A molecular linkage map of cultivated oat

Breeding oat for resistance to the crown rust pathogen Puccinia coronata f. sp. avenae: achievements and prospects

Crown rust, caused by Puccinia coronata f. sp. avenae (Pca), is a significant impediment to global oat production. Some 98 alleles at 92 loci conferring resistance to Pca in Avena have been designated; however, allelic relationships and chromosomal locations of many of these are unknown. Long-term monitoring of Pca in Australia, North America and elsewhere has shown that it is highly variable even in the absence of sexual recombination, likely due to large pathogen populations that cycle between wild oat communities and oat crops. Efforts to develop cultivars with genetic resistance to Pca began in the 1950s. Based almost solely on all all-stage resistance, this has had temporary benefits but very limited success. The inability to eradicate wild oats, and their common occurrence in many oat growing regions, means that future strategies to control Pca must be based on the assumption of a large and variable prevailing pathogen population with high evolutionary potential, even if cultivars with durable resistance are deployed and grown widely. The presence of minor gene, additive APR to Pca in hexaploid oat germplasm opens the possibility of pyramiding several such genes to give high levels of resistance. The recent availability of reference genomes for diploid and hexaploid oat will undoubtedly accelerate efforts to discover, characterise and develop high throughput diagnostic markers to introgress and pyramid resistance to Pca in high yielding adapted oat germplasm.

Phylogenetic analysis of the genus Avena based on chloroplast intergenic spacer psbA-trnH and single-copy nuclear gene Acc1

Two uncorrelated nucleotide sequences, chloroplast intergenic spacer psbA-trnH and acetyl CoA carboxylase gene (Acc1), were used to perform phylogenetic analyses in 75 accessions of the genus Avena, representing 13 diploids, seven tetraploid, and four hexaploids by maximum parsimony and Bayesian inference. Phylogenic analyses based on the chloroplast intergenic spacer psbA-trnH confirmed that the A genome diploid might be the maternal donor of species of the genus Avena. Two haplotypes of the Acc1 gene region were obtained from the AB genome tetraploids, indicating an allopolyploid origin for the tetraploid species. Among the AB genome species, both gene trees revealed differences between Avena agadiriana and the other species, suggesting that an AS genome diploid might be the A genome donor and the other genome diploid donor might be the Ac genome diploid Avena canariensis or the Ad genome diploid Avena damascena. Three haplotypes of the Acc1 gene have been detected among the ACD genome hexaploid species. The haplotype that seems to represent the D genome clustered with the tetraploid species Avena murphyi and Avena maroccana, which supported the CD genomic designation instead of AC for A. murphyi and A. maroccana.

Using genotyping-by-sequencing (GBS) for genomic discovery in cultivated oat

Advances in next-generation sequencing offer high-throughput and cost-effective genotyping alternatives, including genotyping-by-sequencing (GBS). Results have shown that this methodology is efficient for genotyping a variety of species, including those with complex genomes. To assess the utility of GBS in cultivated hexaploid oat (Avena sativa L.), seven bi-parental mapping populations and diverse inbred lines from breeding programs around the world were studied. We examined technical factors that influence GBS SNP calls, established a workflow that combines two bioinformatics pipelines for GBS SNP calling, and provided a nomenclature for oat GBS loci. The high-throughput GBS system enabled us to place 45,117 loci on an oat consensus map, thus establishing a positional reference for further genomic studies. Using the diversity lines, we estimated that a minimum density of one marker per 2 to 2.8 cM would be required for genome-wide association studies (GWAS), and GBS markers met this density requirement in most chromosome regions. We also demonstrated the utility of GBS in additional diagnostic applications related to oat breeding. We conclude that GBS is a powerful and useful approach, which will have many additional applications in oat breeding and genomic studies.

Linkage mapping and identification of QTL affecting deoxynivalenol (DON) content (Fusarium resistance) in oats (Avena sativa L.)

Mycotoxins caused by Fusarium spp. is a major concern on food and feed safety in oats, although Fusarium head blight (FHB) is often less apparent than in other small grain cereals. Breeding resistant cultivars is an economic and environment-friendly way to reduce toxin content, either by the identification of resistance QTL or phenotypic evaluation. Both are little explored in oats. A recombinant-inbred line population, Hurdal × Z595-7 (HZ595, with 184 lines), was used for QTL mapping and was phenotyped for 3 years. Spawn inoculation was applied and deoxynivalenol (DON) content, FHB severity, days to heading and maturity (DH and DM), and plant height (PH) were measured. The population was genotyped with DArTs, AFLPs, SSRs and selected SNPs, and a linkage map of 1,132 cM was constructed, covering all 21 oat chromosomes. A QTL for DON on chromosome 17A/7C, tentatively designated as Qdon.umb-17A/7C, was detected in all experiments using composite interval mapping, with phenotypic effects of 12.2–26.6 %. In addition, QTL for DON were also found on chromosomes 5C, 9D, 13A, 14D and unknown_3, while a QTL for FHB was found on 11A. Several of the DON/FHB QTL coincided with those for DH, DM and/or PH. A half-sib population of HZ595, Hurdal × Z615-4 (HZ615, with 91 lines), was phenotyped in 2011 for validation of QTL found in HZ595, and Qdon.umb-17A/7C was again localized with a phenotypic effect of 12.4 %. Three SNPs closely linked to Qdon.umb-17A/7C were identified in both populations, and one each for QTL on 5C, 11A and 13A were identified in HZ595. These SNPs, together with those yet to be identified, could be useful in marker-assisted selection to pyramiding resistance QTL.

SNP discovery and chromosome anchoring provide the first physically-anchored hexaploid oat map and reveal synteny with model species

A physically anchored consensus map is foundational to modern genomics research; however, construction of such a map in oat (Avena sativa L., 2n = 6x = 42) has been hindered by the size and complexity of the genome, the scarcity of robust molecular markers, and the lack of aneuploid stocks. Resources developed in this study include a modified SNP discovery method for complex genomes, a diverse set of oat SNP markers, and a novel chromosome-deficient SNP anchoring strategy. These resources were applied to build the first complete, physically-anchored consensus map of hexaploid oat. Approximately 11,000 high-confidence in silico SNPs were discovered based on nine million inter-varietal sequence reads of genomic and cDNA origin. GoldenGate genotyping of 3,072 SNP assays yielded 1,311 robust markers, of which 985 were mapped in 390 recombinant-inbred lines from six bi-parental mapping populations ranging in size from 49 to 97 progeny. The consensus map included 985 SNPs and 68 previously-published markers, resolving 21 linkage groups with a total map distance of 1,838.8 cM. Consensus linkage groups were assigned to 21 chromosomes using SNP deletion analysis of chromosome-deficient monosomic hybrid stocks. Alignments with sequenced genomes of rice and Brachypodium provide evidence for extensive conservation of genomic regions, and renewed encouragement for orthology-based genomic discovery in this important hexaploid species. These results also provide a framework for high-resolution genetic analysis in oat, and a model for marker development and map construction in other species with complex genomes and limited resources.

Identification of RFLP and NBS/PK profiling markers for disease resistance loci in genetic maps of oats

Two of the domains most widely shared among R genes are the nucleotide binding site (NBS) and protein kinase (PK) domains. The present study describes and maps a number of new oat resistance gene analogues (RGAs) with two purposes in mind: (1) to identify genetic regions that contain R genes and (2) to determine whether RGAs can be used as molecular markers for qualitative loci and for QTLs affording resistance to Puccinia coronata. Such genes have been mapped in the diploid A. strigosa × A. wiestii (Asw map) and the hexaploid MN841801-1 × Noble-2 (MN map). Genomic and cDNA NBS-RGA probes from oat, barley and wheat were used to produce RFLPs and to obtain markers by motif-directed profiling based on the NBS (NBS profiling) and PK (PK profiling) domains. The efficiency of primers used in NBS/PK profiling to amplify RGA fragments was assessed by sequencing individual marker bands derived from genomic and cDNA fragments. The positions of 184 markers were identified in the Asw map, while those for 99 were identified in the MN map. Large numbers of NBS and PK profiling markers were found in clusters across different linkage groups, with the PK profiling markers more evenly distributed. The location of markers throughout the genetic maps and the composition of marker clusters indicate that NBS- and PK-based markers cover partly complementary regions of oat genomes. Markers of the different classes obtained were found associated with the two resistance loci, PcA and R-284B-2, mapped on Asw, and with five out of eight QTLs for partial resistance in the MN map. 53 RGA-RFLPs and 187 NBS/PK profiling markers were also mapped on the hexaploid map A. byzantina cv. Kanota × A. sativa cv. Ogle. Significant co-localization was seen between the RGA markers in the KO map and other markers closely linked to resistance loci, such as those for P. coronata and barley yellow dwarf virus (Bydv) that were previously mapped in other segregating populations.

Characterization of a library from a single microdissected oat (Avena sativa L.) chromosome

A plasmid library of oat chromosome No21, the smallest chromosome of the complement, was constructed by microdissection and microcloning. The chromosome was deproteinized with proteinase K and digested with Sau3A and linker adaptors were ligated to the DNA fragments. From the single chromosome (less than 0.4 pg), 10 μg of DNA was obtained after 2 rounds of PCR amplification. Cloning experiments with the amplified DNA produced as many as 500 000 recombinant clones from the single chromosome. The 500 clones evaluated ranged in size from 150 to 1700 base pairs (bp) with an average size of 650 bp. These were approximately 41% high-copy and 59% low/unique copy clones. Tandem repeats were absent in the library and may have been selected against by a combination of the Sau3A digestion, which is sensitive to C-methylation, and the PCR amplification. Many low-copy dispersed repetitive sequences were present in the library. These were present primarily on A- and D-genome chromosomes. Southern blot analysis revealed that the unique-copy clones were suitable for restriction fragment length polymorphism analysis and that they mapped to the pertinent oat nullisomic lines.

Transfer of sequence tagged site PCR markers between wheat and barley

Transfer of mapping information between related species has facilitated the development of restriction fragment length polymorphism (RFLP) maps in the cereals. Sequence tagged site (STS) primer sets for use in the polymerase chain reaction may be developed from mapped RFLP clones. For this study, we mapped 97 STS primer sets to chromosomes in wheat and barley to determine the potential transferability of the primer sets and the degree of correspondence between RFLP and STS locations. STS products mapped to the same chromosome group in wheat and barley 75% of the time. RFLP location predicted STS location 69% of the time in wheat and 56% of the time in barley. Southern hybridizations showed that most primer sets amplified sequences homologous to the RFLP clone, although additional sequences were often amplified that did not hybridize to the RFLP clone. Nontarget sequences were often amplified when primer sets were transferred across species. In general, results suggest a good probability of success in transferring STSs between wheat and barley, and that RFLP location can be used to predict STS location. However, transferability of STSs cannot be assumed, suggesting a need for recombinational mapping of STS markers in each species as new primer sets are developed. Key words : sequence tagged sites, PCR, wheat, barley.

Molecular mapping of wheat. Homoeologous group 2

A molecular-marker map of bread wheat having many markers in common with other grasses in the Gramineae family is a prerequisite for molecular level genetic studies and breeding in this crop species. We have constructed restriction fragment length polymorphism maps of the A-, B-, and D-genome chromosomes of homoeologous group 2 of hexaploid wheat (Triticum aestivum L. em. Thell) using 114 F7 lines from a synthetic x bread wheat cross and clones from 11 libraries. Chromosomes 2A, 2B, and 2D comprise 57, 60, and 56 markers and each spans about 200 cM. Comparisons between chromosomes are facilitated by 26 sets of homoeoloci. Genes mapped include a heterologous abscisic acid responsive locus cloned as pBS128, the epidermal waxiness inhibitor W21, and two presumed leaf rust and stem rust resistance genes. Anomalies suggesting ancestral rearrangements in chromosome 2B are pointed out and features of wheat group 2 chromosomes that are common to barley (Hordeum vulgare L.), rice (Oryza spp.), and T. tauschii are discussed.

Molecular genetic maps of the group 6 chromosomes of hexaploid wheat (Triticum aestivum L. em. Thell.)

Restriction fragment length polymorphism (RFLP) maps of chromosomes 6A, 6B, and 6D of hexaploid wheat (Triticum aestivum L. em. Thell.) have been produced. They were constructed using a population of F7-8 recombinant inbred lines derived from a synthetic wheat x bread wheat cross. The maps consist of 74 markers assigned to map positions at a LOD >= 3 (29 markers assigned to 6A, 24 to 6B, and 21 to 6D) and 2 markers assigned to 6D ordered at a LOD of 2.7. Another 78 markers were assigned to intervals on the maps. The maps of 6A, 6B, and 6D span 178, 132, and 206 cM, respectively. Twenty-one clones detected orthologous loci in two homoeologues and 3 detected an orthologous locus in each chromosome. Orthologous loci are located at intervals of from 1.5 to 26 cM throughout 70% of the length of the linkage maps. Within this portion of the maps, colinearity (homosequentiality) among the three homoeologues is strongly indicated. The remainder of the linkage maps consists of three segments ranging in length from 47 to 60 cM. Colinearity among these chromosomes and other Triticeae homoeologous group 6 chromosomes is indicated and a consensus RFLP map derived from maps of the homoeologous group 6 chromosomes of hexaploid wheat, tetraploid wheat, Triticum tauschii, and barley is presented. Key words : RFLP, wheat, linkage maps, molecular markers.

Comparative mapping of the oat Dw6/dw6 dwarfing locus using NILs and association with vacuolar proton ATPase subunit H

Seven pairs of oat near-isogenic lines (NILs) (Kibite in Crop Sci 41:277-278, 2001) contrasting for the Dw6 dwarfing gene were used to test for correlation between tall/dwarf phenotype and polymorphic genotype using restriction fragment length polymorphism (RFLP) and other molecular markers selected from the Kanota × Ogle (K×O) (Wight et al. in Genome 46:28-47, 2003) and Terra × Marion (De Koeyer et al. in Theor Appl Genet 108:1285-1298, 2004) recombination maps. This strategy located the Dw6/dw6 locus to a small chromosomal region on K×O linkage group (LG) KO33, near or at a putative RFLP locus aco245z. Aco245z and other tightly linked flanking markers have potential for use in marker-assisted selection (MAS), and PCR-based markers were developed from several of these. RFLP genotyping of the Dw6 NILs indicated that 13 of the 14 individual lines were homogeneously maternal or paternal for a large genomic region near Dw6/dw6, an unexpected result for NILs. The cDNA clone aco245 codes for a vacuolar proton ATPase subunit H, a potential candidate gene for Dw6. Vacuolar proton ATPase enzymes have a central role in plant growth and development and a mutation in subunit C is responsible for the det3 dwarfing mutation in Arabidopsis thaliana (Schumacher et al. in Genes Dev 13:3259-3270, 1999). Aco245 affords the potential of designing highly precise diagnostic markers for MAS for Dw6. The Dw6 NILs have potential utility to investigate the role of vacuolar proton ATPases in growth and development in plants.

An updated doubled haploid oat linkage map and QTL mapping of agronomic and grain quality traits from Canadian field trials

The first doubled haploid oat linkage map constructed at MTT Agrifood Research Finland was supplemented with additional microsatellites and Diversity Array Technology (DArT) markers to produce a map containing 1058 DNA markers and 34 linkage groups. The map was used to locate quantitative trait loci (QTLs) for 11 important breeding traits analyzed from Finnish and Canadian field trials. The new markers enabled most of the linkage groups to be anchored to the 'Kanota' × 'Ogle' oat ( Avena sativa L.) reference map and allowed comparison of the QTLs located in this study with those found previously. Two to 12 QTLs for each trait were discovered, of which several were expressed consistently across several environments.

Use of tyramide-fluorescence in situ hybridization and chromosome microdissection for ascertaining homology relationships and chromosome linkage group associations in oats

The physical mapping of single locus sequences by tyramide-fluorescence in situ hybridization (Tyr-FISH) and the analysis of sequences obtained from microdissected chromosomes were assayed as potential tools for (1) determining homology and homoeology among chromosome regions of Avena species, and (2) establishing associations between linkage groups and specific chromosomes. Low copy number probes, derived from resistance gene analogues (RGAs) and 2.8-4.5 kb long, successfully produced hybridization signals on specific chromosomes. Four sets of homoeologous chromosome regions were identified in the hexaploids using 3 probes that produced 4 single locus markers in A. strigosa and 2 in A. eriantha. Laser capture microdissection of metaphase I cells of A. sativa monosomic lines allowed the isolation of critical univalents. Sequences derived from 2 RGAs were successfully amplified in DNA extracted from univalents. In one instance, it was possible to map a nucleotide polymorphism specific for 1 chromosome. An association was established between this chromosome and its linkage groups in 2 hexaploid genetic maps. The results indicate that Tyr-FISH is useful in the characterization of homoeologous chromosome segments in hexaploids, whereas chromosome microdissection, as employed in this work, needs to be improved before it can routinely be used with meiotic chromosomes.

Reconstruction of the synthetic W7984 x Opata M85 wheat reference population

Reference populations are valuable resources in genetics studies for determining marker order, marker selection, trait mapping, construction of large-insert libraries, cross-referencing marker platforms, and genome sequencing. Reference populations can be propagated indefinitely, they are polymorphic and have normal segregation. Described are two new reference populations who share the same parents of the original wheat reference population Synthetic W7984 (Altar84/ Aegilops tauschii (219) CIGM86.940) x Opata M85, an F(1)-derived doubled haploid population (SynOpDH) of 215 inbred lines and a recombinant inbred population (SynOpRIL) of 2039 F(6) lines derived by single-plant self-pollinations. A linkage map was constructed for the SynOpDH population using 1446 markers. In addition, a core set of 42 SSR markers was genotyped on SynOpRIL. A new approach to identifying a core set of markers used a step-wise selection protocol based on polymorphism, uniform chromosome distribution, and reliability to create nested sets starting with one marker per chromosome, followed by two, four, and six. It is suggested that researchers use these markers as anchors for all future mapping projects to facilitate cross-referencing markers and chromosome locations. To enhance this public resource, researchers are strongly urged to validate line identities and deposit their data in GrainGenes so that others can benefit from the accumulated information.

Genetic association of crown rust resistance gene Pc68, storage protein loci, and resistance gene analogues in oats

Segregating F(3) families, derived from a cross between oat cultivar Swan and the putative single gene line PC68, were used to determine the association of seed storage protein loci and resistance gene analogues (RGAs) with the crown rust resistance gene Pc68. SDS-PAGE analysis detected three avenin loci, AveX, AveY, and AveZ, closely linked to Pc68. Their diagnostic alleles are linked in coupling to Pc68 and were also detected in three additional lines carrying Pc68. Another protein locus was linked in repulsion to Pc68. In complementary studies, three wheat RGA clones (W2, W4, and W10) detected restriction fragment length polymorphisms (RFLPs) between homozygous resistant and homozygous susceptible F(3) DNA bulks. Four oat homologues of W2 were cloned and sequenced. RFLPs detected with two of them were mapped using F(3) and F(4) populations. Clone 18 detected a locus, Orga2, linked in repulsion to Pc68. Clone 22 detected several RFLPs including Orga1 (the closest locus to Pc68) and three RGA loci (Orga22-2, Orga22-3, and Orga22-4) loosely linked to Pc68. The diagnostic RFLPs linked in coupling to Pc68 were detected by clone 22 in three additional oat lines carrying Pc68 and have potential utility in investigating and improving crown rust resistance of oat.

The evolution pattern of rDNA ITS in Avena and phylogenetic relationship of the Avena species (Poaceae: Aveneae)

Ribosomal ITS sequences are commonly used for phylogenetic reconstruction because they are included in rDNA repeats, and these repeats often undergo rapid concerted evolution within and between arrays. Therefore, the rDNA ITS copies appear to be virtually identical and can sometimes be treated as a single gene. In this paper we examined ITS polymorphism within and among 13 diploid (A and C genomes), seven tetraploid (AB, AC and CC genomes) and four hexaploid (ACD genome) to infer the extent and direction of concerted evolution, and to reveal the phylogenetic and genome relationship among species of Avena. A total of 170 clones of the ITS1-5.8S-ITS2 fragment were sequenced to carry out haplotype and phylogenetic analysis. In addition, 111 Avena ITS sequences retrieved from GenBank were combined with 170 clones to construct a phylogeny and a network. We demonstrate the major divergence between the A and C genomes whereas the distinction among the A and B/D genomes was generally not possible. High affinity among the A(d) genome species A. damascena and the ACD genome species A. fatua was found, whereas the rest of the ACD genome hexaploids and the AACC tetraploids were highly affiliated with the A(l) genome diploid A. longiglumis. One of the AACC species A. murphyi showed the closest relationship with most of the hexaploid species. Both C(v) and C(p) genome species have been proposed as paternal donors of the C-genome carrying polyploids. Incomplete concerted evolution is responsible for the observed differences among different clones of a single Avena individual. The elimination of C-genome rRNA sequences and the resulting evolutionary inference of hexaploid species are discussed.

Phylogenetic inferences in Avena based on analysis of FL intron2 sequences

The development and application of molecular methods in oats has been relatively slow compared with other crops. Results from the previous analyses have left many questions concerning species evolutionary relationships unanswered, especially regarding the origins of the B and D genomes, which are only known to be present in polyploid oat species. To investigate the species and genome relationships in genus Avena, among 13 diploid (A and C genomes), we used the second intron of the nuclear gene FLORICAULA/LEAFY (FL int2) in seven tetraploid (AB and AC genomes), and five hexaploid (ACD genome) species. The Avena FL int2 is rather long, and high levels of variation in length and sequence composition were found. Evidence for more than one copy of the FL int2 sequence was obtained for both the A and C genome groups, and the degree of divergence of the A genome copies was greater than that observed within the C genome sequences. Phylogenetic analysis of the FL int2 sequences resulted in topologies that contained four major groups; these groups reemphasize the major genomic divergence between the A and C genomes, and the close relationship among the A, B, and D genomes. However, the D genome in hexaploids more likely originated from a C genome diploid rather than the generally believed A genome, and the C genome diploid A. clauda may have played an important role in the origination of both the C and D genome in polyploids.

New DArT markers for oat provide enhanced map coverage and global germplasm characterization

Background

Genomic discovery in oat and its application to oat improvement have been hindered by a lack of genetic markers common to different genetic maps, and by the difficulty of conducting whole-genome analysis using high-throughput markers. This study was intended to develop, characterize, and apply a large set of oat genetic markers based on Diversity Array Technology (DArT).

Results

Approximately 19,000 genomic clones were isolated from complexity-reduced genomic representations of pooled DNA samples from 60 oat varieties of global origin. These were screened on three discovery arrays, with more than 2000 polymorphic markers being identified for use in this study, and approximately 2700 potentially polymorphic markers being identified for use in future studies. DNA sequence was obtained for 2573 clones and assembled into a non-redundant set of 1770 contigs and singletons. Of these, 705 showed highly significant (Expectation < 10E-10) BLAST similarity to gene sequences in public databases. Based on marker scores in 80 recombinant inbred lines, 1010 new DArT markers were used to saturate and improve the 'Kanota' × 'Ogle' genetic map. DArT markers provided map coverage approximately equivalent to existing markers. After binning markers from similar clones, as well as those with 99% scoring similarity, a set of 1295 non-redundant markers was used to analyze genetic diversity in 182 accessions of cultivated oat of worldwide origin. Results of this analysis confirmed that major clusters of oat diversity are related to spring vs. winter type, and to the presence of major breeding programs within geographical regions. Secondary clusters revealed groups that were often related to known pedigree structure.

Conclusion

These markers will provide a solid basis for future efforts in genomic discovery, comparative mapping, and the generation of an oat consensus map. They will also provide new opportunities for directed breeding of superior oat varieties, and guidance in the maintenance of oat genetic diversity.

The first doubled haploid linkage map for cultivated oat

To date, all linkage maps of hexaploid oat (Avena sativa L.) have been constructed using recombinant inbred lines (RILs). Doubled haploids (DHs), however, have the advantage over RILs of their comprehensive homozygosity. DHs have been used for mapping in several cereal species, but in oats the production of large DH populations has only recently become an option. A linkage map of hexaploid oat was constructed using an anther culture-derived DH population (137 individuals) from the F1 individuals of a cross between the Finnish cultivar 'Aslak' and the Swedish cultivar 'Matilda'. The map is composed of 28 linkage groups containing 625 DNA markers: 375 AFLPs (amplified fragment length polymorphisms), 3 IRAPs (inter-retrotransposon amplified polymorphisms), 12 ISSRs (inter simple sequence repeats), 12 microsatellites, 57 RAPDs (random amplified polymorphic DNAs), 59 REMAPs (retrotransposon-microsatellite amplified polymorphisms), 105 SRAPs (sequence-related amplified polymorphisms), and 2 SNPs (single-nucleotide polymorphisms). The total map size is 1526 cM. Over half of the markers in the map showed distorted segregation, with alleles from 'Aslak' usually prevailing. This is explained by the better performance of 'Aslak' in anther culture. Quantitative trait loci affecting some important quality and agronomic traits are being localized on the map.

Overview of QTL detection in plants and tests for synergistic epistatic interactions

Improvements in the usefulness of QTL analysis arise from better statistical methods applied to the problem, ability to analyze more complex mating designs, and the fitting of less simplified genetic models. Here we review the advantages of different plant mating designs in QTL analysis and conclude that diallel designs have several favorable properties. We then turn to the detection of systematic genome-wide synergistic epistasis. This form of epistasis has important implications from evolutionary (maintenance of sexual reproduction and concealment of cryptic genetic variation) and practical perspectives (response to pyramided favorable alleles). We develop two methods for detecting systematic synergistic epistasis, one based on analyzing interactions between locus effects and predicted individual genotypic values and one based on analyzing pairwise locus interactions. Using the first method we detect synergistic epistasis in a barley and a wheat dataset but not in a maize dataset. We fail to detect synergistic epistasis with the second method. We discuss our results in the light of theoretical questions concerning the mechanisms of synergistic epistasis.

The origin of the C-genome and cytoplasm of Avena polyploids

The contribution of C-genome diploid species to the evolution of polyploid oats was studied using C-genome ITS-specific primers. SCAR analysis among Avena accessions confirmed the presence of C-genome ITS1-5.8S-ITS2 sequences in the genome of AACC and AACCDD polyploids. In situ hybridization and screening of more than a thousand rRNA clones in Avena polyploid species containing the C-genome revealed substantial C-genome rRNA sequence elimination. C-genome clones sequenced and Maximum Likelihood Parsimony analysis revealed close proximity to Avena ventricosa ITS1-5.8S-ITS2 sequences, providing strong evidence of the latter's active role in the evolution of tetraploid and hexaploid oats. In addition, cloning and sequencing of the chloroplastic trnL intron among the most representative Avena species verified the maternal origin of A-genome for the AACC interspecific hybrid formation, which was the genetic bridge for the establishment of cultivated hexaploid oats.

Molecular diversity of the 5S rRNA gene and genomic relationships in the genus Avena (Poaceae: Aveneae)

The molecular diversity of the rDNA sequences (5S rDNA units) in 71 accessions from 26 taxa of Avena was evaluated. The analyses, based on 553 sequenced clones, indicated that there were 6 unit classes, named according to the haplomes (genomes) they putatively represent, namely the long A1, long B1, long M1, short C1, short D1, and short M1 unit classes. The long and short M1 unit classes were found in the tetraploid A. macrostachya, the only perennial species. The long M1 unit class was closely related to the short C1 unit class, while the short M1 unit class was closely related to the long A1 and long B1 unit classes. However, the short D1 unit class was more divergent from the other unit classes. There was only one unit class per haplome in Avena, whereas haplomes in the Triticeae often have two. Most of the sequences captured belonged to the long A1 unit class. Sequences identified as the long B1 unit class were found in the tetraploids A. abyssinica and A. vaviloviana and the diploids A. atlantica and A. longiglumis. The short C1 unit class was found in the diploid species carrying the C genome, i.e., A. clauda, A. eriantha, and A. ventricosa, and also in the diploid A. longiglumis, the tetraploids A. insularis and A. maroccana, and all the hexaploid species. The short D1 unit class was found in all the hexaploid species and two clones of A. clauda. It is noteworthy that in previous studies the B genome was found only in tetraploid species and the D genome only in hexaploid species. Unexpectedly, we found that various diploid Avena species contained the B1 and D1 units. The long B1 unit class was found in 3 accessions of the diploid A. atlantica (CN25849, CN25864, and CN25887) collected in Morocco and in 2 accessions of A. longiglumis (CIav9087 and CIav9089) collected in Algeria and Libya, respectively, whereas only 1 clone of A. clauda (CN21378) had the short D1 unit. Thus there might be a clue as to where to search for diploids carrying the B and D genomes. Avena longiglumis was found to be the most diverse species, possibly harboring the A, B, and C haplomes. The long M1 and short M1 are the unit classes typical of A. macrostachya. These results could explain the roles of A. clauda, A. longiglumis, and A. atlantica in the evolution of the genus Avena. Furthermore, one clone of the tetraploid A. murphyi was found to have sequences belonging to the short D1 unit class, which could indicate that A. murphyi might have been the progenitor of hexaploid oats and not, as postulated earlier, A. insularis. The evolution of Avena did not follow the molecular clock. The path inferred is that the C genome is more ancient than the A and B genomes and closer to the genome of A. macrostachya, the only existing perennial, which is presumed to be the most ancestral species in the genus.

Loci affecting flowering time in oat under short-day conditions

Flowering time (or days to heading) is an important characteristic in crop plants that affects adaptation to cropping cycles and growing seasons. The objectives of this study were to identify molecular markers associated with flowering time in 3 oat populations developed from Brazilian oat varieties, and to compare their map locations with those of other loci that might influence flowering time. Flowering time was studied in recombinant inbred lines from 3 hexaploid oat populations: UFRGS 8 x Pc68/5*Starter; UFRGS 881971 x Pc68/5*Starter; and UFRGS 8 x UFRGS 930605. Bulked segregant analysis, using amplified fragment length polymorphism, was followed by selective mapping in each population and in a reference population, 'Kanota' x 'Ogle' (KxO). One quantitative trait locus (QTL) with major effects on flowering time was identified in each cross. Comparative mapping showed that a major QTL, with earliness alleles originating from UFRGS 8 and UFRGS 881971, is in a region with close homology to KxO linkage group 17 and to a locus that reportedly confers day-length insensitivity in oat (Di1). This is the first report to identify the map location of the Di1 locus, and putatively confirm the presence of Di1 alleles in new germplasm. Further comparative mapping and the alignment of mapped oat markers with the sequenced rice genome suggest that this QTL and (or) Di1 is orthologous to the Hd1 locus in rice and the CONSTANS gene in Arabidopsis and other species. A different QTL with major effects segregated in the UFRGS 8 x UFRGS 930605 cross, where the early-flowering allele for Di1 was probably fixed. Two additional QTLs with smaller effects were identified in the UFRGS 8 x Pc68/5*Starter population. These results suggest that the Brazilian oat line UFRGS 8 contains an optimal set of alleles conditioning earliness under the short-day conditions of the Brazilian winter growing season, and that molecular selection could be used to introgress these alleles into other breeding material.

A genetic linkage map for tef [Eragrostis tef (Zucc.) Trotter]

Tef [Eragrostis tef (Zucc.) Trotter] is the major cereal crop in Ethiopia. Tef is an allotetraploid with a base chromosome number of 10 (2n = 4x = 40) and a genome size of 730 Mbp. Ninety-four F(9) recombinant inbred lines (RIL) derived from the interspecific cross, Eragrostis tef cv. Kaye Murri x Eragrostis pilosa (accession 30-5), were mapped using restriction fragment length polymorphisms (RFLP), simple sequence repeats derived from expressed sequence tags (EST-SSR), single nucleotide polymorphism/insertion and deletion (SNP/INDEL), intron fragment length polymorphism (IFLP) and inter-simple sequence repeat amplification (ISSR). A total of 156 loci from 121 markers was grouped into 21 linkage groups at LOD 4, and the map covered 2,081.5 cM with a mean density of 12.3 cM per locus. Three putative homoeologous groups were identified based on multi-locus markers. Sixteen percent of the loci deviated from normal segregation with a predominance of E. tef alleles, and a majority of the distorted loci were clustered on three linkage groups. This map will be useful for further genetic studies in tef including mapping of loci controlling quantitative traits (QTL), and comparative analysis with other cereal crops.

An algorithm for analyzing linkages affected by heterozygous translocations: QuadMap

Heterozygous chromosome rearrangements such as reciprocal translocations are most accurately displayed as two-dimensional linkage maps. Standard linkage mapping software packages, such as MapMaker, generate only one-dimensional maps and so reciprocal translocations appear as clusters of markers, even though they originate from two nonhomologous chromosomes. To more accurately map these regions, researchers have developed statistical methods that use the variance in map distance to distinguish among the four segments (two translocation, two interstitial) of the translocation. In this study, we describe modifications to one of these protocols, that proposed by Livingstone et al. (2000). We also introduce QuadMap, a new software application for dissecting heterozygous translocation-affected linkage maps.

Generation and analysis of 9792 EST sequences from cold acclimated oat, Avena sativa

BACKGROUND

Oat is an important crop in North America and northern Europe. In Scandinavia, yields are limited by the fact that oat cannot be used as a winter crop. In order to develop such a crop, more knowledge about mechanisms of cold tolerance in oat is required.

RESULTS

From an oat cDNA library 9792 single-pass EST sequences were obtained. The library was prepared from pooled RNA samples isolated from leaves of four-week old Avena sativa (oat) plants incubated at +4 degrees C for 4, 8, 16 and 32 hours. Exclusion of sequences shorter than 100 bp resulted in 8508 high-quality ESTs with a mean length of 710.7 bp. Clustering and assembly identified a set of 2800 different transcripts denoted the Avena sativa cold induced UniGene set (AsCIUniGene set). Taking advantage of various tools and databases, putative functions were assigned to 1620 (58%) of these genes. Of the remaining 1180 unclassified sequences, 427 appeared to be oat-specific since they lacked any significant sequence similarity (Blast E values > 10(-10)) to any sequence available in the public databases. Of the 2800 UniGene sequences, 398 displayed significant homology (BlastX E values < or = 10(-10)) to genes previously reported to be involved in cold stress related processes. 107 novel oat transcription factors were also identified, out of which 51 were similar to genes previously shown to be cold induced. The CBF transcription factors have a major role in regulating cold acclimation. Four oat CBF sequences were found, belonging to the monocot cluster of DREB family ERF/AP2 domain proteins. Finally in the total EST sequence data (5.3 Mbp) approximately 400 potential SSRs were found, a frequency similar to what has previously been identified in Arabidopsis ESTs.

CONCLUSION

The AsCIUniGene set will now be used to fabricate an oat biochip, to perform various expression studies with different oat cultivars incubated at varying temperatures, to generate molecular markers and provide tools for various genetic transformation experiments in oat. This will lead to a better understanding of the cellular biology of this important crop and will open up new ways to improve its agronomical properties.

Quantitative trait loci for partial resistance to crown rust, Puccinia coronata, in cultivated oat, Avena sativa L

To facilitate the detection of quantitative trait loci (QTLs) for partial resistance to oat crown rust, Puccinia coronata f. sp. avenae Eriks., a genetic map was generated in a population of 158 F(6)-derived oat recombinant inbred lines from a cross of a partial resistance line MN841801-1 by a susceptible cultivar selection 'Noble-2'. The map, developed using 230 marker loci, mostly restriction fragment length polymorphism and amplified fragment length polymorphism markers, spanned 1,509 cM (Haldane) arranged into 30 linkage groups of 2-18 markers each. Four consistently detected major QTLs for partial rust resistance, Prq1a, Prq1b, Prq2, and Prq7, and three minor QTLs, Prq3, Prq5, and Prq6, were found in tests involving three field and two greenhouse environments. In addition, two major QTLs for flowering time, Ftq1 and Ftq7, and five weaker QTLs, Ftq2, Ftq3, Ftq4, Ftq5, and Ftq6, were revealed. Overlapping of the map segments of Ftq1 and Prq1 and of Ftq7 and Prq7 suggested either linkage between the flowering time QTLs and resistance QTLs or a pleiotropic effect of the Ftq QTLs on rust resistance. Relatively low heritability estimates (0.30) obtained for partial resistance to crown rust in the field indicate a potential value for marker-assisted selection.

Isolation and mapping of resistance gene analogs from the Avena strigosa genome

Degenerate primers based on conserved regions of the nucleotide binding site (NBS) domain (encoded by the largest group of cloned plant disease resistance genes) were used to isolate a set of 15 resistance gene analogs (RGA) from the diploid species Avena strigosa Schreb. These were grouped into seven classes on the basis of 60% or greater nucleic acid sequence identity. Representative clones were used for genetic mapping in diploid and hexaploid oats. Two RGAs were mapped at two loci of the linkage group AswBF belonging to the A. strigosa x A. wiestii Steud map, and ten RGAs were mapped at 15 loci in eight linkage groups belonging to the A. byzantina C. Koch cv. Kanota x A. sativa L. cv. Ogle map. A similar approach was used for targeting genes encoding receptor-like kinases. Three different sequences were obtained and mapped to two linkage groups of the hexaploid oat map. Associations were explored between already known disease resistance loci mapped in different populations and the RGAs. Molecular markers previously linked to crown rust and barley yellow dwarf resistance genes or quantitative trait loci were found in the Kanota x Ogle map linked to RGAs at a distance ranging from 0 cM to 20 cM. Homoeologous RGAs were found to be linked to loci either conferring resistance to different isolates of the same pathogen or to different pathogens. This suggests that these RGAs identify genome regions containing resistance gene clusters.

Mapping of 99 new microsatellite-derived loci in rye (Secale cereale L.) including 39 expressed sequence tags

The genetic map of rye contains predominantly restriction fragment length polymorphism (RFLP) markers but also a limited number of microsatellite markers, which are known to be more reliable and easier to apply. We report here the saturation of the genomic map of rye with additional microsatellite-derived markers that we obtained from the rye expressed sequence tag (EST) databases and the Gatersleben collection of wheat microsatellite markers (WMS). A total of 99 loci (39 EST and 60 WMS) were mapped into the RFLP frameworks of four rye mapping populations consisting of 139, 64, 58 and 60 RFLPs, respectively. For another ten WMS loci, which amplified PCR products not polymorphic in any of the mapping populations, chromosome and chromosome arm locations were determined using wheat-rye addition lines. Chromosomes 1R, 2R, 3R, 4R, 5R, 6R and 7R were enriched with 9, 19, 9, 13, 27, 16 and 16 microsatellite loci, respectively. The microsatellite loci mapped were evenly distributed along the chromosomes, which is important for the further application of these markers for gene mapping or diversity studies in rye. Forty-four of the WMS loci mapped in rye were found to be homologous to those mapped in bread wheat ( Triticum aestivum L.).

A molecular linkage map with associated QTLs from a hulless x covered spring oat population

In spring-type oat ( Avena sativa L.), quantitative trait loci (QTLs) detected in adapted populations may have the greatest potential for improving germplasm via marker-assisted selection. An F(6) recombinant inbred (RI) population was developed from a cross between two Canadian spring oat varieties: 'Terra', a hulless line, and 'Marion', an elite covered-seeded line. A molecular linkage map was generated using 430 AFLP, RFLP, RAPD, SCAR, and phenotypic markers scored on 101 RI lines. This map was refined by selecting a robust set of 124 framework markers that mapped to 35 linkage groups and contained 35 unlinked loci. One hundred one lines grown in up to 13 field environments in Canada and the United States between 1992 and 1997 were evaluated for 16 agronomic, kernel, and chemical composition traits. QTLs were localized using three detection methods with an experiment-wide error rate of approximately 0.05 for each trait. In total, 34 main-effect QTLs affecting the following traits were identified: heading date, plant height, lodging, visual score, grain yield, kernel weight, milling yield, test weight, thin and plump kernels, groat beta-glucan concentration, oil concentration, and protein. Several of these correspond to QTLs in homologous or homoeologous regions reported in other oat QTL studies. Twenty-four QTL-by-environment interactions and three epistatic interactions were also detected. The locus controlling the covered/hulless character ( N1) affected most of the traits measured in this study. Additive QTL models with N1 as a covariate were superior to models based on separate covered and hulless sub-populations. This approach is recommended for other populations segregating for major genes. Marker-trait associations identified in this study have considerable potential for use in marker-assisted selection strategies to improve traits within spring oat breeding programs.

Construction of a comparative RFLP map of Echinochloa crus-galli toward QTL analysis of flooding tolerance

To analyze quantitative trait loci (QTLs) affecting flooding tolerance and other physiological and morphological traits in Echinochloa crus-galli, a restriction fragment length polymorphism (RFLP) map was constructed using 55 plants of the F(2) population ( E. crus-galli var. praticola x E. crus-galli var. formosensis). One hundred forty-one loci formed 41 linkage groups. The total map size was 1,468 cM and the average size of linkage groups was 35.8 cM. The average distance between markers was 14.7 cM and the range was 0-37.2 cM. Early comparisons to the genetic maps of other taxa suggest appreciable synteny with buffelgrass ( Pennisetum spp.) and sorghum ( Sorghum spp.). One hundred ninty-one F(2) plants were used to analyze QTLs of flooding tolerance, plant morphology, heading date, number of leaves, and plant height. For flooding tolerance, two QTLs were detected and one was mapped on linkage group 24. Other traits, including plant morphology, heading date, number of leaves, and plant height were highly correlated. Three genomic regions accounted for most of the mapped QTLs, each explaining 2-4 of the significant marker-trait associations. The high observed correlation between the traits appears to result from QTLs with a large contribution to the phenotypic variance at the same or nearby locations.

DNA markers in plant improvement: an overview

The progress made in DNA marker technology has been tremendous and exciting. DNA markers have provided valuable tools in various analyses ranging from phylogenetic analysis to the positional cloning of genes. The development of high-density molecular maps which has been facilitated by PCR-based markers, have made the mapping and tagging of almost any trait possible. Marker-assisted selection has the potential to deploy favorable gene combinations for disease control. Comparative studies between incompatible species using these markers has resulted in synteny maps which are useful not only in predicting genome organization and evolution but also have practical application in plant breeding. DNA marker technology has found application in fingerprinting genotypes, in determining seed purity, in systematic sampling of germplasm, and in phylogenetic analysis. This review discusses the use of this technology for the genetic improvement of plants.

Molecular mapping of genomic regions underlying barley yellow dwarf tolerance in cultivated oat (Avena sativa L.)

Barley yellow dwarf (BYD) is one of the most important viral diseases in small grains, including oat (Avena sativa L.). Breeding for BYD tolerance is an effective and efficient means to control the disease. Characterization of major sources of tolerance, and identification of marker and the trait associations, will directly benefit breeding for BYD tolerance. Genomic regions underlying BYD tolerance were mapped and characterized in an oat population consisting of 152 recombinant inbred lines from the cross of 'Ogle' (tolerant)/MAM17-5 (sensitive). Tolerance was evaluated in replicated field trials across 2 years under artificial inoculation with viruliferous aphids harboring BYD virus isolate PAV-IL. Composite interval mapping was used for quantitative trait loci (QTLs) analysis with a framework map consisting of 272 molecular markers. Four QTLs, BYDq1, BYDq2, BYDq3 and BYDq4, for BYD tolerance were identified on linkage groups OM1, 5, 7 and 24, respectively. All but BYDq2 were consistently detected across both years. Significant epistasis was found between some QTLs. The final model including the epistatic effect explained 50.3 to 58.2% of the total phenotypic variation for BYD tolerance. Some QTLs for BYD tolerance were closely linked to QTLs for plant height and days to heading. Potential problems with QTL mapping for BYD tolerance have been discussed. The identified association of markers and tolerance should be useful to pyramid favorable alleles for BYD tolerance into individual oat lines.

Isolation and identification of Triticeae chromosome 1 receptor-like kinase genes (Lrk10) from diploid, tetraploid, and hexaploid species of the genus Avena

The DNA sequence of an extracellular (EXC) domain of an oat (Avena sativa L.) receptor-like kinase (ALrk10) gene was amplified from 23 accessions of 15 Avena species (6 diploid, 6 tetraploid, and 3 hexaploid). Primers were designed from one partial oat ALrk10 clone that had been used to map the gene in hexaploid oat to linkage groups syntenic to Triticeae chromosome 1 and 3. Cluster (phylogenetic) analyses showed that all of the oat DNA sequences amplified with these primers are orthologous to the wheat and barley sequences that are located on chromosome 1 of the Triticeae species. Triticeae chromosome 3 Lrk10 sequences were not amplified using these primers. Cluster analyses provided evidence for multiple copies at a locus. The analysis divided the ALrk EXC sequences into two groups, one of which included AA and AABB genome species and the other CC, AACC, and CCCC genome species. Both groups of sequences were found in hexaploid AACCDD genome species, but not in all accessions. The C genome group was divided into 3 subgroups: (i) the CC diploids and the perennial autotetraploid, Avena macrostachya (this supports other evidence for the presence of the C in this autotetraploid species); (ii) a sequence from Avena maroccana and Avena murphyi and several sequences from different accessions of A. sativa; and (iii) A. murphyi and sequences from A. sativa and Avena sterilis. This suggests a possible polyphyletic origin for A. sativa from the AACC progenitor tetraploids or an origin from a progenitor of the AACC tetraploids. The sequences of the A genome group were not as clearly divided into subgroups. Although a group of sequences from the accession 'SunII' and a sequence from line Pg3, are clearly different from the others, the A genome diploid sequences were interspersed with tetraploid and hexaploid sequences.

A molecular marker map in 'Kanota' x 'Ogle' hexaploid oat (Avena spp.) enhanced by additional markers and a robust framework

Molecular mapping of cultivated oats was conducted to update the previous reference map constructed using a recombinant inbred (RI) population derived from Avena byzantina C. Koch cv. Kanota x Avena sativa L. cv. Ogle. In the current work, 607 new markers were scored, many on a larger set of RI lines (133 vs. 71) than previously reported. A robust, updated framework map was developed to resolve linkage associations among 286 markers. The remaining 880 markers were placed individually within the most likely framework interval using chi2 tests. This molecular framework incorporates and builds on previous studies, including physical mapping and linkage mapping in additional oat populations. The resulting map provides a common tool for use by oat researchers concerned with structural genomics, functional genomics, and molecular breeding.

Genetic and physical mapping of Lrk10-like receptor kinase sequences in hexaploid oat (Avena sativa L.)

Oat receptor-like kinase gene sequences, homologous to the Lrk10 gene from wheat (Triticum aestivum L.), were mapped in oat (Avena sativa L.). PCR primers designed from the wheat Lrk10 were used to produce ALrk10 from oat. Two DNA sequences, ALrk1A1 and ALrk4A5, were produced from primers designed from coding and noncoding regions of ALrk10. Their use as RFLP probes indicated that the kinase genes mapped to four loci on different hexaploid oat 'Kanota' x 'Ogle' linkage groups (4_12, 5, 6, and 13) and to a fifth locus unlinked to other markers. Three of these linkage groups contain a region homologous to the short arm of chromosome I of wheat and the fourth contains a region homologous to chromosome 3 of wheat. Analysis with several nullisomics of oat indicated that two of the map locations are on satellite chromosomes. RFLP mapping in a 'Dumont' x 'OT328' population indicated that one map location is closely linked to Pg9, a resistance gene to oat stem rust (Puccinia graminis subsp. avenae). Comparative mapping indicates this to be the region of a presumed cluster of crown rust (Puccinia coronata subsp. avenae) and stem rust resistance genes (Pg3, Pg9, Pc44, Pc46, Pc50, Pc68, Pc95, and PcX). The map position of several RGAs located on KO6 and KO3_38 with respect to Lrk10 and storage protein genes are also reported.

Characterization of AFLP markers in damselflies: prevalence of codominant markers and implications for population genetic applications

Amplified fragment length polymorphism (AFLP) analysis is becoming increasingly popular as a method for generating molecular markers for population genetic applications. For practical considerations, it is generally assumed in population studies that AFLPs segregate as dominant markers, i.e., that present and absent are the only possible states of a given locus. We tested the assumption of dominance in natural populations of the damselfly Nehalennia irene (Hagen) (Odonata: Coenagrionidae). Electro-blotted AFLP products from 21 samples were probed with individual markers. Eleven markers were analyzed, of which two were monomorphic and nine were polymorphic. Only two of the polymorphic markers behaved in a strictly dominant manner. The remaining seven polymorphic markers displayed various degrees of codominance, with 2-10 visible alleles in the sample. Of the three markers displaying the highest degree of variability, two contained microsatellite repeat tracts. Our results suggest that the assumption of dominance is unfounded. As a result, AFLP analysis may be unsuitable for estimating several important population genetic parameters, including genetic diversity.

A linkage map of hexaploid oat based on grass anchor DNA clones and its relationship to other oat maps

A cultivated oat linkage map was developed using a recombinant inbred population of 136 F6:7 lines from the cross 'Ogle' × 'TAM O-301'. A total of 441 marker loci, including 355 restriction fragment length polymorphism (RFLP) markers, 40 amplified fragment length polymorphisms (AFLPs), 22 random amplified polymorphic DNAs (RAPDs), 7 sequence-tagged sites (STSs), 1 simple sequence repeat (SSR), 12 isozyme loci, and 4 discrete morphological traits, was mapped. Fifteen loci remained unlinked, and 426 loci produced 34 linkage groups (with 2–43 loci each) spanning 2049 cM of the oat genome (from 4.2 to 174.0 cM per group). Comparisons with other Avena maps revealed 35 genome regions syntenic between hexaploid maps and 16–34 regions conserved between diploid and hexaploid maps. Those portions of hexaploid oat maps that could be compared were completely conserved. Considerable conservation of diploid genome regions on the hexaploid map also was observed (89–95%); however, at the whole-chromosome level, colinearity was much lower. Comparisons among linkage groups, both within and among Avena mapping populations, revealed several putative homoeologous linkage group sets as well as some linkage groups composed of segments from different homoeologous groups. The relationships between many Avena linkage groups remain uncertain, however, due to incomplete coverage by comparative markers and to complications introduced by genomic duplications and rearrangements.Key words: Avena, linkage map, comparative mapping, homoeology.

A restriction fragment length polymorphism based linkage map of a diploid Avena recombinant inbred line population

A population of 100 F6-derived recombinant inbred lines was developed from the cross of two diploid (2n = 14) Avena accessions, CI3815 (A. strigosa) and CI1994 (A. wiestii). Restriction fragment length polymorphism (RFLP) probes previously mapped in other grass species were used to develop a framework linkage map suitable for comparative genetics. Nine linkage groups were identified among the 181 loci mapped, with an average interlocus distance of 5 cM, and a total genetic map length of 880 cM. A cluster of five tightly linked crown rust resistance genes (Pca) was localized on the map, as were five loci identified by disease resistance gene analogs from maize, sorghum, and wheat. None of the five loci identified by the gene analogs were linked to the Pca locus. The linkage map was compared with previously published diploid and hexaploid linkage maps in an attempt to identify homologous or homoeologous chromosomes between populations. Locus orders and linkage relationships were poorly conserved between the A. strigosa × A. wiestii map and other Avena maps. In spite of mapping complications due to duplications within a basic genome as well as the allopolyploid constitution of many Avena species, such map comparisons within Avena provide further evidence of substantial chromosomal rearrangement between species within Avena.Key words: grasses, oat, genetic mapping, homoeology.

An anchored AFLP- and retrotransposon-based map of diploid Avena

A saturated genetic map of diploid oat was constructed based on a recombinant inbred (RI) population developed from a cross between Avena strigosa (Cereal Introduction, C.I. 3815) and A. wiestii (C.I. 1994). This 513-locus map includes 372 AFLP (amplified fragment length polymorphism) and 78 S-SAP (sequence-specific-amplification polymorphism) markers, 6 crown-rust resistance loci, 8 resistance-gene analogs (RGAs), one morphological marker, one RAPD (random amplified polymorphic DNA) marker, and is anchored by 45 grass-genome RFLP (restriction fragment length polymorphism) markers. This new A. strigosa × A. wiestii RI map is colinear with a diploid Avena map from an A. atlantica × A. hirtula F2 population. However, some linkage blocks were rearranged as compared to the RFLP map derived from the progenitor A. strigosa × A. wiestii F2 population. Mapping of Bare-1-like sequences via sequence-specific AFLP indicated that related retrotransposons had considerable heterogeneity and widespread distribution in the diploid Avena genome. Novel amplified fragments detected in the RI population suggested that some of these retrotransposon-like sequences are active in diploid Avena. Three markers closely linked to the Pca crown-rust resistance cluster were identified via AFLP-based bulk-segregant analysis. The derived STS (sequence-tagged-site) marker, Agx4, cosegregates with Pc85, the gene that provides resistance specificity to crown-rust isolate 202 at the end of the cluster. This framework map will be useful in gene cloning, genetic mapping of qualitative genes, and positioning QTL (quantitative trait loci) of agricultural importance.Key words: AFLP, Bare-1 retrotransposon, sequence-specific-amplification polymorphism (S-SAP), resistance-gene analog, crown-rust resistance, Pca, Gramineae, grass anchor probe.

Combined AFLP and RFLP mapping in two hexaploid oat recombinant inbred populations

A combined RFLP and AFLP map was constructed for hexaploid oat (Avena spp.). The segregation of AFLP markers was scored in two hexaploid oat recombinant inbred line (RIL) populations, the 'Kanota' × 'Ogle' RFLP population, and a population derived from 'Clintland64' and 'IL86-5698', barley yellow dwarf virus (BYDV)-sensitive and BYDV-tolerant lines, respectively. More than 300 AFLP markers were scored in each population, of which 97 could be scored in both populations. AFLP markers were linked to RFLP markers in 32 of 36 'Kanota' × 'Ogle' RFLP linkage groups. The addition of the AFLP markers to the 'Kanota' × 'Ogle' RFLP data set combined markers from four pairs of linkage groups and increased the size of the map from 1402 cM to 2351 cM. Thirty linkage groups were observed in the 'Clintland64' × 'IL86-5698' population, two of which could be consolidated by comparing the maps from both populations. The AFLP and RFLP markers showed very similar distributions in the 'Kanota' × 'Ogle' population with a tendency of each type of marker to cluster with markers of the same type. The placement of a set of AFLP markers on the 'Kanota' × 'Ogle' linkage map will enrich the RFLP map and allow others to relate AFLP markers for agronomically important genes to the reference 'Kanota' × 'Ogle' linkage map. Key words: amplified fragment length polymorphism, Avena, comparative mapping.

Genome structure and evolution in the allohexaploid weed Avena fatua L. (Poaceae)

Allohexaploid wild oat, Avena fatua L. (Poaceae; 2n = 6x = 42), is one of the world's worst weeds, yet unlike some of the other Avena hexaploids, its genomic structure has been relatively little researched. Consequently, in situ hybridisation was carried out on one accession of A. fatua using an 18S-25S ribosomal DNA (rDNA) sequence and genomic DNA fromA. strigosa (AA-genome diploid) and A. clauda (CC-genome diploid) as probes. Comparing these results with those for other hexaploids studied previously: (i) confirmed that the genomic composition of A. fatua was similar to the other hexaploid Avena taxa (i.e., AACCDD), (ii) identified major sites of rDNA on three pairs of A/D-genome chromosomes, in common with other Avena hexaploids, and (iii) revealed eight chromosome pairs carrying intergenomic translocations between the A/D- and C-genomes in the accession studied. Based on karyotype structure, the identity of some of these recombinant chromosomes was proposed, and this showed that some of these could be divided into two types, (i) those common to all hexaploid Avena species analysed (3 translocations) and (ii) one translocation in this A. fatua accession not previously observed in reports on other hexaploid Avena species. If this translocation is found to be unique to A. fatua, then this information, combined with more traditional morphological data, will add support to the view that A. fatua is genetically distinct from other hexaploid Avena species and thus should retain its full specific status.Key words: wild oats, Avena, genomic in situ hybridisation (GISH), intergenomic translocations, ribosomal DNA.

Discrimination of the closely related A and D genomes of the hexaploid oat Avena sativa L

A satellite DNA sequence, As120a, specific to the A-genome chromosomes in the hexaploid oat, Avena sativa L., was isolated by subcloning a fragment with internal tandem repeats from a plasmid, pAs120, that had been obtained from an Avena strigosa (As genome) genomic library. Southern and in situ hybridization showed that sequences with homology to sequences within pAs120 were dispersed throughout the genome of diploid (A and C genomes), tetraploid (AC genomes), and hexaploid (ACD genomes) Avena species. In contrast, sequences homologous to As120a were found in two A-genome species (A. strigosa and Avena longiglumis) and in the hexaploid A. sativa whereas this sequence was little amplified in the tetraploid Avena murphyi and was absent in the remaining A- and C-genome diploid species. In situ hybridization of pAs120a to hexaploid oat species revealed the distribution of elements of the As120a repeated family over both arms of 14 of 42 chromosomes of this species. By using double in situ hybridization with pAs120a and a C genome-specific probe, three sets of 14 chromosomes were revealed corresponding to the A, C, and D genomes of the hexaploid species. Simultaneous in situ hybridizations with pAs120a and ribosomal probes were used to assign the SAT chromosomes of hexaploid species to their correct genomes. This work reports a sequence able to distinguish between the closely related A and D genomes of hexaploid oats. This sequence offers new opportunities to analyze the relationships of Avena species and to explore the possible evolution of various polyploid oat species.

Transgenic DNA integrated into the oat genome is frequently interspersed by host DNA

Integration of transgenic DNA into the plant genome was investigated in 13 transgenic oat (Avena sativa L.) lines produced using microprojectile bombardment with one or two cotransformed plasmids. In all transformation events, the transgenic DNA integrated into the plant genome consisted of intact transgene copies that were accompanied by multiple, rearranged, and/or truncated transgene fragments. All fragments of transgenic DNA cosegregated, indicating that they were integrated at single gene loci. Analysis of the structure of the transgenic loci indicated that the transgenic DNA was interspersed by the host genomic DNA. The number of insertions of transgenic DNA within the transgene loci varied from 2 to 12 among the 13 lines. Restriction endonucleases that do not cleave the introduced plasmids produced restriction fragments ranging from 3.6 to about 60 kb in length hybridizing to a probe comprising the introduced plasmids. Although the size of the interspersing host DNA within the transgene locus is unknown, the sizes of the transgene-hybridizing restriction fragments indicated that the entire transgene locus must be at least from 35-280 kb. The observation that all transgenic lines analyzed exhibited genomic interspersion of multiple clustered transgenes suggests a predominating integration mechanism. We propose that transgene integration at multiple clustered DNA replication forks could account for the observed interspersion of transgenic DNA with host genomic DNA within transgenic loci.

Identification of quantitative Loci for tolerance to barley yellow dwarf virus in oat

ABSTRACT Molecular markers linked to quantitative trait loci conditioning tolerance to barley yellow dwarf virus (BYDV) were identified in oat (Avena sativa) using amplified fragment length polymorphism (AFLP) analysis. Near-isogenic and recombinant inbred lines (NILs and RILs, respectively) derived from a cross of Clintland64 (BYDV-sensitive) and IL86-5698 (BYDV-tolerant) were evaluated for their responses to an Illinois isolate of the PAV strain of BYDV. Individual markers identified in the analysis of the NILs explained up to 35% of the variability seen in the tolerance response. Single-point analysis of the marker data from the RIL population identified 24 markers in three linkage groups that were associated with tolerance to BYDV infection at P </= 0.001. These markers defined three major loci, A, C, and E, that were contributed by the tolerant parent (IL86-5698) and explained 35.0, 20.6, and 17.0% of the variability, respectively. Three minor loci G, H(1), and R) were identified at P </= 0.01. These loci were contributed by the sensitive parent (Clintland64) and explained 5.8, 5.6, and 5.6% of the variability, respectively. Interval analysis showed that only the loci A, C, and E are associated significantly with BYDV tolerance at log of the likelihood ratio >/= 3.0. These loci explained about 50% total of the variation in BYDV tolerance in multimarker regression analysis in both years. The BYDV tolerance loci A, C, E, and R were mapped to hexaploid oat restriction fragment length polymorphism linkage groups 2, 8, 36, and 5, respectively, by analyzing the segregation of the AFLP markers in the Kanota x Ogle RIL population.