嗜卷书虱低氧耐性品系的选育及其遗传稳定性和生态适合度研究

室内恒温条件下以人工饲料饲养的嗜卷书虱Ｌｉｐｏｓｃｅｌｉｓ ｂｏｓｔｒｙｃｈｏｐｈｉｌａ Ｂａｄｏｎｎｅｌ成虫,分别在０．５％和１％Ｏ２组配的气调环境中以７％左右的选择压力处理,以选育其耐低氧气调性。选育至３０低氧品系ＬＯＣ１和ＬＯＣ２,以ＬＴ５０为标准其性倍数分别达４．７和３．９倍。整个选育过程中,气调暴露时间对数值与死亡率机率值间的回归直线基本平行,其值均低于敏感基线的斜率值,这表明嗜卷书虱对     

嗜卷书虱高CO_2抗性品系的选育及其适合度研究(英文)

室内恒温条件下以人工饲料饲养的嗜卷书虱 Ｌｉｐｏｓｃｅｌｉｓ ｂｏｓｔｒｙｃｈｏｐｈｉｌａ Ｂａｄｏｎｎｅｌ成虫,分别在３５％和５５％ＣＯ_２。（２１％Ｏ_２,Ｎ_２作为平衡气体使用）组配的气调环境中以７０％的选择压力处理,以选育其抗高ＣＯ_２气调性。选育至３０代获得抗性品系ＨＣＣ_１和ＨＣＣ_２,以ＬＴ_５０为标准,其抗性倍数分别达４．６和５．３倍。整个选育过程中,气调暴露时间对数值与死亡率机率值间的回归直线基本平行,且斜率值均低于敏感基线的斜率值,这表明嗜卷书虱对高ＣＯ_２抗性发展的基因潜能至第３０代仍未耗尽。两个抗性品系的抗性均不太稳定,去除选择压力后抗性显著下降,且两抗性品系均具有繁殖不利性即适合度缺陷。以净增值率为指标衡量,ＨＣＣ_１和ＨＣＣ_２相对于敏感品系分别具有０．５２和０．４５的适合度。     

Adaptation to the fitness costs of antibiotic resistance in Escherichia coli.

Policies aimed at alleviating the growing problem of drug-resistant pathogens by restricting antimicrobial usage implicitly assume that resistance reduces the Darwinian fitness of pathogens in the absence of drugs. While fitness costs have been demonstrated for bacteria and viruses resistant to some chemotherapeutic agents, these costs are anticipated to decline during subsequent evolution. This has recently been observed in pathogens as diverse as HIV and Escherichia coli. Here we present evidence that these gentic adaptations to the costs of resistance can virtually preclude resistant lineages from reverting to sensitivity. We show that second site mutations which compensate for the substantial (14 and 18% per generation) fitness costs of streptomycin resistant (rpsL) mutations in E. coli create a genetic background in which streptomycin sensitive, rpsL+ alleles have a 4-30% per generation selective disadvantage relative to adapted, resistant strains. We also present evidence that similar compensatory mutations have been fixed in long-term streptomycin-resistant laboratory strains of E. coli and may account for the persistence of rpsL streptomycin resistance in populations maintained for more than 10,000 generations in the absence of the antibiotic. We discuss the public health implications of these and other experimental results that question whether the more prudent use of antimicrobial chemotherapy will lead to declines in the incidence of drug-resistant pathogenic microbes.     

Control of Rhizoctonia solani and Cotton Seedling Disease by Laetisaria arvalis and a Mycophagous Insect Proisotoma minuta (Collembola)

Population changes in Pseudocercosporella herpotrichoides with and without fungicide selection A conidium mixture of W (wheat-type) and R (rye-type) strains of Pseudocercosporella herpotrichoides (W:R = 1:1) was incubated on PD-agar without fungicide or amended with the imidazole prochloraz or the triazole cyproconazole. The W:R ratio was assessed over four generations using the different benomyl sensitivity of the strains as marker. In untreated mixtures, the R strain completely dominated the population already after two generations. Similarly, in cyproconazole-treated mixtures, the R strain increased to 100% after the second generation, whereas in prochloraz-treated mixtures the R strain either followed the same pattern as in untreated mixtures or fluctuated around the initial proportion of 50% over four generations. The W and R strains displayed different fitness properties. The average hyphal growth rate was 8.5 mm/10 d and 4.5 mm/10 d for the W and the R strains, respectively; the sporulation capacity was 80 and 400 conidia/colony, respectively, for the two strains. The different fitness resulted in a strong dominance of the R strains both in vitro and in field populations towards the end of the vegetation period. This shift towards R-dominance occurred about equally fast with or without fungicide selection.     

Reduced Genetic Load Revealed by Slow Inbreeding in Drosophila Melanogaster

The rate of decline in reproductive fitness in populations of Drosophilia melanogaster inbred at an initial rate of ~1% per generation has been investigated under both competitive and noncompetitive conditions. Breeding population size was variable in the inbred lines with an estimated harmonic mean of 66.7 +/- 2.2. Of the 60 lines maintained without reserves, 75% survived a period of 210 generations of slow inbreeding and were then rapidly inbred by full-sib mating to near-homozygosity. The initial rate of inbreeding was estimated to be 0.96 +/- 0.16% per generation, corresponding to an effective population size of ~50. However, the rate of inbreeding declined significantly with time to average only 0.52 +/- 0.08% per generation over the 210 generation period, most likely due to associative overdominance built up by genetic sampling and selection in the small populations. The total inbreeding depression in fitness was estimated to be 87 +/- 3% for competitive ability and 27 +/- 5% for fitness under uncrowded conditions, corresponding to rates of decline of 2.0 +/- 0.3 and 0.32 +/- 0.07%, respectively, per 1% increase in the inbreeding coefficient. The frequency of lethal second chromosomes in the resultant near-homozygous lines was of the order of 5%, lethal free second chromosomes showed a mean viability under both crowded and uncrowded conditions of ~95%, and their population cage fitness was 60% that of Cy/+ heterozygotes. It can be concluded that homozygous genotypes from which deleterious genes of major effect have been eliminated during slow inbreeding may show far less depression in reproductive fitness than suggested by earlier studies of wild chromosome homozygotes. The loss in fitness due to homozygosity throughout the entire genome may be as little as 85-90% under competitive conditions, and 25-30% in an optimal environment.     

Pest control by the introduction of a conditional lethal trait on multiple loci: potential, limitations, and optimal strategies

Advances in genetics have made it feasible to genetically engineer insect strains carrying a conditional lethal trait on multiple loci. We model the release into a target pest population of insects carrying a dominant and fully penetrant conditional lethal trait on 1-20 loci. Delaying the lethality for several generations after release allows the trait to become widely spread in the target population before being activated. To determine effectiveness and optimal strategies for such releases, we vary release size, number of generations until the conditional lethality, nonconditional fitness cost resulting from gene insertions, and fitness reduction associated with laboratory rearing. We show that conditional lethal releases are potentially orders of magnitude more effective than sterile male releases of equal size, and that far smaller release sizes may be required for this approach than necessary with sterile males. For example, a release of male insects carrying a conditional lethal allele that is activated in the F4 generation on 10 loci reduces the target populatioin to 10(-4) of no-release size if there are initially two released males for every wild male. We show how the effectiveness of conditional lethal releases decreases as the nonconditional fitness reduction (i.e., fitness reduction before the trait becomes lethal) associated with the conditional lethal genes increases. For example, if there is a 5% nonconditional fitness cost per conditional lethal allele, then a 2:1 (released male:wild male) release with conditional lethal alleles that are activated in the F4 generation reduces the population to 2-5% (depending on the degree of density dependence) of the no-release size. If there is a per-allele reduction in fitness, then as the number of loci is increased there is a trade-off between the fraction of offspring carrying at least one conditional lethal allele and the fitness of the released insects. We calculate the optimal number of loci on which to insert the conditional lethal gene given various conditions. In addition, we show how laboratory-rearing fitness costs, density-dependence, and all-male versus male-female releases affect the efficiency of conditional lethal releases.     

SELECTION OF RESISTANCE STRAINS IN LIPOSCELIS BOSTRYCHOPHILA BADONNEL TO CO2-ENRICHED ATMOSPHERES*

Abstract Adults of the psocid Liposcelis bostrychophila Badonnel were exposed to atmospheres containing 35% and 55% CO₂ for 30 generations to select strains resistant to high CO₂ content (HCC). Selection pressure was maintained at around 70% of mortality. At the 30th generation, comparison of sensitivities between the selected strains (HCC₁ and HCC₂) and the original susceptible strain (MA-S) revealed a resistance factor (RF) at the 50% mortality level (LT₅₀) of 4. 6-and 5. 3-folds, respectively. Throughout the selection process, log-time against probit-mortality lines remained roughly parallel and the slopes remained lower than that of MA-S strain until the last generation. It is inplied that the genetic potential of L. bostrychophila to develop resistance to CO₂ was not exhausted until the 30th generation. Removal of selection pressure for 5 generations from 2 sub-populations of two selected strains from the 25th generation caused significant reduction in resistance. In the absence of MAs exposure, the two selected strains (HCC₁ and HCC₂) possessed the fitness defect. HCC₁ and HCC₂ were calculated by R₀ to have a fitness value of 0. 52 and 0. 45 relative to MA-S.     

Long-term effect of mutagenic DNA repair on accumulation of mutations in Pseudomonas syringae B86-17

Forty replicate lineages of Pseudomonas syringae B86-17 cells expressing the rulAB mutagenic DNA repair (MDR) determinant or the rulB::Km MDR-deficient mutant GWS242 were passaged through single-cell bottlenecks (60 cycles), with a UV radiation (UVR) exposure given to half of the lineages at the beginning of each cycle. After every 10th bottleneck cycle, single-colony isolates from all 80 lineages were subjected to 39 phenotypic screens, with newly arising mutations detected in 60 and 0% of UVR-exposed or non-UVR-exposed B86-17 lineages, respectively, by the 60th cycle. Cellular fitness, measured as growth rate in a minimal medium, of UVR-exposed lineages of both B86-17 and GWS242 after 60 cycles was not significantly different from that of the ancestral strains. Although UVR exposure and MDR activity increased the occurrence of mutations in cells, a significant reduction in overall fitness was not observed.     

Does fitness erode in the absence of selection? An experimental test with Tribolium

In the absence of natural selection, average fitness in the population is expected to decline due to the accumulation of deleterious mutations. Replicate populations of flour beetles (Tribolium confusum) were maintained for 22 generations in the virtual absence of selection (random mating, favorable environment, excess of food, and mortality of only 3%). Larva-to-adult survivorship rates were similar in the stock population and selection-free populations. In contrast, starvation resistance of adult beetles from selection-free populations was significantly reduced (by more than 2% per generation). When tested in the favorable environment, beetles in one selection-free population had significantly slower development and smaller sizes of females than beetles from the stock population. Since such changes in these fitness components are usually maladaptive, they indicate possible erosion of fitness under relaxed selection at the rate of 0.1-0.2% per generation. No fitness erosion was detectable in the second selection-free population.     

Widespread fitness alignment in the legume-rhizobium symbiosis

Although 'cheaters' potentially destabilize the legume-rhizobium mutualism, we lack a comprehensive review of host-symbiont fitness correlations. Studies measuring rhizobium relative or absolute fitness and host benefit are surveyed. Mutant studies are tallied for evidence of pleiotropy; studies of natural strains are analyzed with meta-analysis. Of 80 rhizobium mutations, 19 decrease both partners' fitness, four increase both, two increase host fitness but decrease symbiont fitness and none increase symbiont fitness at the host's expense. The pooled correlation between rhizobium nodulation competitiveness and plant aboveground biomass is 0.65 across five experiments that compete natural strains against a reference, whereas, across 14 experiments that compete rhizobia against soil populations or each other, the pooled correlation is 0.24. Pooled correlations between aboveground biomass and nodule number and nodule biomass are 0.76 and 0.83. Positive correlations between legume and rhizobium fitness imply that most ineffective rhizobia are 'defective' rather than 'defectors'; this extends to natural variants, with only one significant fitness conflict. Most studies involve non-coevolved associations, indicating that fitness alignment is the default state. Rhizobium mutations that increase both host and symbiont fitness suggest that some plants maladaptively restrict symbiosis with novel strains.     

Virulence and fitness of the fungal pathogen Entomophaga maimaiga in its host Lymantria dispar, for pathogen and host strains originating from Asia, Europe, and North America

In this study, we tested (1) whether non-North American gypsy moth strains are susceptible to North American isolates of Entomophaga maimaiga and (2) the potential for erosion in the efficacy of E. maimaiga in controlling gypsy moth. We used bioassays to assess the variability in virulence (measured as time to death) as well as fitness of the pathogen (measured as spore production) in four gypsy strains challenged with six E. maimaiga isolates, using host and pathogen strains originating from Asia, Europe, and North America. We found that all E. maimaiga isolates tested were pathogenic to all strains of Lymantria dispar, regardless of the geographical origin of the fungal isolate, with at least 86% mortality for all combinations of fungal isolate and gypsy moth strain. We therefore conclude that Asian gypsy moths are susceptible to North American strains of E. maimaiga. No significant interactions between fungal isolates and gypsy moth strains with regard to time to death were found, indicating that each fungal isolate had the same overall effect on all the gypsy moth strains tested. However, fungal isolates differed significantly with regard to virulence, with a Russian isolate being the slowest to kill gypsy moth (5.1+/-0.1 days) and a Japanese isolate being the overall fastest to kill its host (4.0+/-0.1 days). Fungal isolates also differed in fitness, with variability in types of spores produced. These differences in virulence and fitness were, however, not correlated with geographical origin of the fungal isolate. Gypsy moth strains had no or only little effect on fungal virulence and fitness. Based on our studies with laboratory-reared gypsy moth strains, erosion of successful control of gypsy moth by E. maimaiga seems unlikely.     

Recombination between homologous autosomes in medfly (Ceratitis capitata) males: type-1 recombination and the implications for the stability of genetic sexing strains

The sterile insect technique (SIT) is an environmentally safe technology to control insect pests. To improve this technology, genetic sexing strains (GSS) have been developed for the Mediterranean fruit fly, Ceratitis capitata. Such strains are based on Y-autosome translocations linking a selectable marker to the male sex and their long-term stability, especially under large-scale mass rearing conditions, is threatened by genetic recombination in the heterozygous males. We have measured male recombination in order to be able to construct GSS that are more stable. Our results show that male recombination occurs at very low frequencies, that is, below 1% per generation. Furthermore, recombination in medfly males occurs premeiotically. By selecting strains where the Y-autosome translocation breakpoint and the selectable marker are closely linked, the deleterious effects of recombination on the stability of GSS can be minimized. In such strains recombination is reduced by ca. 80% as compared to previously studied GSS. Although recombinants still occur at very low frequencies they still pose a threat to the integrity of the sexing system if they possess a selective advantage. Under mass rearing condition such recombinants will accumulate according to their relative fitness and additional measures, such as improved mass rearing strategies, are required to preserve the accuracy of the sexing system. As a conclusion it is shown that current GSS are stable enough to allow mass rearing at levels exceeding 1000 million male medflies per week.     

Effect of cryopreservation and lyophilization on viability and growth of strict anaerobic human gut microbes

Strict anaerobic gut microbes have been suggested as ‘next‐generation probiotics’ for treating several intestinal disorders. The development of preservation techniques is of major importance for therapeutic application. This study investigated cryopreservation (?80°C) and lyophilization survival and storage stability (4°C for 3 months) of the strict anaerobic gut microbes Bacteroides thetaiotaomicron, Faecalibacterium prausnitzii, Roseburia intestinalis, Anaerostipes caccae, Eubacterium hallii and Blautia obeum. To improve preservation survival, protectants sucrose and inulin (both 5% w/v) were added for lyophilization and were also combined with glycerol (15% v/v) for cryopreservation. Bacterial fitness, evaluated by maximum growth rate and lag phase, viability and membrane integrity were determined using a standardized growth assay and by flow cytometry as markers for preservation resistance. Lyophilization was more detrimental to viability and fitness than cryopreservation, but led to better storage stability. Adding sucrose and inulin enhanced viability and the proportion of intact cells during lyophilization of all strains. Viability of protectant‐free B. thetaiotaomicron, A. caccae and F. prausnitzii was above 50% after cryopreservation and storage and increased to above 80% if protectants were present. The addition of glycerol, sucrose and inulin strongly enhanced the viability of B. obeum, E. hallii and R. intestinalis from 0.03–2% in protectant‐free cultures to 11–37%. This is the first study that quantitatively compared the effect of cryopreservation and lyophilization and the addition of selected protectants on viability and fitness of six strict anaerobic gut microbes. Our results suggest that efficiency of protectants is process‐ and species‐specific.     

Impact of inbreeding on performance of the predator Podisus maculiventris

Effects of inbreeding on the development and reproduction of the predatory pentatomid Podisus maculiventris were assessed throughout 30 generations following introduction in the laboratory. Developmental and reproductive fitness of two mildly inbred lines did not differ dramatically from that of a reference population. In most generations, egg weight, egg hatch and developmental rate of nymphs were greater in the reference strain than in both inbred strains, but fecundity and longevity did not differ among strains. In the 30th generation, fecundity of all strains had dropped to about half of that observed in the 15th generation, which was presumably related to non-genetic factors. There were consistent differences among strains for adult body weight and size, indicating heritable variation. A positive relationship between weight at day 14 and humeral width of P. maculiventris females was found, but neither of these parameters was associated with total fecundity. In the 30th generation, predators from both inbred lines had generally similar predation capacities to those from the reference population. No significant relationship could be detected between body weights and predation rates of either nymphs or adults. The use of inbreeding to minimize selective adaptation during rearing or to select for favourable traits in P. maculiventris is discussed.     

Studies on the color variation in larvae ofEphestia kühniella Zeller (Lepidoptera: Phycitidae) III. Fitness of different larval color strains

The population fitness in terms of the intrinsic rate of increase r_m was measured in eight pure (homogenic) strains of Ephestia kühniellaZeller with different larva color at an optimal temperature of 25°C, and in three strains at unfavorble temperatures of 15, 17, 28, and 30°C, to understand a mechanism of maintenance of a larval color variation found in wild populations. The survival rate, hatchability, and gross rate of reproduction were poorly correlated with the fitness but the mean generation time and net reproduction rate were correlated with the fitness significantly at 25°C. Intermediate color (pink) strain(s) grew faster, initiated reproduction earlier and had shorter longevities than other strain(s) under the range of 15 and 28°C; the fitness was highest in the intermediate larval color strains except at 30°C in which all strains had a negative rate of increase. The results were discussed with reference to the relationship between the larval color and fitness along with a maintenance mechanism of the variation.     

Multigenerational versus single generation studies to estimate herbicide resistance fitness cost in Arabidopsis thaliana

The evolution of resistance in response to pesticide selection is expected to be delayed if fitness costs are associated with resistance genes. The estimate of fitness costs usually involves comparing major growth traits of resistant versus susceptible individuals in the absence of pesticide. Ideally, a measure of changes in resistance allele frequency over several generations would allow the best estimate of the overall fitness cost of a resistance gene. In greenhouse conditions, we monitored the dynamics of the evolution of the frequencies of six herbicide-resistant mutations (acetolactate synthase, cellulose synthase, and auxin-induced target genes) in the model species Arabidopsis thaliana in a multigenerational study covering five to seven nonoverlapping generations. The microevolutionary dynamics in experimental populations indicated a mean fitness cost of 38%, 73%, and 94% for the ixr1-2, axr1-3, and axr2-1 resistances, respectively; no fitness cost for the csr1-1, and ixr2-1 resistances; and a transient advantage for the aux1-7 resistance. The result for the csr1-1 resistance contrasts with a cost of 37% based on total seed number in a previous study, demonstrating that single generation studies could have limitation for detecting cost. A positive frequency dependence for the fitness cost was also detected for the ixr1-2 resistance. The results are discussed in relation to the maintenance of polymorphism at resistance loci.     

原生质体融合技术构建棕榈油酸高产酵母菌株

采用原生质体融合技术进行产棕榈油酸酵母Saccharomy cescerevisiaeNo.12.926和产脂酵母RhodotorulaNo.12.908的融合研究,获得了棕榈油酸高产酵母工程菌株。实验结果表明,原生质体形成的最佳条件为:对数期酵母No.12.926和No.12.908用2%蜗牛酶于30℃分别酶解1.5和2h。在最佳条件下,酵母No.12.926和No.12.908原生质体形成率分别为94%和80%,再生率分别为75%和60%。原生质体融合由聚乙二醇诱导。将得到的融合子进行多次传代培养优选,获得了遗传性状稳定的融合菌株。融合子的生物量为亲株的两倍多,其细胞形态和菌落颜色与亲株有差别。产脂和产棕榈油酸分析表明,融合子的产脂量为菌体干重的48.53%,其中棕榈油酸占油脂总量的47.29%,为菌体干重的22.95%。     

Heterozygous Effects on Fitness of Ems-Treated Chromosomes in DROSOPHILA MELANOGASTER

The heterozygous effects on fitness of second chromosomes carrying mutants induced with different doses of EMS were ascertained by monitoring changes in chromosome frequencies over time. These changes were observed in populations in which the treated chromosomes, as well as untreated competitors, remained heterozygous in males generation after generation. This situation was achieved by using a translocation which links the second chromosome to the X chromosome; however, only untranslocated second chromosomes were mutagenized. Chromosomes were classified according to their effects on viability in homozygous condition. A preliminary homozygosis identified completely lethal chromosomes; secondary tests distinguished between drastic (viability index < 0.1) and nondrastic chromosomes. Chromosomes that were nondrastic after treatment were found to reduce the fitness of their heterozygous carriers by 3-5%. The data show that flies homozygous for these chromosomes were about 2.7% less viable per treatment with 1 mm EMS than flies homozygous for untreated chromosomes. By comparing the fitness-depressing effects of nondrastic EMS-induced mutants in heterozygous condition with the corresponding viability-depressing effects measured by Temin, it is apparent that the total fitness effects are several times larger than the viability effects alone. Completely lethal chromosomes derived from the most heavily treated material reduced fitness by 11% in heterozygous condition; approximately half of this reduction was due to the lethal mutations themselves.     

Frequency of bacteriocin resistance development and associated fitness costs in Listeria monocytogenes

Bacteriocin-producing starter cultures have been suggested as natural food preservatives; however, development of resistance in the target organism is a major concern. We investigated the development of resistance in Listeria monocytogenes to the two major bacteriocins pediocin PA-1 and nisin A, with a focus on the variations between strains and the influence of environmental conditions. While considerable strain-specific variations in the frequency of resistance development and associated fitness costs were observed, the influence of environmental stress seemed to be bacteriocin specific. Pediocin resistance frequencies were determined for 20 strains and were in most cases ca. 10(-6). However, two strains with intermediate pediocin sensitivity had 100-fold-higher pediocin resistance frequencies. Nisin resistance frequencies (14 strains) were in the range of 10(-7) to 10(-2). Strains with intermediate nisin sensitivity were among those with the highest frequencies. Environmental stress in the form of low temperature (10 degrees C), reduced pH (5.5), or the presence of NaCl (6.5%) did not influence the frequency of pediocin resistance development; in contrast, the nisin resistance frequency was considerably reduced (<5 x 10(-8)). Pediocin resistance in all spontaneous mutants was very stable, but the stability of nisin resistance varied. Pediocin-resistant mutants had fitness costs in the form of reduction down to 44% of the maximum specific growth rate of the wild-type strain. Nisin-resistant mutants had fewer and less-pronounced growth rate reductions. The fitness costs were not increased upon applying environmental stress (5 degrees C, 6.5% NaCl, or pH 5.5), indicating that the bacteriocin-resistant mutants were not more stress sensitive than the wild-type strains. In a saveloy-type meat model at 5 degrees C, however, the growth differences seemed to be negligible. The applicational perspectives of the results are discussed.     

Sexual reproduction facilitates the adaptation of parasites to antagonistic host environments: Evidence from empirical study in the wheat-Mycosphaerella graminicola system

Most eukaryotes use sexual reproduction to transmit genetic information from generation to generation despite the advantages offered by asexual reproduction. One theory to explain the origin and maintenance of sexual reproduction hypothesises that sexual recombination generates genetic variation that allows faster adaptation to fluctuating and/or stressful environments. We used a combination of ecological, molecular genetic, statistical and experimental evolution approaches to test this hypothesis in an agricultural plant-pathogen system. We inoculated wheat hosts with 10 strains of the fungal pathogen Mycosphaerella graminicola in a field experiment and estimated the contributions of sexual reproduction, asexual reproduction and immigration to the genetic composition of fungal populations sampled from moderately resistant and susceptible hosts through the course of an epidemic cycle. We found that a significant proportion of the M. graminicola population in the late phase of the epidemic originated from sexual reproduction among isolates that had been introduced into the field plots at the beginning of the epidemic. Recombinants were recovered at a higher frequency on the moderately resistant plant host Madsen than on the susceptible host Stephens. By the end of the growing season, we estimated that approximately 13% of the strains sampled from the resistant host were recombinants, compared with 9% in the samples collected from the susceptible host. We also found that pathogen strains originating from the resistant cultivar displayed higher levels of fitness, virulence and fungicide tolerance than those originating from the susceptible cultivar. Our results provide empirical support for the hypothesis that sexual reproduction facilitates the evolution of parasites to overcome host resistance.