Phylogenomic Analysis of a 55.1-kb 19-Gene Dataset Resolves a Monophyletic Fusarium that Includes the Fusarium solani Species Complex.

Scientific communication is facilitated by a data-driven, scientifically sound taxonomy that considers the end-user's needs and established successful practice. In 2013, the Fusarium community voiced near unanimous support for a concept of Fusarium that represented a clade comprising all agriculturally and clinically important Fusarium species, including the F. solani species complex (FSSC). Subsequently, this concept was challenged in 2015 by one research group who proposed dividing the genus Fusarium into seven genera, including the FSSC described as members of the genus Neocosmospora, with subsequent justification in 2018 based on claims that the 2013 concept of Fusarium is polyphyletic. Here, we test this claim and provide a phylogeny based on exonic nucleotide sequences of 19 orthologous protein-coding genes that strongly support the monophyly of Fusarium including the FSSC. We reassert the practical and scientific argument in support of a genus Fusarium that includes the FSSC and several other basal lineages, consistent with the longstanding use of this name among plant pathologists, medical mycologists, quarantine officials, regulatory agencies, students, and researchers with a stake in its taxonomy. In recognition of this monophyly, 40 species described as genus Neocosmospora were recombined in genus Fusarium, and nine others were renamed Fusarium. Here the global Fusarium community voices strong support for the inclusion of the FSSC in Fusarium, as it remains the best scientific, nomenclatural, and practical taxonomic option available.

Development and characterization of wheat-sea wheatgrass (Thinopyrum junceiforme) amphiploids for biotic stress resistance and abiotic stress tolerance.

KEY MESSAGE: Development of a complete wheat-Thinopyrum junceiforme amphiploid facilitated identification of resistance to multiple pests and abiotic stress derived from the wild species and shed new light on its genome composition. Wheat production is facing numerous challenges from biotic and abiotic stresses. Alien gene transfer has been an effective approach for wheat germplasm enhancement. Thinopyrum junceiforme, also known as sea wheatgrass (SWG), is a distant relative of wheat and a relatively untapped source for wheat improvement. In the present study, we developed a complete amphiploid, 13G819, between emmer wheat and SWG for the first time. Analysis of the chromosome constitution of the wheat-SWG amphiploid by multiple-color genomic in situ hybridization indicated that SWG is an allotetraploid with its J1 genome closely related to Th. bessarabicum and Th. elongatum, and its J2 genome was derived from an unknown source. Two SWG-derived perennial wheat lines, 14F3516 and 14F3536, are partial amphiploids and carry 13 SWG chromosomes of mixed J1 and J2 genome composition, suggesting cytological instability. We challenged the amphiploid 13G819 with various abiotic and biotic stress treatments together with its emmer wheat parent. Compared to its emmer wheat parent, the amphiploid showed high tolerance to waterlogging, manganese toxicity and salinity, low nitrogen and possibly to heat as well. The amphiploid 13G819 is also highly resistant to the wheat streak mosaic virus (temperature insensitive) and Fusarium head blight. All three amphiploids had solid stems, which confer resistance to wheat stem sawflies. All these traits make SWG an excellent source for improving wheat resistance to diseases and insects and tolerance to abiotic stress.

Divergent and convergent modes of interaction between wheat and Puccinia graminis f. sp. tritici isolates revealed by the comparative gene co-expression network and genome analyses.

BACKGROUND: Two opposing evolutionary constraints exert pressure on plant pathogens: one to diversify virulence factors in order to evade plant defenses, and the other to retain virulence factors critical for maintaining a compatible interaction with the plant host. To better understand how the diversified arsenals of fungal genes promote interaction with the same compatible wheat line, we performed a comparative genomic analysis of two North American isolates of Puccinia graminis f. sp. tritici (Pgt). RESULTS: The patterns of inter-isolate divergence in the secreted candidate effector genes were compared with the levels of conservation and divergence of plant-pathogen gene co-expression networks (GCN) developed for each isolate. Comprative genomic analyses revealed substantial level of interisolate divergence in effector gene complement and sequence divergence. Gene Ontology (GO) analyses of the conserved and unique parts of the isolate-specific GCNs identified a number of conserved host pathways targeted by both isolates. Interestingly, the degree of inter-isolate sub-network conservation varied widely for the different host pathways and was positively associated with the proportion of conserved effector candidates associated with each sub-network. While different Pgt isolates tended to exploit similar wheat pathways for infection, the mode of plant-pathogen interaction varied for different pathways with some pathways being associated with the conserved set of effectors and others being linked with the diverged or isolate-specific effectors. CONCLUSIONS: Our data suggest that at the intra-species level pathogen populations likely maintain divergent sets of effectors capable of targeting the same plant host pathways. This functional redundancy may play an important role in the dynamic of the "arms-race" between host and pathogen serving as the basis for diverse virulence strategies and creating conditions where mutations in certain effector groups will not have a major effect on the pathogen's ability to infect the host.

Quantitative Trait Loci for Slow-Rusting Resistance to Leaf Rust in Doubled-Haploid Wheat Population CI13227 × Lakin.

CI13227 is a U.S. winter wheat line with adult-plant slow-rusting resistance that has been the subject of several studies on the characteristics and components of slow rusting. Previous genetic studies used different populations and approaches and came to different conclusions about the genetic basis of resistance in CI13227. To clarify the situation, a new doubled-haploid (DH) population of CI13227 × Lakin was produced and a linkage map was constructed using 5,570 single-nucleotide polymorphism (SNP) markers derived from wheat 90K SNP assays and 84 simple sequence repeat markers. Three quantitative trait loci (QTL) were identified for three slow-rusting traits on chromosome arms 2DS, 7AL, and 7BL from CI13227. A fourth QTL mapped on chromosome 3BS was from Lakin. The QTL on 2DS, designated QLr.hwwg-2DS, explained 11.2 to 25.6% of the phenotypic variation. It was found in the same position as a slow-rusting QTL in the CI13227 × Suwon 92 population in a previous study and, thus, verified the 2DS QTL. The QTL on chromosome 7BL explained 8.1 and 19.3% of the phenotypic variation and is likely to be Lr68. The other two QTL showed a minor effect on some of the traits evaluated in a single experiment. Flanking SNP closely linked to all QTL were converted to Kompetitive allele-specific polymerase chain reaction markers that can be used in marker-assisted selection to transfer these QTL into adapted wheat cultivars.

TaXA21-A1 on chromosome 5AL is associated with resistance to multiple pests in wheat.

KEY MESSAGE: The wheat ortholog of the rice gene OsXA21 against bacterial leaf blight showed resistance to multiple pests in bread wheat but different interacting proteins. ABSTRACT: A quantitative trait locus QYr.osu-5A on the long arm of chromosome 5A in bread wheat (Triticum aestivum L., 2n = 6x = 42; AABBDD) was previously reported to confer consistent resistance in adult plants to predominant stripe rust races, but the gene causing the quantitative trait locus (QTL) is not known. Single-nucleotide polymorphism (SNP) markers were used to saturate the QTL region. Comparative and syntenic regions between wheat and rice (Oryza sativa) were applied to identify candidate genes for QYr.osu-5A. TaXA21-A1, which is referred to as a wheat ortholog of OsXA21-like gene on chromosome 9 in rice, was mapped under the peak of the QYr.osu-5A. TaXA21-A1 not only explained the phenotypic variation in reaction to different stripe rust races but also showed significant effects on resistance to powdery mildew and Hessian fly biotype BP. The natural allelic variation resulted in the alternations of four amino acids in deduced TaXA21-A1 proteins. The interacting proteins of TaXA21-A1 were different from those identified by OsXA21 on rice chromosome 11 against bacterial leaf blight. TaXA21-A1 confers unique resistance against multiple pests in wheat but might not have common protein interactors or thus overlapping functions with OsXA21 in rice. XA21 function has diverged during evolution of cereal crops. The molecular marker developed for TaXA21-A1 would accelerate its application of the candidate gene at the QYr.osu-5A locus in wheat breeding programs.

Spiked GBS: a unified, open platform for single marker genotyping and whole-genome profiling.

BACKGROUND: In plant breeding, there are two primary applications for DNA markers in selection: 1) selection of known genes using a single marker assay (marker-assisted selection; MAS); and 2) whole-genome profiling and prediction (genomic selection; GS). Typically, marker platforms have addressed only one of these objectives. RESULTS: We have developed spiked genotyping-by-sequencing (sGBS), which combines targeted amplicon sequencing with reduced representation genotyping-by-sequencing. To minimize the cost of targeted assays, we utilize a small percent of sequencing capacity available in runs of GBS libraries to "spike" amplified targets of a priori alleles tagged with a different set of unique barcodes. This open platform allows multiple, single-target loci to be assayed while simultaneously generating a whole-genome profile. This dual-genotyping approach allows different sets of samples to be evaluated for single markers or whole genome-profiling. Here, we report the application of sGBS on a winter wheat panel that was screened for converted KASP markers and newly-designed markers targeting known polymorphisms in the leaf rust resistance gene Lr34. CONCLUSIONS: The flexibility and low-cost of sGBS will enable a range of applications across genetics research. Specifically in breeding applications, the sGBS approach will allow breeders to obtain a whole-genome profile of important individuals while simultaneously targeting specific genes for a range of selection strategies across the breeding program.

Introgression of stem rust resistance genes SrTA10187 and SrTA10171 from Aegilops tauschii to wheat.

Aegilops tauschii, the diploid progenitor of the wheat D genome, is a readily accessible germplasm pool for wheat breeding as genes can be transferred to elite wheat cultivars through direct hybridization followed by backcrossing. Gene transfer and genetic mapping can be integrated by developing mapping populations during backcrossing. Using direct crossing, two genes for resistance to the African stem rust fungus race TTKSK (Ug99), were transferred from the Ae. tauschii accessions TA10187 and TA10171 to an elite hard winter wheat line, KS05HW14. BC2 mapping populations were created concurrently with developing advanced backcross lines carrying rust resistance. Bulked segregant analysis on the BC2 populations identified marker loci on 6DS and 7DS linked to stem rust resistance genes transferred from TA10187 and TA10171, respectively. Linkage maps were developed for both genes and closely linked markers reported in this study will be useful for selection and pyramiding with other Ug99-effective stem rust resistance genes. The Ae. tauschii-derived resistance genes were temporarily designated SrTA10187 and SrTA10171 and will serve as valuable resources for stem rust resistance breeding.

Simultaneous transfer, introgression, and genomic localization of genes for resistance to stem rust race TTKSK (Ug99) from Aegilops tauschii to wheat.

Wheat production is currently threatened by widely virulent races of the wheat stem rust fungus, Puccinia graminis f. sp. tritici, that are part of the TTKSK (also known as 'Ug99') race group. The diploid D genome donor species Aegilops tauschii (2n = 2x = 14, DD) is a readily accessible source of resistance to TTKSK and its derivatives that can be transferred to hexaploid wheat, Triticum aestivum (2n = 6x = 42, AABBDD). To expedite transfer of TTKSK resistance from Ae. tauschii, a direct hybridization approach was undertaken that integrates gene transfer, mapping, and introgression into one process. Direct crossing of Ae. tauschii accessions with an elite wheat breeding line combines the steps of gene transfer and introgression while development of mapping populations during gene transfer enables the identification of closely linked markers. Direct crosses were made using TTKSK-resistant Ae. tauschii accessions TA1662 and PI 603225 as males and a stem rust-susceptible T. aestivum breeding line, KS05HW14, as a female. Embryo rescue enabled recovery of F1 (ABDD) plants that were backcrossed as females to the hexaploid recurrent parent. Stem rust-resistant BC1F1 plants from each Ae. tauschii donor source were used as males to generate BC2F1 mapping populations. Bulked segregant analysis of BC2F1 genotypes was performed using 70 SSR loci distributed across the D genome. Using this approach, stem rust resistance genes from both accessions were located on chromosome arm 1DS and mapped using SSR and EST-STS markers. An allelism test indicated the stem rust resistance gene transferred from PI 603225 is Sr33. Race specificity suggests the stem rust resistance gene transferred from TA1662 is unique and this gene has been temporarily designated SrTA1662. Stem rust resistance genes derived from TA1662 and PI 603225 have been made available with selectable molecular markers in genetic backgrounds suitable for stem rust resistance breeding.

Development and characterization of a compensating wheat-Thinopyrum intermedium Robertsonian translocation with Sr44 resistance to stem rust (Ug99).

The emergence of the highly virulent Ug99 race complex of the stem rust fungus (Puccinia graminis Pers. f. sp. tritici Eriks. and Henn.) threatens wheat (Triticum aestivum L.) production worldwide. One of the effective genes against the Ug99 race complex is Sr44, which was derived from Thinopyrum intermedium (Host) Barkworth and D.R. Dewey and mapped to the short arm of 7J (designated 7J#1S) present in the noncompensating T7DS-7J#1L∙7J#1S translocation. Noncompensating wheat-alien translocations are known to cause genomic duplications and deficiencies leading to poor agronomic performance, precluding their direct use in wheat improvement. The present study was initiated to produce compensating wheat-Th. intermedium Robertsonian translocations with Sr44 resistance. One compensating RobT was identified consisting of the wheat 7DL arm translocated to the Th. intermedium 7J#1S arm resulting in T7DL∙7J#1S. The T7DL∙7J#1S stock was designated as TA5657. The 7DL∙7J#1S stock carries Sr44 and has resistance to the Ug99 race complex. This compensating RobT with Sr44 resistance may be useful in wheat improvement. In addition, we identified an unnamed stem rust resistance gene located on the 7J#1L arm that confers resistance not only to Ug99, but also to race TRTTF, which is virulent to Sr44. However, the action of the second gene can be modified by the presence of suppressors in the recipient wheat cultivars.

Genetic variation for terminal heat stress tolerance in winter wheat.

In many regions worldwide wheat (Triticum aestivum L.) plants experience terminal high temperature stress during the grain filling stage, which is a leading cause for single seed weight decrease and consequently for grain yield reduction. An approach to mitigate high temperature damage is to develop tolerant cultivars using the conventional breeding approach which involves identifying tolerant lines and then incorporating the tolerant traits in commercial varieties. In this study, we evaluated the terminal heat stress tolerance of 304 diverse elite winter wheat lines from wheat breeding programs in the US, Australia, and Serbia in controlled environmental conditions. Chlorophyll content and yield traits were measured and calculated as the percentage of non-stress control. The results showed that there was significant genetic variation for chlorophyll retention and seed weight under heat stress conditions. The positive correlation between the percent of chlorophyll content and the percent of single seed weight was significant. Two possible mechanisms of heat tolerance during grain filling were proposed. One represented by wheat line OK05723W might be mainly through the current photosynthesis since the high percentage of single seed weight was accompanied with high percentages of chlorophyll content and high shoot dry weight, and the other represented by wheat Line TX04M410164 might be mainly through the relocation of reserves since the high percentage of single seed weight was accompanied with low percentages of chlorophyll content and low shoot dry weight under heat stress. The tolerant genotypes identified in this study should be useful for breeding programs after further validation.

Skim exome capture genotyping in wheat.

Next-generation sequencing (NGS) technology advancements continue to reduce the cost of high-throughput genome-wide genotyping for breeding and genetics research. Skim sequencing, which surveys the entire genome at low coverage, has become feasible for quantitative trait locus (QTL) mapping and genomic selection in various crops. However, the genome complexity of allopolyploid crops such as wheat (Triticum aestivum L.) still poses a significant challenge for genome-wide genotyping. Targeted sequencing of the protein-coding regions (i.e., exome) reduces sequencing costs compared to whole genome re-sequencing and can be used for marker discovery and genotyping. We developed a method called skim exome capture (SEC) that combines the strengths of these existing technologies and produces targeted genotyping data while decreasing the cost on a per-sample basis compared to traditional exome capture. Specifically, we fragmented genomic DNA using a tagmentation approach, then enriched those fragments for the low-copy genic portion of the genome using commercial wheat exome baits and multiplexed the sequencing at different levels to achieve desired coverage. We demonstrated that for a library of 48 samples, ∼7-8× target coverage was sufficient for high-quality variant detection. For higher multiplexing levels of 528 and 1056 samples per library, we achieved an average coverage of 0.76× and 0.32×, respectively. Combining these lower coverage SEC sequencing data with genotype imputation using a customized wheat practical haplotype graph database that we developed, we identified hundreds of thousands of high-quality genic variants across the genome. The SEC method can be used for high-resolution QTL mapping, genome-wide association studies, genomic selection, and other downstream applications.

Population genomics of Puccinia graminis f.sp. tritici highlights the role of admixture in the origin of virulent wheat rust races.

Puccinia graminis f.sp. tritici (Pgt) causes stem rust disease in wheat that can result in severe yield losses. The factors driving the evolution of its virulence and adaptation remain poorly characterized. We utilize long-read sequencing to develop a haplotype-resolved genome assembly of a U.S. isolate of Pgt. Using Pgt haplotypes as a reference, we characterize the structural variants (SVs) and single nucleotide polymorphisms in a diverse panel of isolates. SVs impact the repertoire of predicted effectors, secreted proteins involved in host-pathogen interaction, and show evidence of purifying selection. By analyzing global and local genomic ancestry we demonstrate that the origin of 8 out of 12 Pgt clades is linked with either somatic hybridization or sexual recombination between the diverged donor populations. Our study shows that SVs and admixture events appear to play an important role in broadening Pgt virulence and the origin of highly virulent races, creating a resource for studying the evolution of Pgt virulence and preventing future epidemic outbreaks.

Cloning of the broadly effective wheat leaf rust resistance gene Lr42 transferred from Aegilops tauschii.

The wheat wild relative Aegilops tauschii was previously used to transfer the Lr42 leaf rust resistance gene into bread wheat. Lr42 confers resistance at both seedling and adult stages, and it is broadly effective against all leaf rust races tested to date. Lr42 has been used extensively in the CIMMYT international wheat breeding program with resulting cultivars deployed in several countries. Here, using a bulked segregant RNA-Seq (BSR-Seq) mapping strategy, we identify three candidate genes for Lr42. Overexpression of a nucleotide-binding site leucine-rich repeat (NLR) gene AET1Gv20040300 induces strong resistance to leaf rust in wheat and a mutation of the gene disrupted the resistance. The Lr42 resistance allele is rare in Ae. tauschii and likely arose from ectopic recombination. Cloning of Lr42 provides diagnostic markers and over 1000 CIMMYT wheat lines carrying Lr42 have been developed documenting its widespread use and impact in crop improvement.

Analysis of Induction and Establishment of Dwarf Bunt of Wheat Under Marginal Climatic Conditions.

Dwarf bunt caused by Tilletia contraversa is a disease of winter wheat that has a limited geographic distribution due to specific winter climate requirements. The pathogen is listed as a quarantine organism by several countries that may have wheat production areas with inadequate or marginal climate for the disease-in particular the People's Republic of China. Field experiments were conducted in the United States in an area of Kansas that is a climatic analog to the northern winter wheat areas of China to evaluate the risk of disease introduction into such areas. The soil surface of four replicate 2.8 × 9.75 m plots, planted with a highly susceptible cultivar, was inoculated with six teliospore concentrations ranging from 0.88 to 88,400 teliospores/cm2. A single initial inoculation was done in each of three nurseries planted during separate seasons followed by examination for disease for 4 to 6 years afterward. Any diseased spikes produced were crushed and returned to the plots where they were produced. One nursery had no disease during all six seasons. In two nurseries, the disease was induced at trace levels at the three highest inoculation rates. Disease carryover to the second year occurred during one year in one nursery in plots at the highest inoculation rate, but no disease occurred the following three seasons. A duplicate nursery planted in a disease conducive area in Utah demonstrated that the highest rate of inoculum used in the experiments was sufficient to cause almost 100% infection. This study demonstrated that in an area with marginal climatic conditions it was possible to induce transient trace levels of dwarf bunt, but the disease was not established even with a highly susceptible cultivar and high levels of inoculum. Our results support the conclusions of the 1999 Agreement on U.S.-China Agricultural Cooperation which set a tolerance for teliospores in grain, and supports the Risk Assessment Model for Importation of United States Milling Wheat Containing T. contraversa.

Creation and judicious application of a wheat resistance gene atlas.

Disease-resistance (R) gene cloning in wheat (Triticum aestivum) has been accelerated by the recent surge of genomic resources, facilitated by advances in sequencing technologies and bioinformatics. However, with the challenges of population growth and climate change, it is vital not only to clone and functionally characterize a few handfuls of R genes, but also to do so at a scale that would facilitate the breeding and deployment of crops that can recognize the wide range of pathogen effectors that threaten agroecosystems. Pathogen populations are continually changing, and breeders must have tools and resources available to rapidly respond to those changes if we are to safeguard our daily bread. To meet this challenge, we propose the creation of a wheat R-gene atlas by an international community of researchers and breeders. The atlas would consist of an online directory from which sources of resistance could be identified and deployed to achieve more durable resistance to the major wheat pathogens, such as wheat rusts, blotch diseases, powdery mildew, and wheat blast. We present a costed proposal detailing how the interacting molecular components governing disease resistance could be captured from both the host and the pathogen through biparental mapping, mutational genomics, and whole-genome association genetics. We explore options for the configuration and genotyping of diversity panels of hexaploid and tetraploid wheat, as well as their wild relatives and major pathogens, and discuss how the atlas could inform a dynamic, durable approach to R-gene deployment. Set against the current magnitude of wheat yield losses worldwide, recently estimated at 21%, this endeavor presents one route for bringing R genes from the lab to the field at a considerable speed and quantity.

Variation and transgression of aggressiveness among two Gibberella zeae crosses developed from highly aggressive parental isolates.

Gibberella zeae (anamorph: Fusarium graminearum) is the most common cause of Fusarium head blight (FHB) of wheat (Triticum aestivum) worldwide. Aggressiveness is the most important fungal trait affecting disease severity and stability of host resistance. Objectives were to analyze in two field experiments (i) segregation for aggressiveness among 120 progenies from each of two crosses of highly aggressive parents and (ii) stability of FHB resistance of seven moderately to highly resistant winter wheat cultivars against isolates varying for aggressiveness. Aggressiveness was measured as FHB severity per plot, Fusarium exoantigen absorbance, and deoxynivalenol content. In the first experiment, mean FHB ratings were 20 to 49% across environments and progeny. Significant genotypic variation was detected in both crosses (P < 0.01). Isolate-environment interaction explained approximately half of the total variance. Two transgressive segregants were found in cross B across environments. Traits were significantly (P < 0.05) intercorrelated. In the second experiment, despite significant (P < 0.05) genotypic variance for cultivar and isolate, no significant (P > 0.05) interaction was observed for any trait. In conclusion, progeny of highly aggressive parents might exhibit increased aggressiveness due to recombination and may, therefore, adapt nonspecifically to increased quantitative host resistance.

Discovery and mapping of single feature polymorphisms in wheat using Affymetrix arrays.

BACKGROUND: Wheat (Triticum aestivum L.) is a staple food crop worldwide. The wheat genome has not yet been sequenced due to its huge genome size (approximately 17,000 Mb) and high levels of repetitive sequences; the whole genome sequence may not be expected in the near future. Available linkage maps have low marker density due to limitation in available markers; therefore new technologies that detect genome-wide polymorphisms are still needed to discover a large number of new markers for construction of high-resolution maps. A high-resolution map is a critical tool for gene isolation, molecular breeding and genomic research. Single feature polymorphism (SFP) is a new microarray-based type of marker that is detected by hybridization of DNA or cRNA to oligonucleotide probes. This study was conducted to explore the feasibility of using the Affymetrix GeneChip to discover and map SFPs in the large hexaploid wheat genome. RESULTS: Six wheat varieties of diverse origins (Ning 7840, Clark, Jagger, Encruzilhada, Chinese Spring, and Opata 85) were analyzed for significant probe by variety interactions and 396 probe sets with SFPs were identified. A subset of 164 unigenes was sequenced and 54% showed polymorphism within probes. Microarray analysis of 71 recombinant inbred lines from the cross Ning 7840/Clark identified 955 SFPs and 877 of them were mapped together with 269 simple sequence repeat markers. The SFPs were randomly distributed within a chromosome but were unevenly distributed among different genomes. The B genome had the most SFPs, and the D genome had the least. Map positions of a selected set of SFPs were validated by mapping single nucleotide polymorphism using SNaPshot and comparing with expressed sequence tags mapping data. CONCLUSION: The Affymetrix array is a cost-effective platform for SFP discovery and SFP mapping in wheat. The new high-density map constructed in this study will be a useful tool for genetic and genomic research in wheat.

Expression and function of sex pheromones and receptors in the homothallic ascomycete Gibberella zeae.

In heterothallic ascomycete fungi, idiomorphic alleles at the MAT locus control two sex pheromone-receptor pairs that function in the recognition and chemoattraction of strains with opposite mating types. In the ascomycete Gibberella zeae, the MAT locus is rearranged such that both alleles are adjacent on the same chromosome. Strains of G. zeae are self-fertile but can outcross facultatively. Our objective was to determine if pheromones retain a role in sexual reproduction in this homothallic fungus. Putative pheromone precursor genes (ppg1 and ppg2) and their corresponding pheromone receptor genes (pre2 and pre1) were identified in the genomic sequence of G. zeae by sequence similarity and microsynteny with other ascomycetes. ppg1, a homolog of the Saccharomyces alpha-factor pheromone precursor gene, was expressed in germinating conidia and mature ascospores. Expression of ppg2, a homolog of the a-factor pheromone precursor gene, was not detected in any cells. pre2 was expressed in all cells, but pre1 was expressed weakly and only in mature ascospores. ppg1 or pre2 deletion mutations reduced fertility in self-fertilization tests by approximately 50%. Deltappg1 reduced male fertility and Deltapre2 reduced female fertility in outcrossing tests. In contrast, Deltappg2 and Deltapre1 had no discernible effects on sexual function. Deltappg1/Deltappg2 and Deltapre1/Deltapre2 double mutants had the same phenotype as the Deltappg1 and Deltapre2 single mutants. Thus, one of the putative pheromone-receptor pairs (ppg1/pre2) enhances, but is not essential for, selfing and outcrossing in G. zeae whereas no functional role was found for the other pair (ppg2/pre1).

Alignment of genetic and physical maps of Gibberella zeae.

We previously published a genetic map of Gibberella zeae (Fusarium graminearum sensu lato) based on a cross between Kansas strain Z-3639 (lineage 7) and Japanese strain R-5470 (lineage 6). In this study, that genetic map was aligned with the third assembly of the genomic sequence of G. zeae strain PH-1 (lineage 7) using seven structural genes and 108 sequenced amplified fragment length polymorphism markers. Several linkage groups were combined based on the alignments, the nine original linkage groups were reduced to six groups, and the total size of the genetic map was reduced from 1,286 to 1,140 centimorgans. Nine supercontigs, comprising 99.2% of the genomic sequence assembly, were anchored to the genetic map. Eight markers (four markers from each parent) were not found in the genome assembly, and four of these markers were closely linked, suggesting that >150 kb of DNA sequence is missing from the PH-1 genome assembly. The alignments of the linkage groups and supercontigs yielded four independent sets, which is consistent with the four chromosomes reported for this fungus. Two proposed heterozygous inversions were confirmed by the alignments; otherwise, the colinearity of the genetic and physical maps was high. Two of four regions with segregation distortion were explained by the two selectable markers employed in making the cross. The average recombination rates for each chromosome were similar to those previously reported for G. zeae. Despite an inferred history of genetic isolation of lineage 6 and lineage 7, the chromosomes of these lineages remain homologous and are capable of recombination along their entire lengths, even within the inversions. This genetic map can now be used in conjunction with the physical sequence to study phenotypes (e.g., fertility and fitness) and genetic features (e.g., centromeres and recombination frequency) that do not have a known molecular signature in the genome.

A Novel Source of Resistance in Wheat to Pyrenophora tritici-repentis Race 1.

Tan spot, caused by the fungus Pyrenophora tritici-repentis, causes serious yield losses in wheat (Triticum aestivum) and many other grasses. Race 1 of the fungus, which produces the necrosis toxin Ptr ToxA and the chlorosis toxin Ptr ToxC, is the most prevalent race in the Great Plains of the United States. Wheat genotypes with useful levels of resistance to race 1 have been deployed, but this resistance reduces damage by only 50 to 75%. Therefore, new sources of resistance to P. tritici-repentis are needed. Recombinant inbred lines developed from a cross between the Indian spring wheat cvs. WH542 (resistant) and HD29 (moderately susceptible) were evaluated for reaction to race 1 of the fungus. Composite interval mapping revealed quantitative trait loci (QTL) on the short arm of chromosome 3A explaining 23% of the phenotypic variation, and the long arm of chromosome 5B explaining 27% of the variation. Both resistance alleles were contributed by the WH542 parent. The QTL on 5BL is probably tsn1, which was described previously. The 3AS QTL (QTs.ksu-3AS) on 3AS is a novel QTL for resistance to P. tritici-repentis race 1. The QTL region is located in the most distal bin of chromosome 3AS in a 2.2-centimorgan marker interval. Flanking markers Xbarc45 and Xbarc86 are suitable for marker-assisted selection for tan spot resistance.