Challenges of metamorphosis in invertebrate hosts: maintaining parasite resistance across life‐history stages

1. Insects lack the acquired immune system of vertebrates, but there is some evidence that insect immunity can be primed against an encountered pathogen to mitigate the intensity of future infections within a life stage. 2. Many invertebrates have multiple life‐history stages separated by complete metamorphosis, but different life stages can often be infected by the same pathogens, and the potential loss of immune priming during metamorphosis could therefore have detrimental effects on the host. Evidence that invertebrate immune priming can persist through metamorphosis is still missing, and consequently it is unclear how host–parasite interactions change across different life‐history stages in the context of infection history. 3. By experimentally manipulating the infection history of the flour beetle Tribolium confusum, we show that intestinal gregarine parasite infections during the larval stage reduced parasite load in adults, demonstrating that a host‐controlled mechanism for parasite resistance can persist through complete metamorphosis in insects. 4. Infections reduced larval developmental rates and increased host mortality but only during the crucial metamorphic stage, indicating that parasites impact multiple life stages. In general, our results demonstrate that invertebrates can show surprisingly robust immune priming despite dramatic physiological changes and protect hosts across completely different life‐history stages.     

Seasonal Patterns of Hormones,Macroparasites, and Microparasites in Wild African Ungulates: The Interplay among Stress,Reproduction, and Disease

Sex hormones, reproductive status, and pathogen load all affect stress. Together with stress, these factors can modulate the immune system and affect disease incidence. Thus, it is important to concurrently measure these factors, along with their seasonal fluctuations, to better understand their complex interactions. Using steroid hormone metabolites from fecal samples, we examined seasonal correlations among zebra and springbok stress, reproduction, gastrointestinal (GI) parasite infections, and anthrax infection signatures in zebra and springbok in Etosha National Park (ENP), Namibia, and found strong seasonal effects. Infection intensities of all three GI macroparasites examined (strongyle helminths, Strongyloides helminths, and Eimeria coccidia) were highest in the wet season, concurrent with the timing of anthrax outbreaks. Parasites also declined with increased acquired immune responses. We found hormonal evidence that both mares and ewes are overwhelmingly seasonal breeders in ENP, and that reproductive hormones are correlated with immunosuppression and higher susceptibility to GI parasite infections. Stress hormones largely peak in the dry season, particularly in zebra, when parasite infection intensities are lowest, and are most strongly correlated with host mid-gestation rather than with parasite infection intensity. Given the evidence that GI parasites can cause host pathology, immunomodulation, and immunosuppression, their persistence in ENP hosts without inducing chronic stress responses supports the hypothesis that hosts are tolerant of their parasites. Such tolerance would help to explain the ubiquity of these organisms in ENP herbivores, even in the face of their potential immunomodulatory trade-offs with anti-anthrax immunity.     

The Plasmodium parasite—a ‘new’ challenge for insect innate immunity

Though lacking adaptive immunity, insects possess a powerful innate immune system, a genome-encoded defence machinery used to confront infections. Studies in the fruit fly Drosophila melanogaster revealed a remarkable capacity of the innate immune system to differentiate between and subsequently respond to different bacteria and fungi. However, hematophagous compared to non-hematophagous insects encounter additional blood-borne infectious agents, such as parasites and viruses, during their lifetime. Anopheles mosquitoes become infected with the malaria parasite Plasmodium during feeding on infected human hosts and may then transmit the parasite to new hosts during subsequent bites. Whether Anopheles has developed mechanisms to confront these infections is the subject of this review. Initially, we review our current understanding of innate immune reactions and give an overview of the Anopheles immune system as revealed through comparative genomic analyses. Then, we examine and discuss the capacity of mosquitoes to recognize and respond to infections, especially to Plasmodium, and finally, we explore approaches to investigate and potentially utilize the vector immune competence to prevent pathogen transmission. Such approaches constitute a new challenge for insect immunity research, a challenge for global health.     

A narrow hybrid zone between the grasshoppers Stenobothrus clavatus and Stenobothrus rubicundus (Orthoptera: Gomphocerinae): female preferences for courtship songs

Two grasshopper species, Stenobothrus rubicundus and Stenobothrus clavatus, were previously shown to hybridize in a narrow contact zone on Mount Tomaros in northern Greece. The species are characterized by complex and completely different courtship songs. In the present study, we investigated female preferences for the courtship songs of S. rubicundus, S. clavatus and hybrids in playback experiments. Playback of the courtship songs revealed assortative preferences in females of the parental species: they significantly more often preferred the songs of conspecific males. Hybrid females showed a lower selectivity than parental females, responding somewhat equally eager to playback of the songs of S. clavatus, S. rubicundus, and natural hybrid song, although less actively to the F₁ hybrid song. The results suggest that hybrid males may lose to males of parental species, whereas hybrid females would even have an advantage over parental females. Comparison of responses of females from allopatric populations and Mount Tomaros to different song types shows no evidence for reinforcement. Asymmetry found in female preferences may have implications for the structure of the hybrid zone. © 2013 The Linnean Society of London     

Hybridization leads to host-use divergence in a polyphagous butterfly sibling species pair

Climate warming has lead to increased genetic introgression across a narrow hybrid zone separating the eastern and Canadian tiger swallowtails (Papilio glaucus and Papilio canadensis). This situation has led to the formation of an allochronically separated hybrid population with a delayed emerging phenotype or “late flight”. Here, we assess how the recombination of the parental genomes that lead to this phenotype may have facilitated another major ecological shift, host-use divergence. We first contrast the ovipositional profiles of the late flight population to that of the parental species P. glaucus and P. canadensis. Subsequently we contrast the larval survival and growth of the late flight, a P. canadensis and a P. glaucus population, and a population from the northern edge of the hybrid zone on five hosts. Our results indicate that the ovipositional preference of this hybrid swarm is identical to that of the introgressing parental species, P. glaucus. Due to the absence of the preferred hosts of P. glaucus (Liriodendron tulipifera L. and Ptelea trifoliata L.) where the late flight occurs, this ovipositional pattern implies a functional specialization onto a secondary host of both parental species, Fraxinus americana L. In contrast, the larval host-use abilities represent a mixture of P. glaucus and P. canadensis, indicating divergence in larval host-use abilities has not taken place. However, high genetic variability (genetic coefficient of variation) is present for growth on F. americana in the late flight hybrid swarm and tradeoffs for larval performance on the preferred hosts of the parental species are evident; indicating a strong potential for future specialization in larval host-use abilities. This current scenario represents an instance where a shift in a major ecological trait, host use, is likely occurring as a byproduct of a shift in an unrelated trait (delayed emergence) leading to partial reproductive isolation.     

HYBRID DYSFUNCTION IN FIRE-BELLIED TOADS (BOMBINA)

Reproductive isolation between two taxa may be due to endogenous selection, which is generated by incompatibilities between the respective genomes, to exogenous selection, which is generated by differential adaptations to alternative environments, or to both. The continuing debate over the relative importance of either mode of selection has highlighted the need for unambiguous data on the fitness of hybrid genotypes. The hybrid zone between the fire-bellied toad (Bombina bombina) and the yellow-bellied toad (B. variegata) in central Europe involves adaptation to different environments, but evidence of hybrid dysfunction is equivocal. In this study, we followed the development under laboratory conditions of naturally laid eggs collected from a transect across the Bombina hybrid zone in Croatia. Fitness was significantly reduced in hybrid populations: Egg batches from the center of the hybrid zone showed significantly higher embryonic and larval mortality and higher frequencies of morphological abnormalities relative to either parental type. Overall mortality from day of egg collection to three weeks after hatching reached 20% in central hybrid populations, compared to 2% in pure populations. There was no significant difference in fitness between two parental types. Within hybrid populations, there was considerable variation in fitness, with some genotypes showing no evidence of reduced viability. We discuss the implications of these findings for our understanding of barriers to gene flow between species.     

VIRULENCE OF MIXED-CLONE AND SINGLE-CLONE INFECTIONS OF THE RODENT MALARIA PLASMODIUM CHABAUDI

Most evolutionary models treat virulence as an unavoidable consequence of microparasite replication and have predicted that in mixed-genotype infections, natural selection should favor higher levels of virulence than is optimal in genetically uniform infections. Increased virulence may evolve as a genetically fixed strategy, appropriate for the frequency of mixed infections in the population, or may occur as a conditional response to mixed infection, that is, a facultative strategy. Here we test whether facultative alterations in replication rates in the presence of competing genotypes occur and generate greater virulence. An important alternative, not currently incorporated in models of the evolution of virulence, is that host responses mounted against genetically diverse parasites may be more costly or less effective than those against genetically uniform parasites. If so, mixed clone infections will be more virulent for a given parasite replication rate. Two groups of mice were infected with one of two clones of Plasmodium chabaudi parasites, and three groups of mice were infected with 1:9, 5:5, or 9:1 mixtures of the same two clones. Virulence was assessed by monitoring mouse body weight and red blood cell density. Transmission stage densities were significantly higher in mixed- than in single-clone infections. Within treatment groups, transmission stage production increased with the virulence of the infection, a phenotypic correlation consistent with the genetic correlation assumed by much of the theoretical work on the evolution of virulence. Consistent with theoretical predictions of facultative alterations in virulence, we found that mice infected with both parasite clones lost more weight and had on average lower blood counts than those infected with single-clone infections. However, there was no consistent evidence of the mechanism invoked by evolutionary models that predict this effect. Replication rates and parasite densities were not always higher in ???mixed-clone infections, and for a given replication rate or parasite density, mixed-clone infections were still more virulent. Instead, prolonged anemia and increased transmission may have occured because genetically diverse infections are less rapidly cleared by hosts. Differences in maximum weight loss occured even when there were comparable parasite densities in mixed- and single-clone infections. We suggest that mounting an immune response against more that one parasite genotype is more costly for hosts, which therefore suffer higher virulence.     

Tissue-Dwelling Intracellular Parasites: Infection and Immune Responses in the Mammalian Host to Toxoplasma, Sarcocystis and Trichinella

The cycles, pathogenicity and immune processes observed in 3intracellular infections, with the protozoa Toxoplasma gondii,several Sarcocystis species, and the nematode Trichinella spirahsare described. All three organisms have cycles that have co-existedwith their hosts for an evolutionarily long period. Naturalhosts of the protozoa generally are able to develop immunityafter primary infection in a timely manner. However infectionis often fatal in certain unusual hosts not naturally exposedto Toxoplasma and Sarcocystis. It is suggested that whereashosts may have selected strains of intermediate pathogenicity,the two protozoa may have selected for hosts that can developtimely immune responses and survive with chronic persistentinfection. Stability of cycle is maintained by co-evolutionin the definitive host through which the cycle must pass almostregularly. Toxoplasmosis and sarcosporidiosis join malaria andAfrican trypanosomiasis as infections that co-evolved with theirhosts, selecting for immunocompetency and intermediate to lowpathogenicity (Allison, 1982). The plea is made to considerthe probable evolutionary experience of experimental hosts whenstudying their specific immune mechanisms and before possiblygeneralizing from such studies, because the immune responseis dependent on evolutionary experience. In respect to infectionwith Trichinella spiralis one might consider carnivores to havebeen selected and pure herbivores unselected. However, infectionoccurs in a wide variety of hosts, with carnivorous, scavengingand accidental transmission, some of which are of recent modificationby man. Because infection is of low prevalence, and involvesa wide variety of hosts, and pathogenicity is more related tothe degree of hypersensitivity, than to frequency of transmissionand worm load, no uniform effects of selection are apparent.Trichinella infected athymic mice have a high worm load withlittle pathogenic effect. Immunologic processes, while restrictingthe number of worms, are much more important in making the hostsick. In all three intracellular infections both protectiveimmunity and delayed type hypersensitivity are operative. Inbalance, these immune responses are protective in Toxoplasmaand Sarcocystis infection, but are pathogenic in trichinellosis,often causing the death of the infected host.     

Effects of hybridization on the immunity of collared Ficedula albicollis and pied flycatchers F. hypoleuca, and their infection by haemosporidians

Because they are ubiquitous and typically reduce the fitness of hosts, parasites may play important roles in hybrid zone dynamics. Despite much work on herbivores and hybrid plants, the effect of parasites on the fitness of animal hybrids is poorly known. In an attempt to partly fill this gap, we examined the prevalence of avian haemosporidians Haemoproteus in a hybrid zone between collared Ficedula albicollis and pied flycatchers F. hypoleuca . 40 species-informative genetic markers allowed us to identify F1 hybrids, thus avoiding problems inherent in many studies that group hybrid genotypes. Furthermore, naturally occurring extra-pair paternity allowed us to test the immune responses of pure and hybrid nestlings to a novel antigen (phytohaemagglutinin) in a shared environment. In contrast to previous suggestions that animal hybrids may more often display resistance against parasites than plant hybrids, F1 hybrids exhibited prevalence of parasitism and immune responses that were intermediate between the two parental species. We also detected differences between the two parental species in their prevalence of infection, with the competitively dominant species (collared flycatcher) being less often infected by Haemoproteus . Overall, our results contribute to other recent data supporting the idea that the resistance of animals to parasites is variously and unpredictably affected by hybridization, and that there is a concordance in the general patterns observed in plants and animals. Haemosporidians in avian hybrids provide a useful system for investigating the interactions between hosts and parasites that characterize host contact zones.     

Comparison of spleen transcriptomes of two wild rodent species reveals differences in the immune response against Borrelia afzelii

Different host species often differ considerably in susceptibility to a given pathogen, but the causes of such differences are rarely known. The natural hosts of the tick‐transmitted bacterium Borrelia afzelii, which is one of causative agents of Lyme borreliosis in humans, include a variety of small mammals like voles and mice. Previous studies have shown that B. afzelii‐infected bank voles (Myodes glareolus) have about ten times higher bacterial load than infected yellow‐necked mice (Apodemus flavicollis), indicating that these two species differ in resistance. In this study, we compared the immune response to B. afzelii infection in these host species by using RNA sequencing to quantify gene expression in spleen. Gene set enrichment analysis (GSEA) showed that several immune pathways were down‐regulated in infected animals in both bank voles and yellow‐necked mice. Moreover, IFNα response was up‐regulated in B. afzelii‐infected yellow‐necked mice, while IL6 signaling and the complement pathway were down‐regulated in infected bank voles; differences in regulation of these three pathways between bank voles and yellow‐necked mice could thus contribute to the difference in resistance to B. afzelii between the species. This study provides knowledge of gene expression induced by a zoonotic pathogen in its natural host, and possible species‐specific regulation of immune responses associated with resistance.     

MHC-I Affects Infection Intensity but Not Infection Status with a Frequent Avian Malaria Parasite in Blue Tits

Host resistance against parasites depends on three aspects: the ability to prevent, control and clear infections. In vertebrates the immune system consists of innate and adaptive immunity. Innate immunity is particularly important for preventing infection and eradicating established infections at an early stage while adaptive immunity is slow, but powerful, and essential for controlling infection intensities and eventually clearing infections. Major Histocompatibility Complex (MHC) molecules are central in adaptive immunity, and studies on parasite resistance and MHC in wild animals have found effects on both infection intensity (parasite load) and infection status (infected or not). It seems MHC can affect both the ability to control infection intensities and the ability to clear infections. However, these two aspects have rarely been considered simultaneously, and their relative importance in natural populations is therefore unclear. Here we investigate if MHC class I genotype affects infection intensity and infection status with a frequent avian malaria infection Haemoproteus majoris in a natural population of blue tits Cyanistes caeruleus. We found a significant negative association between a single MHC allele and infection intensity but no association with infection status. Blue tits that carry a specific MHC allele seem able to suppress H. majoris infection intensity, while we have no evidence that this allele also has an effect on clearance of the H. majoris infection, a result that is in contrast with some previous studies of MHC and avian malaria. A likely explanation could be that the clearance rate of avian malaria parasites differs between avian malaria lineages and/or between avian hosts.     

Why join groups? Lessons from parasite‐manipulated Artemia

Grouping behaviours (e.g. schooling, shoaling and swarming) are commonly explicated through adaptive hypotheses such as protection against predation, access to mates or improved foraging. However, the hypothesis that aggregation can result from manipulation by parasites to increase their transmission has never been demonstrated. We investigated this hypothesis using natural populations of two crustacean hosts (Artemia franciscana and Artemia parthenogenetica) infected with one cestode and two microsporidian parasites. We found that swarming propensity increased in cestode‐infected hosts and that red colour intensity was higher in swarming compared with non‐swarming infected hosts. These effects likely result in increased cestode transmission to its final avian host. Furthermore, we found that microsporidian‐infected hosts had both increased swarming propensity and surfacing behaviour. Finally, we demonstrated using experimental infections that these concurrent manipulations result in increased spore transmission to new hosts. Hence, this study suggests that parasites can play a prominent role in host grouping behaviours.     

Strong density-dependent competition and acquired immunity constrain parasite establishment: implications for parasite aggregation

The vast majority of parasites exhibit an aggregated frequency distribution within their host population, such that most hosts have few or no parasites while only a minority of hosts are heavily infected. One exception to this rule is the trophically transmitted parasite Pterygodermatites peromysci of the white-footed mouse (Peromyscus leucopus), which is randomly distributed within its host population. Here, we ask: what are the factors generating the random distribution of parasites in this system when the majority of macroparasites exhibit non-random patterns? We hypothesise that tight density-dependent processes constrain parasite establishment and survival, preventing the build-up of parasites within individual hosts, and preclude aggregation within the host population. We first conducted primary infections in a laboratory experiment using white-footed mice to test for density-dependent parasite establishment and survival of adult worms. Secondary or challenge infection experiments were then conducted to investigate underlying mechanisms, including intra-specific competition and host-mediated restrictions (i.e. acquired immunity). The results of our experimental infections show a dose-dependent constraint on within-host-parasite establishment, such that the proportion of mice infected rose initially with exposure, and then dropped off at the highest dose. Additional evidence of density-dependent competition comes from the decrease in worm length with increasing levels of exposure. In the challenge infection experiment, previous exposure to parasites resulted in a lower prevalence and intensity of infection compared with primary infection of naïve mice; the magnitude of this effect was also density-dependent. Host immune response (IgG levels) increased with the level of exposure, but decreased with the number of worms established. Our results suggest that strong intra-specific competition and acquired host immunity operate in a density-dependent manner to constrain parasite establishment, driving down aggregation and ultimately accounting for the observed random distribution of parasites.     

The expression of virulence during double infections by different parasites with conflicting host exploitation and transmission strategies

In many natural populations, hosts are found to be infected by more than one parasite species. When these parasites have different host exploitation strategies and transmission modes, a conflict among them may arise. Such a conflict may reduce the success of both parasites, but could work to the benefit of the host. For example, the less‐virulent parasite may protect the host against the more‐virulent competitor. We examine this conflict using the waterflea Daphnia magna and two of its sympatric parasites: the blood‐infecting bacterium Pasteuria ramosa that transmits horizontally and the intracellular microsporidium Octosporea bayeri that can concurrently transmit horizontally and vertically after infecting ovaries and fat tissues of the host. We quantified host and parasite fitness after exposing Daphnia to one or both parasites, both simultaneously and sequentially. Under conditions of strict horizontal transmission, Pasteuria competitively excluded Octosporea in both simultaneous and sequential double infections, regardless of the order of exposure. Host lifespan, host reproduction and parasite spore production in double infections resembled those of single infection by Pasteuria. When hosts became first vertically (transovarilly) infected with O. bayeri, Octosporea was able to withstand competition with P. ramosa to some degree, but both parasites produced less transmission stages than they did in single infections. At the same time, the host suffered from reduced fecundity and longevity. Our study demonstrates that even when competing parasite species utilize different host tissues to proliferate, double infections lead to the expression of higher virulence and ultimately may select for higher virulence. Furthermore, we found no evidence that the less‐virulent and vertically transmitting O. bayeri protects its host against the highly virulent P. ramosa.     

Studies on toll-like receptor stimuli responsiveness in HIV-1 and HIV-2 infections

Background: HIV-1 and HIV-2 are two related viruses with distinct clinical outcomes, where HIV-1 is more pathogenic and transmissible than HIV-2. The pathogenesis of both infections is influenced by the dysregulation and deterioration of the adaptive immune system. However, their effects on the responsiveness of innate immunity are less well known. Here, we report on toll-like receptor (TLR) stimuli responsiveness in HIV-1 or HIV-2 infections. Methods: Whole blood from 235 individuals living in Guinea-Bissau who were uninfected, infected with HIV-1, infected with HIV-2, and/or infected with HTLV-I, was stimulated with TLR7/8 and TLR9 agonists, R-848 and unmethylated CpG DNA. After TLR7/8 and TLR9 stimuli, the expression levels of IL-12 and IFN-α were related to gender, age, infection status, CD4⁺ T cell counts, and plasma viral load. Results: Defective TLR9 responsiveness was observed in the advanced disease stage, along with CD4⁺ T cell loss in both HIV-1 and HIV-2 infections. Moreover, TLR7/8 responsiveness was reduced in HIV-1 infected individuals compared with uninfected controls. Conclusions: Innate immunity responsiveness can be monitored by whole blood stimulation. Both advanced HIV-1 and HIV-2 infections may cause innate immunity dysregulation.     

Community structure of canopy arthropods associated to Quercus crassifolia×Quercus crassipes complex

Quercus crassifolia and Q. crassipes are dominant species in temperate forests of central Mexico and hybridize between each other when they occur in sympatry. Oak canopies contain a considerable portion of arthropod diversity and the hybrid zones can provide new habitats to epiphyte fauna. We tested if the establishment of hybrids in contact zones with their parental hosts increases the species diversity of canopy arthropods assuming that hybrid trees constitute new genotypes of potential new habitats to small organisms. We examined the effect of hybridization on some community structure parameters (diversity, composition, similarity and density of arthropod fauna) of canopy arthropods compared to their parental species in a hybrid zone located in central Mexico. We employed 17 leaf morphological traits and six diagnostic RAPD primers to identify parental and hybrid plants. The RAPDs marker showed unidirectional introgression towards Q. crassifolia, and were detected hybrid (F₁), backcrosses and introgression individual trees. In total, 30 oak canopies were fogged during rainy and dry season. We recognized 532 taxa of arthropods belonging to 22 orders associated with tree canopies. The taxonomic status of host‐trees may be an important factor in the arthropod community structure and that seasonality (dry and rainy) is not a factor that could modify their organization. Trees of Q. crassipes registered the highest densities of arthropod fauna followed by hybrid hosts (F₁); trees originated by backcrosses towards Q. crassifolia registered a significant less arthropod density than F₁ hybrids; and trees of Q. crassifolia had the lowest density. Hybrid plants and Q. crassipes individuals had higher diversity (H′) of arthropods than Q. crassifolia plants. Hybrid plants had also more rare species in both seasons in comparison with parental species. This study suggests that hybrid oaks act as a center of biodiversity by accumulating arthropods of both parental and different species including a considerable number of rare species.     

ELECTROPHORETIC CONFIRMATION OF INTERSPECIFIC HYBRIDIZATION IN AESCULUS (HIPPOCASTANACEAE) AND THE GENETIC STRUCTURE OF A BROAD HYBRID ZONE

Within a broad (>200 km wide) hybrid zone involving three parapatric species of Aesculus, we observed coincident clines in allele frequency for 6 of 14 electrophoretic loci. The cooccurrence of alleles characteristic of A. pavia, A. sylvatica, and A. flava was used to estimate genetic admixtures in 48 populations involving various hybrids between these taxa in the southeastern United States. High levels of allelic polymorphism (up to 40% greater than the parental taxa) were observed in hybrid populations and also in some populations bordering the hybrid zone. A detailed analysis of a portion of the hybrid zone involving A. pavia and A. sylvatica revealed a highly asymmetrical pattern of gene flow, predominantly from Coastal Plain populations of A. pavia into Piedmont populations of A. sylvatica. Computer simulations were used to generate expected genotypic arrays for parental, F₁; and backcross individuals, which were compared with natural populations using a character index scoring system. In these comparisons, hybrid individuals could be distinguished from either parent, but F₁ and backcross progeny could not be distinguished from each other. Most hybrid populations were found to include hybrids and one of the parental taxa, but never both parents. Three populations appeared to be predominantly hybrids with no identifiable parental individuals. Hybrids occurred commonly at least 150 km beyond the range of A. pavia, but usually not more than 25 km beyond the range of A. sylvatica. Introgression, suggested by genetically hybrid individuals and significant gene admixtures of two or more species in populations lacking morphological evidence of hybridization, may extend the hybrid zone further in both directions. The absence of one or both parental species from hybrid populations implies a selective disadvantage to parentals in the hybrid zone and/or that hybridization has occurred through long-distance gene flow via pollen, primarily from A. pavia into A. sylvatica. Long-distance pollen movement in plants may generate hybrid zones of qualitatively different structure than those observed in animals, where gene flow involves dispersal of individuals.     

Parasitism, immune response and reproductive success in the house martin Delichonurbica

Parasites often exert strong selection pressures on their hosts that have evolved anti-parasite defences to counter the negative effects of parasites. We studied the relationship between intensity of parasitism, one aspect of host immune response, and host reproductive success, using the house martin bug Oeciacushirundinis and its house martin Delichonurbica host as a model system. Experimental manipulation of parasite load of nests during laying of the first clutch altered the intensity of parasitism. Parasites reduced the reproductive success of their hosts measured in terms of body condition and survival of nestlings. Host immune response, measured as the concentration of gammaglobulins and total plasma proteins, was positively associated with parasite reproduction, estimated as the number of juvenile parasites, but was only weakly related to the intensity of adult parasites. The concentration of gammaglobulins was negatively related to nestling body mass, implying a trade-off between immune function and body condition. Parasite reproduction thus exerts a cost on hosts by increasing the immune response. Received: 25 August 1997 / Accepted: 3 November 1997     

Alternative seed defence mechanisms in a palo verde (Fabaceae) hybrid zone: effects on bruchid beetle abundance

Abstract.

• 1 We asked three questions about the patterns of relative abundance of insect herbivores across host plant taxa at a palo verde hybrid zone. (1) What is the morphological structure of the hybrid zone and does this suggest a certain pattern of introgression? (2) How are putative parental seed defence mechanisms expressed in hybrid plants? (3) Do ovipositing females prefer host plant taxa on which their offspring have best survivorship?
• 2 Morphologically, hybrids were either intermediate or tended to resemble one parental species. Previous studies have suggested that unidirectional introgression results in loss of parental defence mechanisms against herbivores. Hybrid plants in general lacked seed coat resistance and early pod abscission which are known to act as plant defence mechanisms against bruchid beetles in the parental palo verde trees.
• 3 All other sources of bruchid mortality that we examined did not vary across parental and hybrid taxa, with the possible exception of egg parasitism which occurred at a lower frequency on one parental palo verde species.
• 4 Thus, survivorship of bruchid offspring should be greater on hybrid palo verdes.
• 5 Patterns of egg densities suggested that females may prefer hybrid hosts in some years but not others. An oviposition choice experiment conducted in the field, however, showed bruchids have no preference for hybrids over one of the parental species.
• 6 These results suggest that some insect herbivores may have higher densities on hybrid host plants because they are less resistant.