Puccinia striiformis f. sp. tritici, the wheat stripe rust pathogen, is a dikaryotic, biotrophic, and macrocyclic fungus. Genetic study of P. striiformis f. sp. tritici virulence was not possible until the recent discovery of Berberis spp. and Mahonia spp. as alternate hosts. To determine inheritance of virulence and map virulence genes, a segregating population of 119 isolates was developed by self-fertilizing P. striiformis f. sp. tritici isolate 08-220 (race PSTv-11) on barberry leaves under controlled greenhouse conditions. The progeny isolates were phenotyped on a set of 29 wheat lines with single genes for race-specific resistance and genotyped with simple sequence repeat (SSR) markers, single nucleotide polymorphism (SNP) markers derived from secreted protein genes, and SNP markers from genotyping-by-sequencing (GBS). Using the GBS technique, 10,163 polymorphic GBS-SNP markers were identified. Clustering and principal component analysis grouped these markers into six genetic groups, and a genetic map, consisting of six linkage groups, was constructed with 805 markers. The six clusters or linkage groups resulting from these analyses indicated a haploid chromosome number of six in P. striiformis f. sp. tritici. Through virulence testing of the progeny isolates, the parental isolate was found to be homozygous for the avirulence loci corresponding to resistance genes Yr5, Yr10, Yr15, Yr24, Yr32, YrSP, YrTr1, Yr45, and Yr53 and homozygous for the virulence locus corresponding to resistance gene Yr41. Segregation was observed for virulence phenotypes in response to the remaining 19 single-gene lines. A single dominant gene or two dominant genes with different nonallelic gene interactions were identified for each of the segregating virulence phenotypes. Of 27 dominant virulence genes identified, 17 were mapped to two chromosomes. Markers tightly linked to some of the virulence loci may facilitate further studies to clone these genes. The virulence genes and their inheritance information are useful for understanding the host-pathogen interactions and for selecting effective resistance genes or gene combinations for developing stripe rust resistant wheat cultivars.

BackgroundPlant fungal pathogens can rapidly evolve and adapt to new environmental conditions in response to sudden changes of host populations in agro-ecosystems. However, the genomic basis of their host adaptation, especially at the forma specialis level, remains unclear.ResultsWe sequenced two isolates each representing Puccinia striiformis f. sp. tritici (Pst) and P. striiformis f. sp. hordei (Psh), different formae speciales of the stripe rust fungus P. striiformis highly adapted to wheat and barley, respectively. The divergence of Pst and Psh, estimated to start 8.12 million years ago, has been driven by high nucleotide mutation rates. The high genomic variation within dikaryotic urediniospores of P. striiformis has provided raw genetic materials for genome evolution. No specific gene families have enriched in either isolate, but extensive gene loss events have occurred in both Pst and Psh after the divergence from their most recent common ancestor. A large number of isolate-specific genes were identified, with unique genomic features compared to the conserved genes, including 1) significantly shorter in length; 2) significantly less expressed; 3) significantly closer to transposable elements; and 4) redundant in pathways. The presence of specific genes in one isolate (or forma specialis) was resulted from the loss of the homologues in the other isolate (or forma specialis) by the replacements of transposable elements or losses of genomic fragments. In addition, different patterns and numbers of telomeric repeats were observed between the isolates.ConclusionsHost adaptation of P. striiformis at the forma specialis level is a complex pathogenic trait, involving not only virulence-related genes but also other genes. Gene loss, which might be adaptive and driven by transposable element activities, provides genomic basis for host adaptation of different formae speciales of P. striiformis.Electronic supplementary materialThe online version of this article (10.1186/s12864-018-5041-y) contains supplementary material, which is available to authorized users.

Stripe (yellow) rust, caused by Puccinia striiformis f. sp. tritici (Pst), is one of the most destructive diseases of wheat worldwide. Planting resistant cultivars is an effective way to control this disease, but race-specific resistance can be overcome quickly due to the rapid evolving Pst population. Studying the pathogenicity mechanisms is critical for understanding how Pst virulence changes and how to develop wheat cultivars with durable resistance to stripe rust. We re-sequenced 7 Pst isolates and included additional 7 previously sequenced isolates to represent balanced virulence/avirulence profiles for several avirulence loci in seretome analyses. We observed an uneven distribution of heterozygosity among the isolates. Secretome comparison of Pst with other rust fungi identified a large portion of species-specific secreted proteins, suggesting that they may have specific roles when interacting with the wheat host. Thirty-two effectors of Pst were identified from its secretome. We identified candidates for Avr genes corresponding to six Yr genes by correlating polymorphisms for effector genes to the virulence/avirulence profiles of the 14 Pst isolates. The putative AvYr76 was present in the avirulent isolates, but absent in the virulent isolates, suggesting that deleting the coding region of the candidate avirulence gene has produced races virulent to resistance gene Yr76. We conclude that incorporating avirulence/virulence phenotypes into correlation analysis with variations in genomic structure and secretome, particularly presence/absence polymorphisms of effectors, is an efficient way to identify candidate Avr genes in Pst. The candidate effector genes provide a rich resource for further studies to determine the evolutionary history of Pst populations and the co-evolutionary arms race between Pst and wheat. The Avr candidates identified in this study will lead to cloning avirulence genes in Pst, which will enable us to understand molecular mechanisms underlying Pst-wheat interactions, to determine the effectiveness of resistance genes and further to develop durable resistance to stripe rust.

Puccinia striiformis causes stripe rust on wheat, barley, and grasses. Natural population studies have indicated that somatic recombination plays a possible role in P. striiformis variation. To determine whether somatic recombination can occur, susceptible wheat or barley plants were inoculated with mixed urediniospores of paired isolates of P. striiformis. Progeny isolates were selected by passing through a series of inoculations of wheat or barley genotypes. Potential recombinant isolates were compared with the parental isolates on the set of 18 wheat or 12 barley genotypes that are used to differentiate races of P. striiformis f. sp. tritici (the wheat stripe rust pathogen) and P. striiformis f. sp. hordei (the barley stripe rust pathogen), respectively, for virulence changes. They were also tested with 51 simple-sequence repeat and 90 single-nucleotide polymorphism markers for genotype changes. From 68 possible recombinant isolates obtained from nine combinations of isolates based on virulence tests, 66 were proven to be recombinant isolates by molecular markers. Various types of recombinants were determined, including lost virulence from both virulent parental isolates, gained virulence from both avirulent isolates, combined virulences from both parents, and inherited virulence from one parent and avirulence from another. Marker data indicate that most of the recombinants were produced through chromosome reassortment and crossover after the hybridization of two parental isolates. The results demonstrate that somatic recombination is a mechanism by which new variants can be generated in P. striiformis.