAC and analyses of the transgenic plants showed that not only did transgenic plants have intact BIBAC DNA, but also could the BIBAC be transmitted stably into progenies and its containing genes be expressed actively. These results suggest that the large-insert BIBAC library, combined with the biolistic bombardment transformation method, could provide a useful tool for large-scale functional analysis of the Arabidopsis genome sequence and applications in plant-molecular breeding.

Development and use of genic molecular markers (GMMs) for construction of a transcript map of chickpea (Cicer arietinum L.)

A transcript map has been constructed by the development and integration of genic molecular markers (GMMs) including single nucleotide polymorphism (SNP), genic microsatellite or simple sequence repeat (SSR) and intron spanning region (ISR)-based markers, on an inter-specific mapping population of chickpea, the third food legume crop of the world and the first food legume crop of India. For SNP discovery through allele re-sequencing, primer pairs were designed for 688 genes/expressed sequence tags (ESTs) of chickpea and 657 genes/ESTs of closely related species of chickpea. High-quality sequence data obtained for 220 candidate genic regions on 2-20 genotypes representing 9 Cicer species provided 1,893 SNPs with an average frequency of 1/35.83 bp and 0.34 PIC (polymorphism information content) value. On an average 2.9 haplotypes were present in 220 candidate genic regions with an average haplotype diversity of 0.6326. SNP2CAPS analysis of 220 sequence alignments, as mentioned above, provided a total of 192 CAPS candidates. Experimental analysis of these 192 CAPS candidates together with 87 CAPS candidates identified earlier through in silico mining of ESTs provided scorable amplification in 173 (62.01%) cases of which predicted assays were validated in 143 (82.66%) cases (CGMM). Alignments of chickpea unigenes with Medicago truncatula genome were used to develop 121 intron spanning region (CISR) markers of which 87 yielded scorable products. In addition, optimization of 77 EST-derived SSR (ICCeM) markers provided 51 scorable markers. Screening of easily assayable 281 markers including 143 CGMMs, 87 CISRs and 51 ICCeMs on 5 parental genotypes of three mapping populations identified 104 polymorphic markers including 90 markers on the inter-specific mapping population. Sixty-two of these GMMs together with 218 earlier published markers (including 64 GMM loci) and 20 other unpublished markers could be integrated into this genetic map. A genetic map developed here, therefore, has a total of 300 loci including 126 GMM loci and spans 766.56 cM, with an average inter-marker distance of 2.55 cM. In summary, this is the first report on the development of large-scale genic markers including development of easily assayable markers and a transcript map of chickpea. These resources should be useful not only for genome analysis and genetics and breeding applications of chickpea, but also for comparative legume genomics.

Assembling a puzzle of dispersed retrotransposable sequences in the genome of chickpea (Cicer arietinum L.)

Several repetitive elements are known to be present in the genome of chickpea (Cicer arietinum L.) including satellite DNA and En/Spm transposons as well as two dispersed, highly repetitive elements, CaRep1 and CaRep2. PCR was used to prove that CaRep1, CaRep2, and previously isolated CaRep3 of C. arietinum represent different segments of a highly repetitive Ty3-gypsy-like retrotransposon (Metaviridae) designated CaRep that makes up large parts of the intercalary heterochromatin. The full sequence of this element including the LTRs and untranslated internal regions was isolated by selective amplification. The restriction pattern of CaRep was different within the annual species of the genus Cicer, suggesting its rearrangement during the evolution of the genus during the last 100 000 years. In addition to CaRep, another LTR and a non-LTR retrotransposon family were isolated, and their restriction patterns and physical localization in the chickpea genome were characterized. The LINE-like element CaLin is only of comparatively low abundance and reveals a considerable heterogeneity. The Ty1-copia-like element (Pseudoviridae) CaTy is located in the distal parts of the intercalary heterochromatin and adjacent euchromatic regions, but it is absent from the centromeric regions. These results together with earlier findings allow to depict the distribution of retroelements on chickpea chromosomes, which extensively resembles the retroelement landscape of the genome of the model legume Medicago truncatula Gaertn.

Advancing the STMS genomic resources for defining new locations on the intraspecific genetic linkage map of chickpea (Cicer arietinum L.)

Background

Chickpea (Cicer arietinum L.) is an economically important cool season grain legume crop that is valued for its nutritive seeds having high protein content. However, several biotic and abiotic stresses and the low genetic variability in the chickpea genome have continuously hindered the chickpea molecular breeding programs. STMS (Sequence Tagged Microsatellite Sites) markers which are preferred for the construction of saturated linkage maps in several crop species, have also emerged as the most efficient and reliable source for detecting allelic diversity in chickpea. However, the number of STMS markers reported in chickpea is still limited and moreover exhibit low rates of both inter and intraspecific polymorphism, thereby limiting the positions of the SSR markers especially on the intraspecific linkage maps of chickpea. Hence, this study was undertaken with the aim of developing additional STMS markers and utilizing them for advancing the genetic linkage map of chickpea which would have applications in QTL identification, MAS and for de novo assembly of high throughput whole genome sequence data.

Results

A microsatellite enriched library of chickpea (enriched for (GT/CA)_n and (GA/CT)_n repeats) was constructed from which 387 putative microsatellite containing clones were identified. From these, 254 STMS primers were designed of which 181 were developed as functional markers. An intraspecific mapping population of chickpea, [ICCV-2 (single podded) × JG-62 (double podded)] and comprising of 126 RILs, was genotyped for mapping. Of the 522 chickpea STMS markers (including the double-podding trait, screened for parental polymorphism, 226 (43.3%) were polymorphic in the parents and were used to genotype the RILs. At a LOD score of 3.5, eight linkage groups defining the position of 138 markers were obtained that spanned 630.9 cM with an average marker density of 4.57 cM. Further, based on the common loci present between the current map and the previously published chickpea intraspecific map, integration of maps was performed which revealed improvement of marker density and saturation of the region in the vicinity of sfl (double-podding) gene thereby bringing about an advancement of the current map.

Conclusion

An arsenal of 181 new chickpea STMS markers was reported. The developed intraspecific linkage map defined map positions of 138 markers which included 101 new locations.Map integration with a previously published map was carried out which revealed an advanced map with improved density. This study is a major contribution towards providing advanced genomic resources which will facilitate chickpea geneticists and molecular breeders in developing superior genotypes with improved traits.

A BAC/BIBAC-based physical map of chickpea, Cicer arietinum L

Background

Chickpea (Cicer arietinum L.) is the third most important pulse crop worldwide. Despite its importance, relatively little is known about its genome. The availability of a genome-wide physical map allows rapid fine mapping of QTL, development of high-density genome maps, and sequencing of the entire genome. However, no such a physical map has been developed in chickpea.

Results

We present a genome-wide, BAC/BIBAC-based physical map of chickpea developed by fingerprint analysis. Four chickpea BAC and BIBAC libraries, two of which were constructed in this study, were used. A total of 67,584 clones were fingerprinted, and 64,211 (~11.7 ×) of the fingerprints validated and used in the physical map assembly. The physical map consists of 1,945 BAC/BIBAC contigs, with each containing an average of 28.3 clones and having an average physical length of 559 kb. The contigs collectively span approximately 1,088 Mb. By using the physical map, we identified the BAC/BIBAC contigs containing or closely linked to QTL4.1 for resistance to Didymella rabiei (RDR) and QTL8 for days to first flower (DTF), thus further verifying the physical map and confirming its utility in fine mapping and cloning of QTL.

Conclusion

The physical map represents the first genome-wide, BAC/BIBAC-based physical map of chickpea. This map, along with other genomic resources previously developed in the species and the genome sequences of related species (soybean, Medicago and Lotus), will provide a foundation necessary for many areas of advanced genomics research in chickpea and other legume species. The inclusion of transformation-ready BIBACs in the map greatly facilitates its utility in functional analysis of the legume genomes.

Integration of novel SSR and gene-based SNP marker loci in the chickpea genetic map and establishment of new anchor points with Medicago truncatula genome

This study presents the development and mapping of simple sequence repeat (SSR) and single nucleotide polymorphism (SNP) markers in chickpea. The mapping population is based on an inter-specific cross between domesticated and non-domesticated genotypes of chickpea (Cicer arietinum ICC 4958 x C. reticulatum PI 489777). This same population has been the focus of previous studies, permitting integration of new and legacy genetic markers into a single genetic map. We report a set of 311 novel SSR markers (designated ICCM-ICRISAT chickpea microsatellite), obtained from an SSR-enriched genomic library of ICC 4958. Screening of these SSR markers on a diverse panel of 48 chickpea accessions provided 147 polymorphic markers with 2-21 alleles and polymorphic information content value 0.04-0.92. Fifty-two of these markers were polymorphic between parental genotypes of the inter-specific population. We also analyzed 233 previously published (H-series) SSR markers that provided another set of 52 polymorphic markers. An additional 71 gene-based SNP markers were developed from transcript sequences that are highly conserved between chickpea and its near relative Medicago truncatula. By using these three approaches, 175 new marker loci along with 407 previously reported marker loci were integrated to yield an improved genetic map of chickpea. The integrated map contains 521 loci organized into eight linkage groups that span 2,602 cM, with an average inter-marker distance of 4.99 cM. Gene-based markers provide anchor points for comparing the genomes of Medicago and chickpea, and reveal extended synteny between these two species. The combined set of genetic markers and their integration into an improved genetic map should facilitate chickpea genetics and breeding, as well as translational studies between chickpea and Medicago.

A large insert Thellungiella halophila BIBAC library for genomics and identification of stress tolerance genes

Salt cress (Thellungiella halophila), a salt-tolerant relative of Arabidopsis, has turned to be an important model plant for studying abiotic stress tolerance. One binary bacterial artificial chromosome (BIBAC) library was constructed which represents the first plant-transformation-competent large-insert DNA library generated for Thellungiella halophila. The BIBAC library was constructed in BamHI site of binary vector pBIBAC2 by ligation of partial digested nuclear DNA of Thellungiella halophila. This library consists of 23,040 clones with an average insert size of 75 kb, and covers 4x Thellungiella halophila haploid genomes. BIBAC clones which contain inserts over 50 kb were selected and transformed into Arabidopsis for salt tolerant plant screening. One transgenic line was found to be more salt tolerant than wild type plants from the screen of 200 lines. It was demonstrated that the library contains candidates of stress tolerance genes and the approach is suitable for the transformation of stress susceptible plants for genetic improvement.

A comprehensive resource of drought- and salinity- responsive ESTs for gene discovery and marker development in chickpea (Cicer arietinum L.)

BACKGROUND

Chickpea (Cicer arietinum L.), an important grain legume crop of the world is seriously challenged by terminal drought and salinity stresses. However, very limited number of molecular markers and candidate genes are available for undertaking molecular breeding in chickpea to tackle these stresses. This study reports generation and analysis of comprehensive resource of drought- and salinity-responsive expressed sequence tags (ESTs) and gene-based markers.

RESULTS

A total of 20,162 (18,435 high quality) drought- and salinity- responsive ESTs were generated from ten different root tissue cDNA libraries of chickpea. Sequence editing, clustering and assembly analysis resulted in 6,404 unigenes (1,590 contigs and 4,814 singletons). Functional annotation of unigenes based on BLASTX analysis showed that 46.3% (2,965) had significant similarity (< or =1E-05) to sequences in the non-redundant UniProt database. BLASTN analysis of unique sequences with ESTs of four legume species (Medicago, Lotus, soybean and groundnut) and three model plant species (rice, Arabidopsis and poplar) provided insights on conserved genes across legumes as well as novel transcripts for chickpea. Of 2,965 (46.3%) significant unigenes, only 2,071 (32.3%) unigenes could be functionally categorised according to Gene Ontology (GO) descriptions. A total of 2,029 sequences containing 3,728 simple sequence repeats (SSRs) were identified and 177 new EST-SSR markers were developed. Experimental validation of a set of 77 SSR markers on 24 genotypes revealed 230 alleles with an average of 4.6 alleles per marker and average polymorphism information content (PIC) value of 0.43. Besides SSR markers, 21,405 high confidence single nucleotide polymorphisms (SNPs) in 742 contigs (with > or = 5 ESTs) were also identified. Recognition sites for restriction enzymes were identified for 7,884 SNPs in 240 contigs. Hierarchical clustering of 105 selected contigs provided clues about stress- responsive candidate genes and their expression profile showed predominance in specific stress-challenged libraries.

CONCLUSION

Generated set of chickpea ESTs serves as a resource of high quality transcripts for gene discovery and development of functional markers associated with abiotic stress tolerance that will be helpful to facilitate chickpea breeding. Mapping of gene-based markers in chickpea will also add more anchoring points to align genomes of chickpea and other legume species.

Development of chickpea EST-SSR markers and analysis of allelic variation across related species

Despite chickpea being the third important grain legume, there is a limited availability of genomic resources, especially of the expressed sequence tag (EST)-based markers. In this study, we generated 822 chickpea ESTs from immature seeds as well as exploited 1,309 ESTs from the chickpea database, thus utilizing a total of 2,131 EST sequences for development of functional EST-SSR markers. Two hundred and forty-six simple sequence repeat (SSR) motifs were identified from which 183 primer pairs were designed and 60 validated as functional markers. Genetic diversity analysis across 30 chickpea accessions revealed ten markers to be polymorphic producing a total of 29 alleles and an observed heterozygosity average of 0.16 thereby exhibiting low levels of intra-specific polymorphism. However, the markers exhibited high cross-species transferability ranging from 68.3 to 96.6% across the six annual Cicer species and from 29.4 to 61.7% across the seven legume genera. Sequence analysis of size variant amplicons from various species revealed that size polymorphism was due to multiple events such as copy number variation, point mutations and insertions/deletions in the microsatellite repeat as well as in the flanking regions. Interestingly, a wide prevalence of crossability-group-specific sequence variations were observed among Cicer species that were phylogenetically informative. The neighbor joining dendrogram clearly separated the chickpea cultivars from the wild Cicer and validated the proximity of C. judaicum with C. pinnatifidum. Hence, this study for the first time provides an insight into the distribution of SSRs in the chickpea transcribed regions and also demonstrates the development and utilization of genic-SSRs. In addition to proving their suitability for genetic diversity analysis, their high rates of transferability also proved their potential for comparative genomic studies and for following gene introgressions and evolution in wild species, which constitute the valuable secondary genepool in chickpea.

Analysis of genome organization, composition and microsynteny using 500 kb BAC sequences in chickpea

The small genome size (740 Mb), short life cycle (3 months) and high economic importance as a food crop legume make chickpea (Cicer arietinum L.) an important system for genomics research. Although several genetic linkage maps using various markers and genomic tools have become available, sequencing efforts and their use are limited in chickpea genomic research. In this study, we explored the genome organization of chickpea by sequencing approximately 500 kb from 11 BAC clones (three representing ascochyta blight resistance QTL1 (ABR-QTL1) and eight randomly selected BAC clones). Our analysis revealed that these sequenced chickpea genomic regions have a gene density of one per 9.2 kb, an average gene length of 2,500 bp, an average of 4.7 exons per gene, with an average exon and intron size of 401 and 316 bp, respectively, and approximately 8.6% repetitive elements. Other features analyzed included exon and intron length, number of exons per gene, protein length and %GC content. Although there are reports on high synteny among legume genomes, the microsynteny between the 500 kb chickpea and available Medicago truncatula genomic sequences varied depending on the region analyzed. The GBrowse-based annotation of these BACs is available at http://www.genome.ou.edu/plants_totals.html . We believe that our work provides significant information that supports a chickpea genome sequencing effort in the future.

Functional genomics in chickpea: an emerging frontier for molecular-assisted breeding

Chickpea is a valuable and important agricultural crop, but yield potential is limited by a series of biotic and abiotic stresses, including Ascochyta blight, Fusarium wilt, drought, cold and salinity. To accelerate molecular breeding efforts for the discovery and introgression of stress tolerance genes into cultivated chickpea, functional genomics approaches are rapidly growing. Recently a series of genetic tools for chickpea have become available that have allowed high-powered functional genomics studies to proceed, including a dense genetic map, large insert genome libraries, expressed sequence tag libraries, microarrays, serial analysis of gene expression, transgenics and reverse genetics. This review summarises the development of these genomic tools and the achievements made in initial and emerging functional genomics studies. Much of the initial research focused on Ascochyta blight resistance, and a resistance model has been synthesised based on the results of various studies. Use of the rich comparative genomics resources from the model legumes Medicago truncatula and Lotus japonicus is also discussed. Finally, perspectives on the future directions for chickpea functional genomics, with the goal of developing elite chickpea cultivars, are discussed.

Genetic mapping of ascochyta blight resistance in chickpea (Cicer arietinum L.) using a simple sequence repeat linkage map

Ascochyta blight, caused by the fungus Ascochyta rabiei (Pass.) Lab., is one of the most devastating diseases of chickpea (Cicer arietinum L.) worldwide. Research was conducted to map genetic factors for resistance to ascochyta blight using a linkage map constructed with 144 simple sequence repeat markers and 1 morphological marker (fc, flower colour). Stem cutting was used to vegetatively propagate 186 F2 plants derived from a cross between Cicer arietinum L. 'ICCV96029' and 'CDC Frontier'. A total of 556 cutting-derived plants were evaluated for their reaction to ascochyta blight under controlled conditions. Disease reaction of the F1 and F2 plants demonstrated that the resistance was dominantly inherited. A Fain's test based on the means and variances of the ascochyta blight reaction of the F3 families showed that a few genes were segregating in the population. Composite interval mapping identified 3 genomic regions that were associated with the reaction to ascochyta blight. One quantitative trait locus (QTL) on each of LG3, LG4, and LG6 accounted for 13%, 29%, and 12%, respectively, of the total estimated phenotypic variation for the reaction to ascochyta blight. Together, these loci controlled 56% of the total estimated phenotypic variation. The QTL on LG4 and LG6 were in common with the previously reported QTL for ascochyta blight resistance, whereas the QTL on LG3 was unique to the current population.

Development of an integrated intraspecific map of chickpea (Cicer arietinum L.) using two recombinant inbred line populations

A composite intraspecific linkage map of chickpea was developed by integrating individual maps developed from two F(8:9) RIL populations with one common parent. Different molecular markers viz. RAPD, ISSR, RGA, SSR and ASAP were analyzed along with three yield related traits: double podding, seeds per pod and seed weight. A total of 273 markers and 186 RILs were used to generate the map with eight linkage groups at a LOD score of >/=3.0 and maximum recombination fraction of 0.4. The map spanned 739.6 cM with 230 markers at an average distance of 3.2 cM between markers. The predominantly used SSR markers facilitated identification of homologous linkage groups from the previously published interspecific linkage map of chickpea and confirmed conservation of the SSR markers across the two maps as well as the variation in terms of marker distance and order. The double podding gene was tagged by the markers NCPGR33 and UBC249z at 2.0 and 1.1 cM, respectively. Whereas, seeds per pod, was tagged by the markers TA2x and UBC465 at 0.1 and 1.8 cM, respectively. Eight QTLs were identified that influence seed weight. The joint map approach allowed mapping a large number of markers with a moderate coverage of the chickpea genome and few linkage gaps.

Construction and characterization of the first bacterial artificial chromosome library for the cotton species Gossypium barbadense L

As the second most widely cultivated cotton, Gossypium barbadense is well known for its superior fiber properties and its high levels of resistance to Fusarium and Verticillium wilts. To enhance our ability to exploit these properties in breeding programs, we constructed the first bacterial artificial chromosome (BAC) library for this species. The library contains 167 424 clones (49 920 BamHI and 117 504 HindIII clones), with an estimated average insert size of 130 kb. About 94.0% of the clones had inserts over 100 kb, and the empty clones accounted for less than 4.0%. Contamination of the library with chloroplast clones was very low (0.2%). Screening the library with locus-specific probes showed that BAC clones represent 6.5-fold genome equivalents. This high-quality library provides an additional asset with which to exploit genetic variation for cotton improvement.

Mapping quantitative trait loci in chickpea associated with time to flowering and resistance to Didymella rabiei the causal agent of Ascochyta blight

Drought is the major constraint to chickpea (Cicer arietinum L.) productivity worldwide. Utilizing early-flowering genotypes and advancing sowing from spring to autumn have been suggested as strategies for drought avoidance. However, Ascochyta blight (causal agent: Didymella rabiei (Kov.) v. Arx.) is a major limitation for chickpea winter cultivation. Most efforts to introgress resistance to the pathogen into Kabuli germplasm resulted in relatively late flowering germplasm. With the aim to explore the feasibility of combining earliness and resistance, RILs derived from a cross between a Kabuli cultivar and a Desi accession were evaluated under field conditions and genotyped with SSR markers. Three quantitative trait loci (QTLs) with significant effects on resistance were identified: two linked loci located on LG4 in epistatic interaction and a third locus on LG8. Two QTLs were detected for time to flowering: one in LG1 and another on LG2. When resistance and time to flowering were analyzed together, the significance of the resistance estimates obtained for the LG8 locus increased and the locus effect on days to flowering, previously undetected, was significantly different from zero. The identification of a locus linked both to resistance and time to flowering may account for the correlation observed between these traits in this and other breeding attempts.