Sexual selection reveals a cost of pathogen resistance undetected in life-history assays

Mechanisms of resistance to pathogens and parasites are thought to be costly and thus to lead to evolutionary trade-offs between resistance and life-history traits expressed in the absence of the infective agents. On the other hand, sexually selected traits are often proposed to indicate “good genes” for resistance, which implies a positive genetic correlation between resistance and success in sexual selection. Here I show that experimental evolution of improved resistance to the intestinal pathogen Pseudomonas entomophila in Drosophila melanogaster was associated with a reduction in male sexual success. Males from four resistant populations achieved lower paternity than males from four susceptible control populations in competition with males from a competitor strain, indicating an evolutionary cost of resistance in terms of mating success and/or sperm competition. In contrast, no costs were found in larval viability, larval competitive ability and population productivity assayed under nutritional limitation; together with earlier studies this suggests that the costs of P. entomophila resistance for nonsexual fitness components are negligible. Thus, rather than indicating heritable pathogen resistance, sexually selected traits expressed in the absence of pathogens may be sensitive to costs of resistance, even if no such costs are detected in other fitness traits.     

THE EVOLUTION OF RESISTANCE TO HERBIVORY IN IPOMOEA PURPUREA. II. NATURAL SELECTION BY INSECTS AND COSTS OF RESISTANCE

An important component of the process of coevolution between plants and their insect herbivores is the imposition of selection on plants by insects. Although such selection has been inferred from indirect evidence, little direct evidence for it exists. One goal of this study was to seek direct evidence by determining, for a single plant-herbivore system, whether insect herbivores impose selection on their host plants. A second goal was to determine whether costs are associated with genotypes that confer resistance to herbivores, as has been commonly postulated. The annual morning glory, Ipomoea purpurea, exhibits genetic variation in resistance to four different types of insects. For three of these types, most of the genetic variation is additive. Removal of insect herbivores increased the number of seeds produced by I. purpurea by 20% and eliminated additive genetic variation for seed number (fitness). This result implies that herbivores impose selection on some trait(s) of their host plants. Coupled with selection for decreased damage by corn earworms, as revealed by a negative additive genetic covariance between damage and fitness, this result suggests that insect herbivores impose selection on resistance to corn earworms in I. purpurea. Two types of cost of resistance to herbivores were sought in I. purpurea: 1) internal trade-offs in allocation of resources and 2) ecological trade-offs between resistances to different insects. No costs of either type were detected. This result suggests that cost-benefit arguments that attempt to predict the evolution of levels of resistance to herbivores are not applicable to I. purpurea.     

Direct and ecological costs of resistance and tolerance in the stinging nettle

Plant resistance and tolerance to herbivores, parasites, pathogens, and abiotic factors may involve two types of costs. First, resistance and tolerance may be costly in terms of plant fitness. Second, resistance and tolerance to multiple enemies may involve ecological trade-offs. Our study species, the stinging nettle ( Urtica dioica L.) has significant variation among seed families in resistance and tolerance as well as costs of resistance and tolerance to the holoparasitic plant Cuscuta europaea L. Here we report on variation among seed families (i.e. genetic) in tolerance to nutrient limitation and in resistance to both mammalian herbivores (i.e. number of stinging trichomes) and an invertebrate herbivore (i.e. inverse of the performance of a generalist snail, Arianta arbustorum). Our results indicate direct fitness costs of snail resistance in terms of host reproduction whereas we did not detect fitness costs of mammalian resistance or tolerance to nutrient limitation. We further tested for ecological trade-offs among tolerance or resistance to the parasitic plant, herbivore resistance, and tolerance to nutrient limitation in the stinging nettle. Tolerance of nettles to nutrient limitation and resistance to mammalian herbivores tended to correlate negatively. However, there were no significant correlations among resistance and tolerance to the different natural enemies (i.e. parasitic plants, snails, and mammals). The results of this greenhouse study thus suggest that resistance and tolerance of nettles to diverse enemies are free to evolve independently of each other but not completely without direct costs in terms of plant fitness.     

GENETIC VARIANCE FOR RATE OF POPULATION INCREASE IN NATURAL POPULATIONS OF FLOUR BEETLES,TRIBOLIUM SPP.

The genetic and ecological effects of population subdividsion were investigated for two wild strains of Tribolium castaneum and two wild strains of T. confusum and compared with the effects of population subdivision on the synthetic laboratory strain of T. castaneum (c-SM), used extensively in earlier experiments. For the c-SM strain, it has been shown repeatedly, for a variety of different population structures (different combinations of effective numbers, N_e, and migration rates, m), that large heritable differences in population growth rate arise among demes during 10 to 15 generations of population subdivision. Because this laboratory strain was synthesized by mass mating several “inbred” strains in 1973 (80 to 100 generations ago), it is possible that it has genetic variation for fitness (measured as the heritable variance among demes in the rate of population increase) unusually large compared to natural populations of flour beetles. In this paper, I report that natural populations of flour beetle exhibit as much or more phenotypic and genetic variation in the effects of population structure on fitness than the laboratory strain, c-SM. The observation of substantial heritable variation for fitness in natural populations is unexpected under additive theory and may be indicative of nonadditive genetic variance.     

Genotype and diet affect resistance,survival, and fecundity but not fecundity tolerance

Insects are exposed to a variety of potential pathogens in their environment, many of which can severely impact fitness and health. Consequently, hosts have evolved resistance and tolerance strategies to suppress or cope with infections. Hosts utilizing resistance improve fitness by clearing or reducing pathogen loads, and hosts utilizing tolerance reduce harmful fitness effects per pathogen load. To understand variation in, and selective pressures on, resistance and tolerance, we asked to what degree they are shaped by host genetic background, whether plasticity in these responses depends upon dietary environment, and whether there are interactions between these two factors. Females from ten wild‐type Drosophila melanogaster genotypes were kept on high‐ or low‐protein (yeast) diets and infected with one of two opportunistic bacterial pathogens, Lactococcus lactis or Pseudomonas entomophila. We measured host resistance as the inverse of bacterial load in the early infection phase. The relationship (slope) between fly fecundity and individual‐level bacteria load provided our fecundity tolerance measure. Genotype and dietary yeast determined host fecundity and strongly affected survival after infection with pathogenic P. entomophila. There was considerable genetic variation in host resistance, a commonly found phenomenon resulting from for example varying resistance costs or frequency‐dependent selection. Despite this variation and the reproductive cost of higher P. entomophila loads, fecundity tolerance did not vary across genotypes. The absence of genetic variation in tolerance may suggest that at this early infection stage, fecundity tolerance is fixed or that any evolved tolerance mechanisms are not expressed under these infection conditions.     

SEX-SPECIFIC COSTS OF RESISTANCE TO THE FUNGAL PATHOGEN USTILAGO VIOLACEA (MICROBOTRYUM VIOLACEUM) IN SILENE ALBA

Costs of resistance are often invoked to explain the maintenance of polymorphisms for resistance to fungal pathogens in natural plant populations. To investigate such costs, 27 half-sib families of Silene alba, collected from a single host population, were grown in experimental populations in the presence and absence of the anther-smut fungus Ustilago violacea, a host-sterilizing pathogen transmitted by insects that are both pollinators and vectors of the disease. Host families differed significantly in resistance to inoculation, indicating the presence of genetic variation for mechanisms that impede fungal growth once the disease is encountered (“biochemical” resistance) within the host population. In addition, host families differed significantly in onset of flowering and in flower production in the absence of the disease. Path analysis revealed that late onset of flowering in male host families made a direct contribution to high field resistance (P < 0.01), probably due to a reduced rate of contact between hosts and vectors carrying high spore loads (avoidance, or “phenological” resistance). The contribution of low flower production to field resistance only approached significance (P < 0.10). There was a significantly positive genetic association between biochemical and phenological resistance, suggesting that delayed flowering is either a pleiotropic effect of biochemical resistance, or that genes governing these traits are in linkage disequilibrium. Path analysis revealed that biochemical resistance made both a direct contribution to field resistance (P < 0.01) and a positive indirect contribution via its association with phenology and flower production (P < 0.05) in male hosts. Costs of resistance were sex specific. Male host families with high field resistance had significantly lower reproductive success in healthy populations, indicating a fitness cost of field resistance (P < 0.01), whereas no costs were detected for female hosts. Path analysis revealed that the biochemical component of field resistance made no direct contribution to the observed fitness cost in male hosts, whereas its indirect effect through phenology was only marginally significant (P < 0.10). This finding indicates that fitness costs were mainly due to the phenological component of field resistance. Because the host population had no known history of disease, it is not clear whether the fitness costs are responsible for maintenance of the resistance polymorphism or whether the polymorphism is present for reasons unrelated to pathogen infection. Interactions between host families and pathogen strains with respect to inoculation success were not significant. Hence, there was no evidence for indirect costs of biochemical resistance, that is, reduced resistance to alternative strains. Infection rates in experimental populations with an initially patchy distribution of the pathogen were lower than in populations with a uniform pathogen distribution, suggesting that the effective pathogen pressure and hence the relative success of susceptible and resistant individuals may, in addition to fitness costs of resistance, depend on the spatial population structure of the pathogen.     

COSTS AND BENEFITS OF PLANT RESPONSES TO DISEASE: RESISTANCE AND TOLERANCE

A major assumption of models of the evolution of plant resistance to disease is that plant resistance involves fitness costs. To test this assumption, a field experiment was performed so that a quantitative-genetic analysis could be used to detect fitness costs to Ipomoea purpurea of resistance to different fungal isolates of Colletotrichum dematium, a pathogenic fungus causing the disease anthracnose. This experiment yielded no evidence that resistance to anthracnose involves direct fitness costs. Nevertheless, trade-offs in plant fitness that were unrelated to resistance were detected between different disease environments. Tolerance, defined as the ability to compensate in part for fitness decrements caused by disease, was found to involve fitness costs. Halfsib families that were more tolerant of disease had lower fitness in the absence of disease. The possibility that the cost of tolerance could obscure fitness costs of resistance is explored.     

Starvation Reveals Maintenance Cost of Humoral Immunity

Susceptibility to pathogens and genetic variation in disease resistance is assumed to persist in nature because of the high costs of immunity. Within immunity there are different kinds of costs. Costs of immunological deployment, the costs of mounting an immune response, are measured as a change in fitness following immunological challenge. Maintenance costs of immunity are associated with investments of resources into the infrastructure of an immune system and keeping the system at a given level of readiness in the absence of infection. To demonstrate the costs of immunological maintenance in the absence of infection is considered more difficult. In the present study we examined the maintenance costs of the immune system in lines of Drosophila melanogaster that differed in their antibacterial innate immune response under starved and non-starved conditions. Immunodeficient mutant flies that have to invest less in the immunological maintenance were found to live longer under starvation than wild type flies, whereas the opposite was found when food was provided ad libitum. Our study provides evidence for the physiological cost of immunological maintenance and highlights the importance of environmental variation in the study of evolutionary trade-offs.     

Competitiveness and life‐history characteristics of Daphnia with respect to susceptibility to a bacterial pathogen

Costs of resistance, i.e. trade‐offs between resistance to parasites or pathogens and other fitness components, may prevent the fixation of resistant genotypes and therefore explain the maintenance of genetic polymorphism for resistance in the wild. Using two approaches, the cost of resistance to a sterilizing bacterial pathogen were tested for in the crustacean Daphnia magna. First, groups of susceptible and resistant hosts from each of four natural populations were compared in terms of their life‐history characteristics. Secondly, we examined the competitiveness of nine clones from one population for which more detailed information on genetic variation for resistance was known. In no case did the results show that competitiveness or life history characteristics of resistant Daphnia systematically differed from susceptible ones. These results suggest that costs of resistance are unlikely to explain the maintenance of genetic variation in D. magna populations. We discuss methods for measuring fitness and speculate on which genetic models of host‐parasite co‐evolution may apply to the Daphnia‐microparasite system.     

Reversion of a resistance-breaking mutation shows reversion costs and high virus diversity at necrotic local lesions

An instance of host range evolution relevant to plant virus disease control is resistance breaking. Resistance breaking can be hindered by across-host fitness trade-offs generated by negative effects of resistance-breaking mutations on the virus fitness in susceptible hosts. Different mutations in pepper mild mottle virus (PMMoV) coat protein result in the breaking in pepper plants of the resistance determined by the L³ resistance allele. Of these, mutation M138N is widespread in PMMoV populations, despite associated fitness penalties in within-host multiplication and survival. The stability of mutation M138N was analysed by serial passaging in L³ resistant plants. Appearance on passaging of necrotic local lesions (NLL), indicating an effective L³ resistance, showed reversion to nonresistance-breaking phenotypes was common. Most revertant genotypes had the mutation N138K, which affects the properties of the virus particle, introducing a penalty of reversion. Hence, the costs of reversion may determine the evolution of resistance-breaking in addition to resistance-breaking costs. The genetic diversity of the virus population in NLL was much higher than in systemically infected tissues, and included mutations reported to break L³ resistance other than M138N. Infectivity assays on pepper genotypes with different L alleles showed high phenotypic diversity in respect to L alleles in NLL, including phenotypes not reported in nature. Thus, high diversity at NLL may potentiate the appearance of genotypes that enable the colonization of new host genotypes or species. Collectively, the results of this study contribute to better understanding the evolutionary dynamics of resistance breaking and host-range expansions.     

Effects of host plant and genetic background on the fitness costs of resistance to Bacillus thuringiensis

Novel resistance to pathogens and pesticides is commonly associated with a fitness cost. However, measurements of the fitness costs of insecticide resistance have used diverse methods to control for genetic background and rarely assess the effects of environmental variation. Here, we explored how genetic background interacts with resource quality to affect the expression of the fitness costs associated with resistance. We used a serially backcrossed line of the diamondback moth, Plutella xylostella, resistant to the biopesticide Bacillus thuringiensis, to estimate the costs of resistance for insects feeding on two Brassica species. We found that fitness costs increased on the better-defended Brassica oleracea cultivars. These data were included in two meta-analyses of fitness cost experiments that used standardized protocols (and a common resistant insect stock) but which varied in the methodology used to control for the effects of genetic background. The meta-analysis confirmed that fitness costs were higher on the low-quality host (B. oleracea); and experimental methodology did not influence estimates of fitness costs on that plant species. In contrast, fitness costs were heterogeneous in the Brassica pekinensis studies: fitness costs in genetically homogenized lines were significantly higher than in studies using revertant insects. We hypothesize that fitness modifiers can moderate fitness costs on high-quality plants but may not affect fitness when resource quality is low.     

LIFE-CYCLE COMPONENTS OF SELECTION IN ERIGERON ANNUUS: II. GENETIC VARIATION

Genetic variation for seedling and adult fitness components was measured under natural conditions to determine the relative importance of the seedling stage for lifetime fitness in Erigeron annuus. Variation in lifetime reproductive success can result from both the persistent effects of genetic variation expressed among seedlings and from variation in adult fitness components. Analysis of covariance was used to separate the stage specific from the cumulative effects of genetic variance expressed earlier in the life cycle. E. annuus produces seeds through apomixis, which allowed measurement of the fitness of replicate genotypes from germination through the entire life cycle. There were significant differences among genotypes for date of emergence, seedling size, survivorship and fecundity, but heritabilities were low, indicating slow response to selection. For all characters, environmental components of variance were one to two orders of magnitude larger than genetic variance components, resulting in broad sense heritabilities less than 0.1. For seedling size and fecundity, all of the genetic variance was in the form of genotype-environment interactions, often with large negative genetic correlations across environments. In contrast, genotypes differed in mean survivorship through one year, but there were no genotype-environment interactions for viability. Genetic differences in viability were primarily expressed as differences in overwinter survivorship. Genotype × environment interactions among sites and blocks were generated early in the life cycle while the genotype × environment interactions in response to competitive environment (open, annual cover, perennial cover) first appeared in adult fecundity. Genetic variation in lifetime fitness was not significant, despite a fourfold difference in mean fitness among genotypes.     

Female polymorphisms, sexual conflict and limits to speciation processes in animals

Heritable and visually detectable polymorphisms, such as trophic polymorphisms, ecotypes, or colour morphs, have become classical model systems among ecological geneticists and evolutionary biologists. The relatively simple genetic basis of many polymorphisms (one or a few loci) makes such species well-suited to study evolutionary processes in natural settings. More recently, polymorphic systems have become popular when studying the early stages of the speciation process and mechanisms facilitating or constraining the evolution of reproductive isolation. Although colour polymorphisms have been studied extensively in the past, we argue that they have been underutilized as model systems of constraints on speciation processes. Colouration traits may function as signalling characters in sexual selection contexts, and the maintenance of colour polymorphisms is often due to frequency-dependent selection. One important issue is why there are so few described cases of female polymorphisms. Here we present a synthetic overview of female sexual polymorphisms, drawing from our previous work on female colour polymorphisms in lizards and damselflies. We argue that female sexual polymorphisms have probably been overlooked in the past, since workers have mainly focused on male-male competition over mates and have not realized the ecological sources of genetic variation in female fitness. Recent experimental evolution studies on fruit flies (Drosophila melanogaster) have demonstrated significant heritable variation among female genotypes in the fitness costs of resistance or tolerance to male mating harassment. In addition, female-female competition over resources could also generate genetic variation in female fitness and promote the maintenance of female sexual polymorphisms. Female sexual polymorphisms could subsequently either be maintained as intrapopulational polymorphisms or provide the raw material for the formation of new species.     

Consequence of herbivory for the fitness cost of herbicide resistance: photosynthetic variation in the context of plant-herbivore interactions

The cost of adaptations may depend on environmental conditions. We consider how the fitness cost of resistance to the herbicide triazine in Amaranthus hybridus interacts with folivory from the beetle Disonycha glabrata. Triazine-resistant (TR) genotypes suffer a fitness cost because of a pleiotropic reduction in the light reaction of photosynthesis, which in turn often leads to a reduction in photosynthetic rate. We found that the fitness cost of triazine resistance was 360% greater in the presence than absence of D. glabrata. This resulted from multiple phenotypic trade-offs, with TR plants suffering greater herbivory and displaying a diminished tolerance of damage. Our work highlights the importance of incorporating appropriate ecological variation into the assessment of fitness trade-offs. The results of this study also illustrate the potential for herbivores to impose selection on photosynthetic variation, and for variation in resource acquisition to obscure fitness costs.     

Uncovering symbiont‐driven genetic diversity across North American pea aphids

Heritable genetic variation is required for evolution, and while typically encoded within nuclear and organellar genomes, several groups of invertebrates harbour heritable microbes serving as additional sources of genetic variation. Hailing from the symbiont‐rich insect order Hemiptera, pea aphids (Acyrthosiphon pisum) possess several heritable symbionts with roles in host plant utilization, thermotolerance and protection against natural enemies. As pea aphids vary in the numbers and types of harboured symbionts, these bacteria provide heritable and functionally important variation within field populations. In this study, we quantified the cytoplasmically inherited genetic variation contributed by symbionts within North American pea aphids. Through the use of Denaturing Gradient Gel Electrophoresis (DGGE) and 454 amplicon pyrosequencing of 16S rRNA genes, we explored the diversity of bacteria harboured by pea aphids from five populations, spanning three locations and three host plants. We also characterized strain variation by analysing 16S rRNA, housekeeping and symbiont‐associated bacteriophage genes. Our results identified eight species of facultative symbionts, which often varied in frequency between locations and host plants. We detected 28 cytoplasmic genotypes across 318 surveyed aphids, considering only the various combinations of secondary symbiont species infecting single hosts. Yet the detection of multiple Regiella insecticola, Hamiltonella defensa and Rickettsia strains, and diverse bacteriophage genotypes from H. defensa, suggest even greater diversity. Combined, these findings reveal that heritable bacteria contribute substantially to genetic variation in A. pisum. Given the costs and benefits of these symbionts, it is likely that fluctuating selective forces play a role in the maintenance of this diversity.     

Successfully resisting a pathogen is rarely costly in <Emphasis Type="Italic">Daphnia magna</Emphasis>

Background  

A central hypothesis in the evolutionary ecology of parasitism is that trade-offs exist between resistance to parasites and other fitness components such as fecundity, growth, survival, and predator avoidance, or resistance to other parasites. These trade-offs are called costs of resistance. These costs fall into two broad categories: constitutive costs of resistance, which arise from a negative genetic covariance between immunity and other fitness-related traits, and inducible costs of resistance, which are the physiological costs incurred by hosts when mounting an immune response. We sought to study inducible costs in depth using the crustacean Daphnia magna and its bacterial parasite Pasteuria ramosa.     

EFFECTS OF TEMPERATURE ON THE FITNESS COST OF RESISTANCE TO BACTERIOPHAGE T4 IN ESCHERICHIA COLI.

Resistance to predation, herbivory, or disease often comes at a cost such that resistant genotypes are competitively inferior to their sensitive counterparts in the absence of predators, herbivores, or pathogens. The effects of this trade‐off on natural populations depend on its sensitivity to environmental changes. We used Escherichia coli and bacteriophage T4 as a model predator/prey system to study the effects of temperature on the cost of resistance. An array of independent T4‐resistant mutants, derived from a single ancestral strain of E. coli B, had a mean reduction in competitive fitness that depended strongly on environmental temperature; the cost of resistance generally increased with temperature. Genetic variance for fitness among phage‐resistant mutants also depended on temperature; however, genetic variance increased at high and low thermal extremes. These results suggest that temperature is likely to be an important determinant of the consequences of predation in natural communities. We also discuss the underlying mechanistic basis for the cost of resistance in this system and its interaction with temperature.     

Genetic variation in fitness parameters associated with resistance to Bacillus thuringiensis in male and female Trichoplusia ni

Resistance of insects to insecticides is often associated with reduced fitness in the absence of selection. We examined fitness trade-offs associated with resistance to the microbial insecticide, Bacillus thuringiensis (Bt), across full-sib families in a resistant population of Trichoplusia ni. Significant genetic variation in and heritability of susceptibility to Bt occurred among the full-sib families. Male pupal weight was positively correlated with Bt susceptibility, indicating a potential fitness cost, but no such correlation occurred for females. Significant heritability for pupal weight was present for males but not females. A significant negative genetic correlation existed between development time and Bt susceptibility, indicating that resistant larvae developed more slowly than susceptible larvae. Selection for Bt resistance in T. ni resulted in changes in life-history traits that affected males more than females.     

Cost of trichome production and resistance to a specialist insect herbivore in <Emphasis Type="Italic">Arabidopsis lyrata</Emphasis>

Theory predicts that trade-offs between resistance to herbivory and other traits positively affecting fitness can maintain genetic variation in resistance within plant populations. In the perennial herb Arabidopsis lyrata, trichome production is a resistance trait that exhibits both qualitative and quantitative variation. Using a paternal half-sib design, we conducted two greenhouse experiments to ask whether trichomes confer resistance to oviposition and leaf herbivory by the specialist moth Plutella xylostella, and to examine potential genetic constraints on evolution of increased resistance and trichome density. In addition, we examined whether trichome production is induced by insect herbivory. We found strong positive genetic and phenotypic correlations between leaf trichome density and resistance to leaf herbivory, demonstrating that the production of leaf trichomes increases resistance to leaf damage by P. xylostella. Also resistance to oviposition tended to increase with increasing leaf trichome density, but genetic and phenotypic correlations were not statistically significant. Trichome density and resistance to leaf herbivory were negatively correlated genetically with plant size in the absence of herbivores, but not in the presence of herbivores. There was no evidence of increased trichome production after leaf damage by P. xylostella. The results suggest that trichome production and resistance to leaf herbivory are associated with a cost and that the direction of selection on resistance and trichome density depends on the intensity of herbivory.     

Plant-insect interactions.

Recent research shows partially overlapping signal transduction pathways controlling responses to wounding, insects, and pathogens. Chemical and behavioral assays show that plants release herbivore-specific volatiles, and that parasitic wasps can distinguish between these emission patterns. QTL mapping and candidate gene studies are beginning to identify polymorphic resistance genes, and ecological analyses provide information on the physiological and fitness costs of resistance. Such multidisciplinary approaches can elucidate the physiological causes and ecological consequences of plant-herbivore interactions.