Local adaptation and population structure at a micro-geographical scale of a fungal parasite on its host plant

Local adaptation, which has been detected for several wild pathosystems is influenced by gene flow and recombination. In this study, we investigate local adaptation and population structure at a fine scale in wild populations of a plant-pathogen fungus. We sampled hierarchically strains of Colletotrichum lindemuthianum in a wild population of its host. The analysis of AFLP patterns obtained for 86 strains indicated that: (i) many different haplotypes can be discriminated, although occurrence of recombination could not be shown; (ii) migration between adjacent plants seemed rare during the season; and (iii) neutral diversity is structured according to groups of plants and individual host plants. Furthermore, we tested for the occurrence of local adaptation using a cross-inoculation experiment. Our results showed local adaptation at the scale of the individual host plant. These results indicate that fine-scale dynamics has evolutionary consequences in this pathosystem.     

Polygenic traits and parasite local adaptation

The extent to which parasites are locally adapted to their hosts has important implications for human health and agriculture. A recently developed conceptual framework--the geographic mosaic theory of coevolution--predicts that local maladaptation should be common and largely determined by the interplay between gene flow and spatially variable reciprocal selection. Previous investigation of this theory has predominately focused on genetic systems of infection and resistance characterized by few genes of major effect and particular forms of epistasis. Here we extend existing theory by analyzing mathematical models of host-parasite interactions in which host resistance to parasites is mediated by quantitative traits with an additive polygenic basis. In contrast to previous theoretical studies predicated upon major gene mechanisms, we find that parasite local maladaptation is quite uncommon and restricted to one specific functional form of host resistance. Furthermore, our results show that local maladaptation should be rare or absent in studies that measure local adaptation using reciprocal transplant designs conducted in natural environments. Our results thus narrow the scope over which the predictions of the geographic mosaic theory are likely to hold and provide novel and readily testable predictions about when and where local maladaptation is expected.     

Inference of parasite local adaptation using two different fitness components

Estimating parasite fitness is central to studies aiming to understand parasite evolution. Theoretical models generally use the basic reproductive rate R(0) to express fitness, yet it is very difficult to quantify R(0) empirically and experimental studies often use fitness components such as infection intensity or infectivity as substitutes. These surrogate measures may be biased in several ways. We assessed local adaptation of the microsporidium Ordospora colligata to its host, the crustacean Daphnia magna using two different parasite fitness components: infection persistence over several host generations in experimental populations and infection intensity in individual hosts. We argue that infection persistence is a close estimator of R(0), whereas infection intensity measures only a component of it. Both measures show a pattern that is consistent with parasite local adaptation and they correlate positively. However, several inconsistencies between them suggest that infection intensity may at times provide an inadequate estimate of parasite fitness.     

Experimental evolution of local parasite maladaptation

Sign and magnitude of local adaptation in host–parasite systems may vary with ecological, epidemiological or genetic parameters. To investigate the role of host genetic background, we established long‐term experimental populations of different genotypes of the protozoan Paramecium caudatum, infected with the bacterial parasite Holospora undulata. We observed the evolution of an overall pattern of parasite local maladaptation for infectivity, indicating a general coevolutionary disadvantage of this parasite. Maladaptation extended to host populations with the same genetic background, similar to extending from the local to a higher regional level in natural populations. Patterns for virulence were qualitatively similar, but with less statistical support. A nonsignificant correlation with levels of (mal)adaptation for infectivity suggests independent evolution of these traits. Our results indicate similar (co)evolutionary trajectories in populations with different genetic backgrounds. Nonetheless, the correlated clines of genetic distance and parasite performance illustrate how genetic background can shape spatial gradients of local adaptation.     

Experimental evolution with a multicellular host causes diversification within and between microbial parasite populations—Differences in emerging phenotypes of two different parasite strains

Host‐parasite coevolution is predicted to have complex evolutionary consequences, potentially leading to the emergence of genetic and phenotypic diversity for both antagonists. However, little is known about variation in phenotypic responses to coevolution between different parasite strains exposed to the same experimental conditions. We infected Caenorhabditis elegans with one of two strains of Bacillus thuringiensis and either allowed the host and the parasite to experimentally coevolve (coevolution treatment) or allowed only the parasite to adapt to the host (one‐sided parasite adaptation). By isolating single parasite clones from evolved populations, we found phenotypic diversification of the ancestral strain into distinct clones, which varied in virulence toward ancestral hosts and competitive ability against other parasite genotypes. Parasite phenotypes differed remarkably not only between the two strains, but also between and within different replicate populations, indicating diversification of the clonal population caused by selection. This study highlights that the evolutionary selection pressure mediated by a multicellular host causes phenotypic diversification, but not necessarily with the same phenotypic outcome for different parasite strains.     

Superinfection and the coevolution of parasite virulence and host recovery

Parasite strategies of host exploitation may be affected by host defence strategies and multiple infections. In particular, within‐host competition between multiple parasite strains has been shown to select for higher virulence. However, little is known on how multiple infections could affect the coevolution between host recovery and parasite virulence. Here, we extend a coevolutionary model introduced by van Baalen (Proc. R. Soc. B, 265, 1998, 317) to account for superinfection. When the susceptibility to superinfection is low, we recover van Baalen's results and show that there are two potential evolutionary endpoints: one with avirulent parasites and poorly defended hosts, and another one with high virulence and high recovery. However, when the susceptibility to superinfection is above a threshold, the only possible evolutionary outcome is one with high virulence and high investment into defence. We also show that within‐host competition may select for lower host recovery, as a consequence of selection for more virulent strains. We discuss how different parasite and host strategies (superinfection facilitation, competitive exclusion) as well as demographic and environmental parameters, such as host fecundity or various costs of defence, may affect the interplay between multiple infections and host–parasite coevolution. Our model shows the interplay between coevolutionary dynamics and multiple infections may be affected by crucial mechanistic or ecological details.     

Spatiotemporal variation in local adaptation of a specialist insect herbivore to its long‐lived host plant

Local adaptation of interacting species to one another indicates geographically variable reciprocal selection. This process of adaptation is central in the organization and maintenance of genetic variation across populations. Given that the strength of selection and responses to it often vary in time and space, the strength of local adaptation should in theory vary between generations and among populations. However, such spatiotemporal variation has rarely been explicitly demonstrated in nature and local adaptation is commonly considered to be relatively static. We report persistent local adaptation of the short‐lived herbivore Abrostola asclepiadis to its long‐lived host plant Vincetoxicum hirundinaria over three successive generations in two studied populations and considerable temporal variation in local adaptation in six populations supporting the geographic mosaic theory. The observed variation in local adaptation among populations was best explained by geographic distance and population isolation, suggesting that gene flow reduces local adaptation. Changes in herbivore population size did not conclusively explain temporal variation in local adaptation. Our results also imply that short‐term studies are likely to capture only a part of the existing variation in local adaptation.     

Gas Exchange and Emission of Chlorophyll Fluorescence during the Monocycle of Rust, Angular Leaf Spot and Anthracnose on Bean Leaves as a Function of their Trophic Characteristics

Measurements related to gas exchange and chlorophyll fluorescence emission were taken from healthy and diseased bean leaves with rust, angular leaf spot, and anthracnose during lesion development for each disease. The experiments were performed at different temperatures of plant incubation, and using two bean cultivars. The main effect of temperature of plant incubation was in disease development. There was no significant difference between cultivars in relation to disease development and in magnitude of physiological alterations when disease severity was the same for each cultivar. These diseases reduced the net photosynthetic rate and increased the dark respiration of infected leaves after the appearance of visible symptoms and the differences between healthy and diseased leaves increased with disease development. The transpiration rate and stomatal conductance were stable during the monocycle of rust, however, these two variables decreased in leaves with angular leaf spot and anthracnose beginning with symptom appearance and continuing until lesion development was complete. Carboxylation resistance was probably the main factor related to reduction of photosynthetic rate of the apparently healthy area of leaves with rust and angular leaf spot. Reduction of the intercellular concentration of CO₂, due to higher stomatal resistance, was probably the main factor for leaves with anthracnose. Chlorophyll fluorescence assessments suggested that there was no change in electron transport capacity and generation of ATP and NADPH in apparently healthy areas of diseased leaves, but decreases in chlorophyll fluorescence emission occurred on visibly lesioned areas for all diseases. Minimal fluorescence was remarkably reduced in leaves with angular leaf spot. Maximal fluorescence and optimal quantum yield of photosystem II of leaves were reduced for all three diseases. Bean rust, caused by a biotrophic pathogen, induced less damage to the regulation mechanisms of the physiological processes of the remaining green area of diseased leaves than did bean angular leaf spot or anthracnose, caused by hemibiotrophic pathogens. The magnitude of photosynthesis reduction can be related to the host–pathogen trophic relationships.     

A synthesis of experimental work on parasite local adaptation

The study of parasite local adaptation, whereby parasites perform better on sympatric hosts than on allopatric hosts and/or better on their own host population than do other parasites, is of great importance to both basic and applied biology. Theoretical examination of host-parasite coevolution predicts that parasite migration rate, generation time and virulence all contribute to the pattern of parasite local adaptation, such that parasites with greater dispersal ability, more frequent reproduction and/or high virulence ought to exhibit increased infectivity on local hosts. Here, we present a meta-analysis of experimental work from 57 host-parasite systems across 54 local adaptation studies to directly test theoretical predictions concerning the effect of each attribute on parasite adaptation. As expected, we find that studies of parasites with higher migration rates than their hosts report local adaptation, as measured by infection success, significantly more often than studies of parasites with relatively low migration rates. Furthermore, this synthesis serves to identify biases in the current body of work and highlight areas with the greatest need for further study. We emphasize the importance of unifying the field with regard to experimental methods, local adaptation definitions and reported statistics for cross-infection studies.     

Social learning of a brood parasite by its host

Arms races between brood parasites and their hosts provide model systems for studying the evolutionary repercussions of species interactions. However, how naive hosts identify brood parasites as enemies remains poorly understood, despite its ecological and evolutionary significance. Here, we investigate whether young, cuckoo-naive superb fairy-wrens, Malurus cyaneus, can learn to recognize cuckoos as a threat through social transmission of information. Naive individuals were initially unresponsive to a cuckoo specimen, but after observing conspecifics mob a cuckoo, they made more whining and mobbing alarm calls, and spent more time physically mobbing the cuckoo. This is the first direct evidence that naive hosts can learn to identify brood parasites as enemies via social learning.     

Inheritance of Anthracnose Resistance in the Common Bean Differential Cultivar G 2333 and Identification of a New Molecular Marker Linked to the Co-42 gene

The inheritance of anthracnose resistance of the common bean ( Phaseolus vulgaris L.) differential cultivar G 2333 to Colletotrichum lindemuthianum races 73 and 89 was studied in crosses with the susceptible cultivar Rudá. The segregation ratios of 15 : 1 in the F₂ and 3 : 1 in the backcrosses to Rudá indicate that for each of the races tested there are two independent resistance loci in G 2333. A random amplified polymorphic DNA (RAPD) molecular marker (OPH18_1200C) linked in resistance to race 73 was identified in a BC₃F_2:3 population derived from crosses between Rudá and G 2333. A RAPD molecular marker OPAS13_950C, previously identified as linked to gene Co-4² , was also amplified in this population. Co-segregation analyses showed that these two markers are located at 5.6 (OPH18_1200C) and 11.2 (OPAS13_950C) cM of the Co-4² gene. These markers were not present in BC₁F_2:3 plants resistant to race 89 indicating that this population carries a different resistance gene. DNA amplification of BC₁F_2:3 plants with RAPD molecular marker OPAB_450C, previously identified as linked to gene Co-5 , indicated that this gene is present in this population.     

Dissecting the effect of a heterogeneous environment on the interaction between host and parasite fitness traits

Environmental variation can alter the probability of parasitic infection or the fitness consequence of infection, and thus has the potential to dramatically alter the dynamics of host parasite coevolution. Here we investigated the effect of a changing temperature on host-parasite interactions using the crustacean Daphnia magna and its bacterial parasite Pasteuria ramosa. By reciprocally varying (1) the temperature at which exposure to parasites occurred and (2) the temperature at which within-host parasite growth occurred, and measuring several fitness-related traits, we show that while there are temperature combinations that favour either host or parasite, there are also conditions that favour neither, that is, negative fitness consequences for the host without fitness benefits for the parasite. This result highlights the importance of considering a heterogeneous rather than static environment in coevolutionary studies, while also showing support for an optimal virulence strategy in castrating parasites.     

Symptoms and Prepenetration Events Associated with the Infection of Common Bean by the Anamorph and Teleomorph of Glomerella cingulata f.sp. phaseoli

Glomerella cingulata f.sp. phaseoli and Colletotrichum lindemuthianum are the teleomorph and anamorph, respectively, of the pathogen causing anthracnose in common bean. The mechanisms relating to the sexual reproduction of this plant pathogen are still unclear, as are the infection structures involved and the symptoms produced. In the present study, bean plants were inoculated with ascospores and conidia, and the events taking place within the following 120 h were investigated using light microscopy and scanning electron microscopy. The symptoms exhibited by plants inoculated with the ascospores were milder than in those inoculated with conidia. Microscopy revealed that most of ascospores produced germ tubes and appressoria at an early stage (24 h after inoculation). From 48 h onwards, the formation of hyphae and the production of germ tubes and appressoria were great. In contrast, infections originating from conidia developed more slowly, and at 24 and 48 h, many non‐germinated conidia were present, whereas only few conidia developed germ tubes and appressoria. Ascospore germination and appressorium formation were similar on both resistant and susceptible cultivars. Hence, the symptoms and the temporal sequence of events associated with the infection of bean plants by the two fungal forms differed, although the structures produced were similar. This is the fist report comparing symptoms and prepenetration events between anamorph and teleomorph of G. cingulata f.sp. phaseoli in common bean.     

Geographic and within-population structure in variable resistance to parasite species and strains in a vertebrate host

Host resistance to parasites and parasite infectivity may be subject to significant genetically determined variation within species. However, relatively little is known of how this variability is structured in natural vertebrate populations and their macroparasites. A laboratory experiment on host susceptibility-parasite infectivity variation in a wildlife host-parasite system (subspecies of the anuran X. laevis and their polystome flatworms), including 33 pairwise allopatric and sympatric host-parasite combinations (three parasite geographical isolates x 11 host full-sibling families, n=600), revealed a complex pattern of infection success. Results amongst host sibships from different localities suggested that infection success was subject to a highly significant locality x parasite isolate interaction. Within localities, a highly significant sibship x isolate interaction also occurred in one of two groups of sibships examined. The existence of such interactions suggests a potential for frequency-dependent, Red Queen-like selection. Interaction between locality and isolate was partly due to higher infection levels in sympatric combinations, consistent with a general pattern of host-specific adaptation. However, some allopatric combinations produced unpredictably high infection levels, resulting in very asymmetrical cross-infectivity patterns (where the reciprocal cross-infections produced negligible infection). This phylogeographically structured host-parasite system may, therefore, sometimes generate local parasite strains with high infectivity to allopatric hosts. Secondary contact between populations could thus result in significant, and unequal, transfer of parasites.     

Shifts in diversification rates and host jump frequencies shaped the diversity of host range among Sclerotiniaceae fungal plant pathogens

The range of hosts that a parasite can infect in nature is a trait determined by its own evolutionary history and that of its potential hosts. However, knowledge on host range diversity and evolution at the family level is often lacking. Here, we investigate host range variation and diversification trends within the Sclerotiniaceae, a family of Ascomycete fungi. Using a phylogenetic framework, we associate diversification rates, the frequency of host jump events and host range variation during the evolution of this family. Variations in diversification rate during the evolution of the Sclerotiniaceae define three major macro‐evolutionary regimes with contrasted proportions of species infecting a broad range of hosts. Host–parasite cophylogenetic analyses pointed towards parasite radiation on distant hosts long after host speciation (host jump or duplication events) as the dominant mode of association with plants in the Sclerotiniaceae. The intermediate macro‐evolutionary regime showed a low diversification rate, high frequency of duplication events and the highest proportion of broad host range species. Our findings suggest that the emergence of broad host range fungal pathogens results largely from host jumps, as previously reported for oomycete parasites, probably combined with low speciation rates. These results have important implications for our understanding of fungal parasites evolution and are of particular relevance for the durable management of disease epidemics.     

Genetic diversity,virulence and fitness evolution in an obligate fungal parasite of bees

Within‐host competition is predicted to drive the evolution of virulence in parasites, but the precise outcomes of such interactions are often unpredictable due to many factors including the biology of the host and the parasite, stochastic events and co‐evolutionary interactions. Here, we use a serial passage experiment (SPE) with three strains of a heterothallic fungal parasite (Ascosphaera apis) of the Honey bee (Apis mellifera) to assess how evolving under increasing competitive pressure affects parasite virulence and fitness evolution. The results show an increase in virulence after successive generations of selection and consequently faster production of spores. This faster sporulation, however, did not translate into more spores being produced during this longer window of sporulation; rather, it appeared to induce a loss of fitness in terms of total spore production. There was no evidence to suggest that a greater diversity of competing strains was a driver of this increased virulence and subsequent fitness cost, but rather that strain‐specific competitive interactions influenced the evolutionary outcomes of mixed infections. It is possible that the parasite may have evolved to avoid competition with multiple strains because of its heterothallic mode of reproduction, which highlights the importance of understanding parasite biology when predicting disease dynamics.     

不同寄主植物叶片上蚜虫的形态适应

昆虫对寄主植物之间的形态适应性是两者相互关系的重要组成部分, 本文以植食性蚜虫为研究对象, 研究其对不同种类寄主植物叶片的形态适应。选取寄生于禾本科、 杨柳科、 壳斗科和松科4科7属植物叶片上的26种蚜虫, 基于有翅孤雌蚜和无翅孤雌蚜, 在光学显微镜下观察、 测量并统计比较了37个形态特征及其种内变异。以克隆平均值为基础数据,选取与蚜虫取食和附着于植物表面相关的形态特征（喙末端、跗节Ⅰ、Ⅱ和爪）为变量,分别得出无翅、 有翅孤雌蚜和所有蚜型3个聚类分支图,并将各种蚜虫的寄主植物科、 属分别对应到分支图上。结果表明：体型、腹管和触角的形态在不同蚜虫的科间有一定差别, 喙末端、 跗节Ⅰ、跗节Ⅱ及爪的形态则在不同的寄主植物间存在差异; 在蚜虫种内各形态特征存在一定变异, 变异系数的范围为1.89%~26.08%。3个聚类分析的结果显示,不同种类蚜虫形成的分支基本对应不同科、属的寄主植物;而杨一条角蚜Doraphis populi、三堡瘿绵蚜Epipemphigus imaicus和杨柄叶瘿绵蚜Pemphigus matsumurai则处在较为特殊的分支位置, 那是因为前者为次生寄主型, 后两者均能产生虫瘿。研究结果表明寄生于不同植物相同部位上的蚜虫存在形态特征的明显分异, 对应不同的寄主植物, 蚜虫与寄主植物之间存在着形态适应关系。     

美洲斑潜蝇的寄主植物种类、适合度及其为害性的评价

在山西省调查结果,美洲斑潜蝇的寄生有２９种栽培植物,１１种野生植物,它们分属１６个科,其中以葫芦科,茄科和豆科的种类居多。应用选择系数法和适合性指数法定量测定了１６种常见寄生对美洲斑潜蝇的适合度,建立了选择强度与适合度估测模。根据适合度将美洲斑潜蝇的寄生植物划分为３个类型,即适宜寄主、较适宜寄主和次要寄主,在上述研究的基础上,测定了该在不同寄主上的虫道面积和蛹的重量,明确了不同寄主植物对害虫发育的