Avirulence Alleles Research Articles

A new set of international Leptosphaeria maculans isolates as a resource for elucidation of the basis and evolution of blackleg disease on Brassica napus

AbstractA collection of isolates of the fungi Leptosphaeria maculans and L. biglobosa, which cause blackleg disease on Brassica napus (canola/oilseed rape) and other Brassicaceae species, was assembled to represent the global diversity of these pathogens and a resource for international research. The collection consists of 226 isolates (205 L. maculans and 21 L. biglobosa) from 11 countries. The genomes of all 205 L. maculans isolates were sequenced, and the distribution and identity of avirulence gene alleles were determined based on genotypic information and phenotypic reactions on B. napus lines that hosted specific resistance genes. Whilst the frequencies of some avirulence alleles were consistent across each of the regions, others differed dramatically, potentially reflecting the canola/oilseed rape cultivars grown in those countries. Analyses of the single‐nucleotide polymorphism (SNP) diversity within these L. maculans isolates revealed geographical separation of the populations. This "open access" resource provides a standardized set of isolates that can be used to define the basis for how these fungal pathogens cause disease, and as a tool for discovery of new resistance traits in Brassica species.

Biology and Management of Bacterial Spot of Peppers in Oklahoma.

Bacterial spot, an important disease of bell and chili peppers grown in Oklahoma, is caused by Xanthomonas euvesicatoria. We evaluated isolates from 1995 to 2015 (n = 72) for avirulence alleles and race by assessing hypersensitive responses (HRs) on differentials with resistance genes Bs1, Bs2, Bs3, or Bs4. Most isolates (96%) expressed AvrBs2 (races 1, 3, 7, 8), and only three were virulent on Bs2 (race 6). Chili cultivars, grown locally for capsaicin production, were susceptible to all races. Copper-based spray programs were evaluated on bell pepper hybrids with and without Bs2 resistance from 2008 to 2010 and on bell and jalapeño hybrids with Bs2 or Bs1-2-3 from 2017 to 2018. Bs2 and Bs1-2-3 hybrids generally had lower disease and higher yields (≤21 t/ha) than susceptible entries. Copper reduced disease by 20 to 40% and increased yield by an average of 4 t/ha, but yield responses were not always significant (P = 0.05). In August 2018, disease increased to >50% on Bs2 hybrids but remained low on the Bs1-2-3 hybrid. Despite the breakdown of Bs2 resistance, yields of the Bs2 hybrids were not reduced. Avirulence alleles and race of isolates from susceptible, Bs2, and Bs1-2-3 hybrids at the end of the 2018 trial depended on source plant genetics. AvrBs2 was expressed in 86% of isolates from a susceptible hybrid but not in any isolates from the Bs2 and Bs1-2-3 hybrids. HR resistances effectively protected yield, but their deployment may not be sustainable without cultural practices such as crop rotation that limit pathogen survival and transmission to subsequent crops.

Development of a specific marker for detection of a functional AvrLm9 allele and validating the interaction between AvrLm7 and AvrLm9 in Leptosphaeria maculans.

Blackleg, which is caused by the fungus Leptosphaeria maculans (L. maculans), is a major disease of canola in western Canada and worldwide. Long-term use of one source of resistance could cause the breakdown of its effectiveness. Therefore, appropriate use of R genes is very important, and knowledge about the distribution of avirulence genes is a prerequisite for effectively deploying resistance. Of the 14 avirulence genes identified in L. maculans, AvrLm5 and AvrLm9 were recognized as the two alleles of the same gene based on two single nucleotide polymorphisms, C85T and G164A/C. In this study, a specific marker was developed to identify AvrLm5 and AvrLm9 based on two single nucleotide polymorphisms, C85T and G164A/C, which are responsible for the function of AvrLm9. The specific marker can be used to discriminate the AvrLm9 from avrLm9 accurately in L. maculans isolates, which is consistent with inoculation tests in isolates without AvrLm4-7. This specific marker was used to screen 1229 isolates collected from fields in the years 2014 through 2016 in Manitoba. From 68 to 84% of the isolates were found to contain the AvrLm9 allele; while 4-7% of them were avirulent on the variety Goéland with Rlm9 loci. Furthermore, no isolates having both AvrLm9 and AvrLm7 were detected using a cotyledon test, while 67% to 84% of isolates contained both avirulence genes via PCR detection, implying suppression of AvrLm9 by AvrLm7. In addition, avirulence gene profiles of the other 10 avirulence alleles were examined with the 1229 isolates using cotyledon tests or PCR amplifications. Taken together, this research enables the fast identification of AvrLm5/9, provides the Avr genes' landscape of western Canada and elaborates the relationship between AvrLm9 and AvrLm7 using isolates from grower fields.

Discriminant haplotypes of avirulence genes of Phytophthora sojae lead to a molecular assay to predict phenotypes.

The soybean–Phytophthora sojae interaction operates on a gene‐for‐gene relationship, where the product of a resistance gene (Rps) in the host recognizes that of an avirulence gene (Avr) in the pathogen to generate an incompatible reaction. To exploit this form of resistance, one must match with precision the appropriate Rps gene with the corresponding Avr gene. Currently, this association is evaluated by phenotyping assays that are labour‐intensive and often imprecise. To circumvent this limitation, we sought to develop a molecular assay that would reveal the avirulence allele of the seven main Avr genes (Avr1a, Avr1b, Avr1c, Avr1d, Avr1k, Avr3a, and Avr6) in order to diagnose with precision the pathotypes of P. sojae isolates. For this purpose, we analysed the genomic regions of these Avr genes in 31 recently sequenced isolates with different virulence profiles and identified discriminant mutations between avirulence and virulence alleles. Specific primers were designed to generate amplicons of a distinct size, and polymerase chain reaction conditions were optimized in a final assay of two parallel runs. When tested on the 31 isolates of known virulence, the assay accurately revealed all avirulence alleles. The test was further assessed and compared to a phenotyping assay on 25 isolates of unknown virulence. The two assays matched in 97% (170/175) of the interactions studied. Interestingly, the sole cases of discrepancy were obtained with Avr3a, which suggests a possible imperfect interaction with Rps3a. This molecular assay offers a powerful and reliable tool to exploit and study with greater precision soybean resistance against P. sojae.

Characterization of the Race Structure of Leptosphaeria maculans Causing Blackleg of Winter Canola in Oklahoma and Kansas.

Blackleg, caused by the fungus Leptosphaeria maculans, is a widespread disease of winter canola (Brassica napus) in Oklahoma and Kansas. Deployment of genetic resistance is the primary strategy for managing blackleg. Resistance genes (Rlm) in canola interact with avirulence genes in the fungus (AvrLm) in a gene-for-gene manner. Little is known about the diversity and frequency of avirulence genes and the race structure in the region. Isolates of Leptosphaeria spp. were collected from diseased leaves in nine counties in Oklahoma and one county in Kansas from 2009 to 2013. Based on pathogenicity and PCR amplification of mating type and species-specific internal transcribed spacer loci, most isolates (n = 90) were L. maculans. The presence of avirulence genes was evaluated using phenotypic interactions on cotyledons of differential cultivars with Rlm1, Rlm2, Rlm3, and Rlm4 and amplification of AvrLm1, AvrLm4-7, and AvrLm6 by PCR. The avirulence alleles AvrLm6 and AvrLm7 were present in the entire L. maculans population. AvrLm1 was found in 34% of the population, AvrLm2 in 4%, and AvrLm4 in only 1%. A total of five races, defined as combinations of avirulence alleles, were identified that included AvrLm1-2-6-7, AvrLm2-6-7, AvrLm4-6-7, AvrLm1-6-7, and AvrLm6-7. Races virulent on the most Rlm genes, AvrLm1-6-7 at 32% and AvrLm6-7 at 62%, were predominant. Defining the avirulence allele frequency and race structure of L. maculans should be useful for the identification and development of resistant cultivars and hybrids for blackleg management in the region. The results suggest that Rlm6 and Rlm7 would be effective, although their deployment should be integrated with quantitative resistance and cultural practices, such as crop rotation, that limit selection pressure on Rlm genes.

Novel haplotypes and networks of AVR-Pik alleles in Magnaporthe oryzae

BackgroundRice blast disease is one of the most destructive fungal disease of rice worldwide. The avirulence (AVR) genes of Magnaporthe oryzae are recognized by the cognate resistance (R) genes of rice and trigger race-specific resistance. The variation in AVR is one of the major drivers of new races. Detecting the variation in the AVR gene in isolates from a population of Magnaporthe oryzae collected from rice production fields will aid in evaluating the effectiveness of R genes in rice production areas. The Pik gene contains 5 R alleles (Pik, Pikh, Pikp, Pikm and Piks) corresponding to the AVR alleles (AVR-Pik/kh/kp/km/ks) of M. oryzae. The Pik gene specifically recognizes and prevents infections by isolates of M. oryzae that contain AVR-Pik. The molecular variation in AVR-Pik alleles of M. oryzae and Pik alleles of rice remains unclear.ResultsWe studied the possible evolutionary pathways of AVR-Pik alleles by analyzing their DNA sequence variation and assaying their avirulence to the cognate Pik alleles of resistance genes under field conditions in China. The results of PCR products from genomic DNA showed that 278 of the 366 isolates of M. oryzae collected from Yunnan Province, China, carried AVR-Pik alleles. Among the isolates from six regions of Yunnan, 66.7–90.3% carried AVR-Pik alleles. Moreover, 10 AVR-Pik haplotypes encoding five novel AVR-Pik variants were identified among 201 isolates. The AVR-Pik alleles evolved to virulent from avirulent forms via stepwise base substitution. These findings demonstrate that AVR-Pik alleles are under positive selection and that mutations are responsible for defeating race-specific resistant Pik alleles in nature.ConclusionsWe demonstrated the polymorphism and distribution of AVR-Pik alleles in Yunnan Province, China. By pathogenicity assays used to detect the function of the different haplotypes of AVR-Pik, for the first time, we showed the avoidance and stepwise evolution of AVR-Pik alleles in rice production areas of Yunnan. The functional AVR-Pik possesses diversified sequence structures and is under positive selection in nature.

A Six-Year Investigation of the Dynamics of Avirulence Allele Profiles, Blackleg Incidence, and Mating Type Alleles of Leptosphaeria maculans Populations Associated with Canola Crops in Manitoba, Canada.

Blackleg, caused by the fungal pathogen Leptosphaeria maculans, is one of the most economically important diseases of canola (Brassica napus, oilseed rape) worldwide. This study assessed incidence of blackleg, the avirulence allele, and mating type distributions of L. maculans isolates collected in commercial canola fields in Manitoba, Canada, from 2010 to 2015. A total of 956 L. maculans isolates were collected from 2010 to 2015 to determine the presence of 12 avirulence alleles using differential canola cultivars and/or PCR assays specific for each avirulence allele. AvrLm2, AvrLm4, AvrLm5, AvrLm6, AvrLm7, AvrLm11, and AvrLmS were detected at frequencies ranging from 97 to 33%, where the AvrLm1, AvrLm3, AvrLm9, AvrLepR1, and AvrLepR2 alleles were the least abundant. When the race structure was examined, a total of 170 races were identified among the 956 isolates, with three major races, AvrLm-2-4-5-6-7-11, AvrLm-2-4-5-6-7-11-S, and Avr-1-4-5-6-7-11-(S) accounting for 15, 10, and 6% of the total fungal population, respectively. The distribution of the mating type alleles (MAT1-1 and MAT1-2) indicated that sexual reproduction was not inhibited in any of the nine Manitoba regions in any of the years L. maculans isolates were collected.

Insights into fighting against blackleg disease of Brassica napus in Canada

Blackleg disease, caused by the ascomycete fungal pathogen Leptosphaeria maculans, is a devastating disease of canola (Brassica napus) in Australia, Canada and Europe. Although cultural strategies such as crop rotation, fungicide application, and tillage are adopted to control the disease, the most promising disease control strategy is the utilisation of resistant canola varieties. However, field populations of L. maculans display a high evolutionary potential and are able to overcome major resistance genes within a few years, making disease control relying on resistant varieties challenging. In the early 1990s, blackleg resistance gene Rlm3 was introduced into Canadian canola varieties and provided good resistance against the fungal populations until the early 2000s, when moderate to severe blackleg outbreaks were observed in some areas across western Canada. However, the breakdown of Rlm3 resistance was not reported until recently, based on studies on R genes present in Canadian canola varieties and the avirulence allele frequency in L. maculans populations in western Canada. The fact that Rlm3 was overcome by the evolution of fungal populations demands canola breeding programs in Canada to be prepared to develop canola varieties with diversified and efficient R genes. In addition, frequent monitoring of fungal populations can provide up-to-date guidance for proper resistance genes deployment. This literature review provides insights into the outbreaks and management of blackleg disease in Canada.

Changes in allele frequencies of avirulence genes in the blackleg fungus, Leptosphaeria maculans, over two decades in Australia

Cultivation of canola (oilseed rape, rapeseed; Brassica napus) in many parts of the world relies on the use of cultivars carrying resistance genes that recognise avirulence products of the major canola pathogen, Leptosphaeria maculans. However, widespread cultivation of plants with such resistance provides the potential for evolution of the pathogen population to overcome resistance by altering the proportion of avirulence v. virulence alleles. In this study, the frequencies of avirulence genes were measured for 2091 Australian isolates dating from the late 1980s to present. Frequencies of avirulence genes changed over time. Analysis of isolates from the Eyre Peninsula, where canola is intensively cultivated, indicated that changes in allele frequencies at the AvrLm1, AvrLm4 and AvrLm6 loci could be correlated with the widespread planting of cultivars carrying specific resistance genes. These data show that determining avirulence allele frequencies in L. maculans populations provides power to anticipate which cultivars will be most successful in future growing seasons.

Race spectra of Leptosphaeria maculans, the causal agent of blackleg disease of oilseed rape, in different geographic regions in northern Germany

The fungal pathogen Leptosphaeria maculans (LM), causing blackleg in oilseed rape (OSR), develops gene-for-gene interactions with OSR cultivars. Thus, virulent races can cause severe OSR yield losses in genotypes that depend only on the major resistance genes. LM race spectra were determined using 644 isolates collected from four OSR growing areas in northern Germany in autumn 2011 and 2012. Avirulence alleles (AvrLm) and race spectra of the isolates was identified with OSR cultivars harbouring the major genes Rlm1, Rlm2, Rlm3, Rlm4, Rlm7, and Rlm9. Thirteen LM races were determined, whereby 85 % of the collected isolates belonged to race AvrLm(5)-(6)-7-(8) (virulent to Rlm1–4 and Rlm9) and 10 % belonged to race AvrLm1-(5)-(6)-7-(8) (virulent to Rlm2–4 and Rlm9). Only six of 644 isolates (0.9 %) showed virulence to Rlm7. Using tester isolates that identify Rlm7, we showed that Rlm7 was absent in the most sown cultivars in northern Germany, explaining the low frequency of isolates being virulent to Rlm7. However, blackleg disease incidence (DI) increased significantly in the cultivar Exocet (harbouring Rlm7) between 2011/12 and 2012/13, whereas DI was the same in a susceptible control cultivar across all sampled regions and years. Our results show that Rlm7 is the only known major resistance gene in German OSR cultivars that is still effective against LM, but increasing deployment in commercial cultivars may foster resistance breakdown in the future.

Breakdown of Rlm3 resistance in the Brassica napus–Leptosphaeria maculans pathosystem in western Canada

Blackleg disease, caused by the fungal pathogen Leptosphaeria maculans, is a serious disease of Brassica napus. The disease is mainly controlled by genetic resistance and crop rotation. However, L. maculans has displayed a high evolutionary potential to overcome major resistance genes in B. napus. This study aimed to analyze the major-gene and adult-plant resistance (APR) of Canadian B. napus varieties/lines (accessions) and the avirulence allele frequency in L. maculans populations in western Canada. For resistance identification, a set of L. maculans isolates with known avirulence genes were used to characterize major resistance (R) genes in 104 Canadian B. napus accessions and 102 seed samples collected from growers’ fields; with 104 B. napus accessions further evaluated for APR under controlled conditions. In addition, avirulence genes of 300 L. maculans isolates collected from infected canola stubbles in growers’ fields were determined by cotyledon inoculation and gene-specific PCR assays. The results indicated that R genes were present in the majority of these B. napus accessions, with the Rlm3 gene being predominant while other R genes were rarely detected. APR was identified in more than 50 % of the accessions. Predominance of Rlm3 in 102 seed samples from growers’ fields suggested Rlm3-carrying B. napus varieties were currently widely used in western Canada. Avirulence allele frequency identification of field L. maculans isolates revealed the scarcity of the avirulence allele towards Rlm3, AvrLm3. This indicated the breakdown of Rlm3 resistance, which could be due to the over use of this single resistance gene in Canadian B. napus germplasm.

Virulence Characterization of Venturia inaequalis Reference Isolates on the Differential Set of Malus Hosts.

A set of differential hosts has recently been identified for 17 apple scab resistance genes in an updated system for defining gene-for-gene (GfG) relationships in the Venturia inaequalis-Malus pathosystem. However, a set of reference isolates characterized for their complementary avirulence alleles is not yet available. In this paper, we report on improving the set of differential hosts for h(7) and propose the apple genotype LPG3-29 as carrying the single major resistance gene Rvi7. We characterized a reference set of 23 V. inaequalis isolates on 14 differential apple hosts carrying major resistance genes under controlled conditions. We identified isolates that were virulent on at least one of the following defined resistance gene hosts: h(1), h(2), h(3), h(4), h(5), h(6), h(7), h(8), h(9), h(10), and h(13). Sixteen different virulence patterns were observed. In general, the isolates carried one to three virulences, but some of them were more complex, with up to six virulences. This set of well-characterized isolates will be helpful for the identification of additional apple scab resistance genes in apple germplasm and the characterization of new GfG relationships to help improve our understanding of the host-pathogen interactions in the V. inaequalis-Malus pathosystem.

Cryptic virulence and avirulence alleles revealed by controlled sexual recombination in pea aphids.

Although aphids are worldwide crop pests, little is known about aphid effector genes underlying virulence and avirulence. Here we show that controlling the genetics of both aphid and host can reveal novel recombinant genotypes with previously undetected allelic variation in both virulence and avirulence functions. Clonal F1 progeny populations were derived from reciprocal crosses and self-matings between two parental genotypes of pea aphid (Acyrthosiphon pisum) differing in virulence on a Medicago truncatula host carrying the RAP1 and RAP2 resistance genes. These populations showed Mendelian segregation consistent with aphid performance being controlled largely by a dominant virulence allele derived from only one parent. Altered segregation ratios on near-isogenic host genotypes differing in the region carrying RAP1 were indicative of additional heritable functions likely related to avirulence genes originating from both parents. Unexpectedly, some virulent F1 progeny were recovered from selfing of an avirulent parent, suggesting a reservoir of cryptic alleles. Host chlorosis was associated with virulence, whereas necrotic hypersensitive-like response was not. No maternal inheritance was found for any of these characteristics, ruling out sex-linked, cytoplasmic, and endosymbiotic factors. Our results demonstrate the tractability of dissecting the genetic basis of pest-host resistance mechanisms and indicate that the annual sexual cycle in aphids may lead to frequent novel genotypes with both increased and decreased virulence. Availability of genomes for both pest and host can facilitate definition of cognate gene-for-gene relationships, potentially leading to selection of crop genotypes with multiple resistance traits.

Molecular screening for avirulence alleles AvrLm1 and AvrLm6 in airborne inoculum of Leptosphaeria maculans and winter oilseed rape (Brassica napus) plants from Poland and the UK.

A combination of staining, light microscopy and SYBR green- and dual-labelled fluorescent probe-based qPCR chemistries with species- and gene-specific primers was employed to evaluate fluctuations in the aerial biomass of Leptosphaeria maculans spores captured by volumetric spore trappings in Poznan, Poland (2006, 2008) and Harpenden, UK (2002, 2006). Arising from these surveys, DNA samples extracted from Burkard spore-trap tapes were screened for fluctuation patterns in the frequencies of AvrLm1 and AvrLm6, the most prominent of the 15 genes that code for avirulence effectors in this Dothideomycete cause of the destructive phoma stem canker disease of oilseed rape worldwide. In Poznan, very low frequencies of AvrLm1 allele were found in the autumn of both 2006 and 2008, reflecting significantly increased cultivation of rape seed with Rlm1-based resistance. In contrast, at least six folds-higher frequencies of AvrLm6, which were also confirmed by end-point PCR bioassays on phoma-infected leaves from the same region of Poland, were obtained during both years. In the UK, however, relatively higher AvrLm1 allele titres were found in L. maculans spores captured in air samples from the autumn of 2002 on the experimental fields of Rothamsted Research, Harpenden, that were historically sown to genetically heterogeneous B. napus cultivars. In the 2006 screen these levels had plummeted, to a 1:4 ratio, in favour of frequencies of the AvrLm6 allele. Patterns of fluctuations in erg11 (CYP51) fragments coding for sterol 14α-demethylase suggest October as the month with the most viable wind-dispersed L. maculans propagules of each season of the screens.

A dominant avirulence gene in Orobanche cumana triggers Or5 resistance in sunflower

SummaryOrobanche cumana is a weed that grows as a root parasite on sunflower. In general, the O. cumana–sunflower parasitic system is regarded to follow the gene‐for‐gene model, although this has never been demonstrated at the genetic level in O. cumana. The Or5 dominant gene in sunflower confers resistance to O. cumana race E, but not to race F. The objective of this research was to study the inheritance of avirulence/virulence in crosses between plants of O. cumana lines classified as races E and F. Four race E and three race F lines were developed, from which four race E × race F cross‐combinations were made, in three cases including reciprocals. In all cases, F1 seeds did not have the ability to parasitise sunflower line P‐1380 carrying the Or5 gene, indicating dominance of race E avirulence allele(s). Five F2 populations comprising a total of 387 F2:3 families were evaluated on sunflower line P‐1380. In all cases, one‐fourth of the F2:3 families did not possess the ability to parasitise P‐1380 plants, suggesting that race E avirulence and race F virulence on P‐1380 are allelic and controlled by a single locus. This study demonstrated the gene‐for‐gene interaction in the O. cumana–sunflower parasite system and provided useful information to identify genes involved in O. cumana virulence. The approach followed in this research can contribute to define precisely races of the parasite on the basis of the presence of avirulence genes.

Pathogen populations evolve to greater race complexity in agricultural systems--evidence from analysis of Rhynchosporium secalis virulence data.

Fitness cost associated with pathogens carrying unnecessary virulence alleles is the fundamental assumption for preventing the emergence of complex races in plant pathogen populations but this hypothesis has rarely been tested empirically on a temporal and spatial scale which is sufficient to distinguish evolutionary signals from experimental error. We analyzed virulence characteristics of ∼1000 isolates of the barley pathogen Rhynchosporium secalis collected from different parts of the United Kingdom between 1984 and 2005. We found a gradual increase in race complexity over time with a significant correlation between sampling date and race complexity of the pathogen (r20 = 0.71, p = 0.0002) and an average loss of 0.1 avirulence alleles (corresponding to an average gain of 0.1 virulence alleles) each year. We also found a positive and significant correlation between barley cultivar diversity and R. secalis virulence variation. The conditions assumed to favour complex races were not present in the United Kingdom and we hypothesize that the increase in race complexity is attributable to the combination of natural selection and genetic drift. Host resistance selects for corresponding virulence alleles to fixation or dominant frequency. Because of the weak fitness penalty of carrying the unnecessary virulence alleles, genetic drift associated with other evolutionary forces such as hitch-hiking maintains the frequency of the dominant virulence alleles even after the corresponding resistance factors cease to be used.

A Reproductive Fitness Cost Associated With Hessian Fly (Diptera: Cecidomyiidae) Virulence to Wheat's <I>H</I> Gene-Mediated Resistance

We studied whether adaptation of the Hessian fly, Mayetiola destructor (Say) (Diptera: Cecidomyiidae), to plant resistance incurs fitness costs. In this gene-for-gene interaction, adaptation to a single H resistance gene occurs via loss of a single effector encoded by an Avirulence gene. By losing the effector, the adapted larva now survives on the H gene plant, presumably because it evades the plant's H gene-mediated surveillance system. The problem is the Hessian fly larva needs its effectors for colonization. Thus, for adapted individuals, there may be a cost for losing the effector, with this then creating a trade-off between surviving on H-resistant plants and growing on plants that lack H genes. In two different tests, we used wheat lacking H genes to compare the survival and growth of a nonadapted strain to two H-adapted strains. The two adapted strains differed in that one had been selected for adaptation to H9, whereas the other strain had been selected for adaptation to H13. Tests showed that two H-adapted strains were similar to the nonadapted strain in egg-to-adult survival but that they differed in producing adults with smaller wings. By using known relationships between wing length and reproductive potential, we found that losses in wing length underestimate losses in reproductive potential. For example, H9- and H13-adapted females had 9 and 3% wing losses, respectively, but they were estimated to have 32 and 12% losses in egg production. Fitness costs of adaptation will be investigated further via selection experiments comparing Avirulence allele frequencies for Hessian fly populations exposed or not exposed to H genes.

Spatial heterogeneity, frequency-dependent selection and polymorphism in host-parasite interactions

BackgroundGenomic and pathology analysis has revealed enormous diversity in genes involved in disease, including those encoding host resistance and parasite effectors (also known in plant pathology as avirulence genes). It has been proposed that such variation may persist when an organism exists in a spatially structured metapopulation, following the geographic mosaic of coevolution. Here, we study gene-for-gene relationships governing the outcome of plant-parasite interactions in a spatially structured system and, in particular, investigate the population genetic processes which maintain balanced polymorphism in both species.ResultsFollowing previous theory on the effect of heterogeneous environments on maintenance of polymorphism, we analysed a model with two demes in which the demes have different environments and are coupled by gene flow. Environmental variation is manifested by different coefficients of natural selection, the costs to the host of resistance and to the parasite of virulence, the cost to the host of being diseased and the cost to an avirulent parasite of unsuccessfully attacking a resistant host. We show that migration generates negative direct frequency-dependent selection, a condition for maintenance of stable polymorphism in each deme. Balanced polymorphism occurs preferentially if there is heterogeneity for costs of resistance and virulence alleles among populations and to a lesser extent if there is variation in the cost to the host of being diseased. We show that the four fitness costs control the natural frequency of oscillation of host resistance and parasite avirulence alleles. If demes have different costs, their frequencies of oscillation differ and when coupled by gene flow, there is amplitude death of the oscillations in each deme. Numerical simulations show that for a multiple deme island model, costs of resistance and virulence need not to be present in each deme for stable polymorphism to occur.ConclusionsOur theoretical results confirm the importance of empirical studies for measuring the environmental heterogeneity for genetic costs of resistance and virulence alleles. We suggest that such studies should be developed to investigate the generality of this mechanism for the long-term maintenance of genetic diversity at host and parasite genes.

PWT1, an Avirulence Gene of Magnaporthe oryzae Tightly Linked to the rDNA Locus, Is Recognized by Two Staple Crops, Common Wheat and Barley

The pathogenicity to wheat (Pwt1) locus conditions host species specificity of Magnaporthe oryzae on wheat. GFSI1-7-2 (Setaria isolate) carries the avirulence allele (PWT1) at this locus while Br48 (Triticum isolate) carries the virulence allele (pwt1). An F(1) culture derived from a cross between GFSI1-7-2 and Br48 was backcrossed with Br48 to produce a tester population in which PWT1 alone segregated. When hexaploid wheat lines were inoculated with the BC(1)F(1) testers, they were all resistant to all PWT1 carriers and susceptible to all pwt1 carriers, suggesting that they recognize PWT1. When barley cultivars were inoculated with the testers, they showed the same pattern of reactions as the hexaploid lines, suggesting that the barley cultivars also recognize PWT1. These results suggest that PWT1 is a fundamental gene that universally conditions the avirulence of Setaria isolates on two staple crops, hexaploid wheat and barley. Interestingly, tetraploid wheat lines did not recognize PWT1. Molecular mapping using the F(1) and BC(1)F(1) populations revealed that the Pwt1 locus is located on chromosome 2 and tightly linked to the ribosomal DNA locus and a telomere.

Frequency of avirulence genes in Leptosphaeria maculans in western Canada

Changes in pathogenicity of populations of Leptosphaeria maculans, the cause of blackleg disease of canola and other Brassica spp., have been observed in western Canada. These changes in the population are believed to have resulted from the use of specific resistance (Rlm) genes in Brassica spp., many of which have only recently been identified. We determined the frequency of 10 avirulence alleles for 96 isolates of L. maculans collected between 1997 and 2005 from Alberta, Saskatchewan and Manitoba, Canada from infested canola stubble in field plots or farmers’ fields. Cotyledon interaction phenotypes were scored after inoculation of each isolate on a differential set of Brassica varieties or lines carrying the known resistance genes LepR3 and Rlm1 to Rlm10, except for Rlm8. The avirulence (AvrLm) alleles present in each isolate were inferred from the data. Due to the presence of confounding resistance genes in the host differentials, AvrLm1 and AvrLm2 could not be determined in nine of the isolates, AvrLm6 in two isolates and AvrLepR3 in 49. Based on the number of isolates in which the avirulence genes were confirmed, AvrLm1 was detected in 46.0% and AvrLm9 in 60.4% of the isolates in this collection. A high proportion of isolates carried AvrLm2 (96.6%) and AvrLepR3 (97.9%), and all isolates carried AvrLm6 and AvrLm10. The proportion of L. maculans isolates carrying AvrLm3, AvrLm4, AvrLm5 and AvrLm7 was much lower and varied between 10.4 and 29.2%. In total, 16 races of L. maculans were identified based on 10 avirulence alleles, with seven races accounting for 89.7% of the isolates. Information on the avirulence frequency in the pathogen population will be useful for development of resistance management strategies to control blackleg disease in B. napus oilseed rape in western Canada.