Parasitic Offspring Research Articles

Co-dependent formation of the Toxoplasma gondii sub-pellicular microtubules and inner membrane skeleton.

One of the defining features of apicomplexan parasites is their cytoskeleton composed of alveolar vesicles, known as the inner membrane complex (IMC) undergirded by intermediate-like filament network and an array of subpellicular microtubules (SPMTs). In Toxoplasma gondii, this specialized cytoskeleton is involved in all aspects of the disease-causing lytic cycle, and notably acting as a scaffold for parasite offspring in the internal budding process. Despite advances in our understanding of the architecture and molecular composition, insights pertaining to the coordinated assembly of the scaffold are still largely elusive. Here, T. gondii tachyzoites were dissected by advanced, iterative expansion microscopy (pan-ExM) revealing new insights into the very early sequential formation steps of the tubulin scaffold. A comparative study of the related parasite Sarcocystis neurona revealed that different MT bundling organizations of the nascent SPMTs correlate with the number of central and basal alveolar vesicles. In absence of a so far identified MT nucleation mechanism, we genetically dissected T. gondii γ-tubulin and γ-tubulin complex protein 4 (GCP4). While γ-tubulin depletion abolished the formation of the tubulin scaffold, a set of MTs still formed that suggests SPMTs are nucleated at the outer core of the centrosome. Depletion of GCP4 interfered with the correct assembly of SPMTs into the forming daughter buds, further indicating that the parasite utilizes the γ-tubulin complex in tubulin scaffold formation .

Limited evidence of biased offspring sex allocation in a cavity-nesting conspecific brood parasite.

Sex allocation theory predicts that mothers should bias investment in offspring toward the sex that yields higher fitness returns; one such bias may be a skewed offspring sex ratio. Sex allocation is well-studied in birds with cooperative breeding systems, with theory on local resource enhancement and production of helpers at the nest, but little theoretical or empirical work has focused on birds with brood parasitic breeding systems. Wood ducks (Aix sponsa) are a conspecific brood parasite, and rates of parasitism appear to increase with density. Because female wood ducks show high natal philopatry and nest sites are often limiting, local resource competition (LRC) theory predicts that females should overproduce male offspring-the dispersing sex-when competition (density) is high. However, the unique features of conspecific brood parasitism generate alternative predictions from other sex allocation theory, which we develop and test here. We experimentally manipulated nesting density of female wood ducks in 4 populations from 2013 to 2016, and analyzed the resulting sex allocation of >2000 ducklings. In contrast to predictions we did not find overproduction of male offspring by females in high-density populations, females in better condition, or parasitic females; modest support for LRC was found in overproduction of only female parasitic offspring with higher nest box availability. The lack of evidence for sex ratio biases, as expected for LRC and some aspects of brood parasitism, could reflect conflicting selection pressures from nest competition and brood parasitism, or that mechanisms of adaptive sex ratio bias are not possible.

Expressing Parasitoid Venom Protein VRF1 in an Entomopathogen Beauveria bassiana Enhances Virulence toward Cotton Bollworm Helicoverpa armigera.

Despite entomopathogenic fungi being used in various insect pest control, it is recognized that they could replace more chemical insecticides if they were more efficient. We have found that cotton bollworm Helicoverpa armigera responded to the infection of entomopathogenic fungus Beauveria bassiana by activating the Toll pathway. Koinobiont wasps also regulate host immunity and development to ensure the survival of their progeny. Previously, venom protein VRF1 was identified in Microplitis mediator. It enters H. armigera hemocytes, suppresses the expression of antimicrobial peptides (AMPs) by inhibiting the Toll pathway, and prevents parasite offspring from being encapsulated. With this in mind, we thought that it might be feasible to increase the virulence of B. bassiana by embedding VRF1 into its genome. Compared with that of wild-type (WT) B. bassiana, the median lethal dose (LD50) of the transformant expressing VRF1 (named BbVRF1) decreased approximately 2.36-fold, and the median time to lethality (LT50) was shortened to 84% when infecting H. armigera (a natural host of M. mediator). The AMP expression level of hemocytes in H. armigera infected with BbVRF1 strain was significantly downregulated compared to that in the control group infected with the WT. In addition, the LD50 of BbVRF1 against the fall armyworm Spodoptera frugiperda (an unnatural host of M. mediator) was decreased 3.45-fold and the LT50 was shortened to 73%, showing a greater virulence. Our research indicated that BbVRF1, an engineered strain of B. bassiana, has greater efficacy against pest insects both within and outside its host range (M. mediator), expanding the utilization of parasitoid wasp virulence effectors. IMPORTANCE Mycoinsecticides are essential for the development of integrated pest management as substitutes to chemical insecticides, but their usage is limited by their inferior virulence. Thus, genetically engineered bioinsecticides, including recombinant entomopathogenic fungi, have been regarded as a breakthrough to rapidly control pests. Deep knowledge of parasitoid wasps allows us to take advantage of this natural enemy of pest insects beyond raising them for field release. Our transformant BbVRF1 (Beauveria bassiana integrated with a venom protein VRF1 from Microplitis mediator) showed a higher virulence in H. armigera and S. frugiperda, demonstrating its potential for managing natural or unnatural hosts of M. mediator. This result provides a new strategy regarding which venom protein of parasitoid wasps can become part of the arsenal with which to equip entomopathogenic fungi. Utilizing parasitoid wasps with this approach could easily overcome the difficulties of artificial culture and enhance the virulence of other biocontrol agents.

Fledgling discrimination in the hoopoe, a potential host species of the great spotted cuckoo

Obligate brood parasites lay their eggs in nests of other species, with host parents bearing the cost of raising their offspring. These costs imposed on hosts select for the evolution of host defenses against parasitism at all stages of the reproductive cycle. The most effective defense is egg rejection at early stages of the breeding cycle, with later-stage defenses (nestling and fledgling discrimination) being less common. In this study, we tested whether the hoopoe (Upupa epops), a potential host of the great spotted cuckoo (Clamator glandarius) without egg rejection ability, presents defenses after the egg stage. We experimentally parasitized hoopoe nests with great spotted cuckoo nestlings creating mixed broods (with hoopoe and cuckoo nestlings) and broods with only cuckoo nestlings and measured parental feeding behavior and survival of nestlings and fledglings of both species. Cuckoo fledglings were fed fewer often than hoopoe fledglings in mixed broods, and adults approached more often to feed hoopoe fledglings than cuckoo fledglings. Consequently, the survival of cuckoo fledglings in both mixed and only-cuckoo-broods, was significantly lower than that of hoopoe fledglings. These results suggest that hoopoes would discriminate great spotted cuckoo fledglings, with or without direct comparison with their own fledglings. However, the survival of some cuckoos suggests that hoopoes have not reached highly efficient defenses so, other life history traits hindering parasitism by cuckoos may explain low parasitism rates and low levels of defenses in this species.Significance statementBrood parasites lay their eggs in nests of other species, tricking hosts into raising their parasitic offspring. However, hosts may fight back impeding successful parasitism by developing defences at any of the stages of their breeding cycle. We investigated why the hoopoe is not parasitized by the great spotted cuckoo despite this potential host apparently does not show such anti-parasitic defenses. We found that hoopoes have evolved the less common host defense: discrimination of parasite fledglings, even in the absence of their own fledgling for comparison. Our study supports the idea that discrimination during the later stages of the nesting cycle (i.e. nestling and fledgling periods) may be more common that previously assumed.

Low incidence of cannibalism among brood parasitic cuckoo catfish embryos.

Brood parasites have demanding needs of host resources. Brood parasitic offspring are highly competitive and frequently cause the failure of host broods and the survival of a single parasitic offspring. Accordingly, virulent brood parasites lay a single egg in the same host nest to avoid sibling competition. In the cuckoo catfish (Synodontis multipunctatus), which parasitize mouthbrooding cichlid fishes in Lake Tanganyika, the modes of host and parasite oviposition lead to frequent cases of multiple parasitism. We experimentally tested the prediction that multiple parasitism leads to frequent cannibalism among offspring. Cuckoo catfish embryos prey upon host offspring to obtain nourishment during their 3-week development in the host buccal cavity and may also consume conspecific embryos. The potential benefits of cannibalism in the system are, therefore, twofold; to decrease competition for limited resources (i.e., host brood with rich yolk sacs) and to directly obtain nourishment by consuming rivals. We found that cannibalism indeed provided measurable benefits in terms of increased growth of the cannibals, but cannibalism was rare and typically occurred once all host offspring had been consumed. This implies that cannibalism among cuckoo catfish embryos emerges to mitigate starvation rather than eliminate competition.

Egg Rejection: Getting Rid of a Parasite in the Nest

Some species of birds, called brood parasites, lay their eggs in the nests of other species (called the hosts). Male and female hosts take care of the parasitic offspring. In most cases, the hosts raise only the parasitic chicks, which reduces the breeding success of the host birds. However, hosts can defend themselves from brood parasitism by developing the ability to recognize and reject parasitic eggs from their nests. In this article, we explore how host age influences the ability to reject foreign eggs. We show that female magpies improve their ability to reject foreign eggs added to their nests as they age, and we discuss what this might mean for both the magpies and cuckoos. Our results might ultimately help us to better understand how animals and humans defend themselves against the disease-causing parasites that infect them.

Unbalanced Arginine pathway and altered maturation of pleural macrophages in Th2-deficient mice during Litomosoides sigmodontis filarial infection.

Filarial parasites are tissue dwelling worms transmitted by hematophagous vectors. Understanding the mechanisms regulating microfilariae (the parasite offspring) development is a prerequisite for controlling transmission in filarial infections. Th2 immune responses are key for building efficient anti-parasite responses but have been shown to also lead to detrimental tissue damage in the presence of microfilariae. Litomosoides sigmodontis, a rodent filaria residing in the pleural cavity was therefore used to characterize pleuropulmonary pathology and associated immune responses in wild-type and Th2 deficient mice. Wild-type and Th2-deficient mice (Il-4rα-/-/Il-5-/- ) were infected with L. sigmodontis and parasite outcome was analyzed during the patent phase (when microfilariae are in the general circulation). Pleuropulmonary manifestations were investigated and pleural and bronchoalveolar cells were characterized by RNA analysis, imaging and/or flow cytometry focusing on macrophages. Il-4rα-/-/Il-5-/- mice were hypermicrofilaremic and showed an enhanced filarial survival but also displayed a drastic reduction of microfilaria-driven pleural cavity pathologies. In parallel, pleural macrophages from Il-4rα-/-/Il-5-/- mice lacked expression of prototypical alternative activation markers RELMα and Chil3 and showed an altered balance of some markers of the arginine metabolic pathway. In addition, monocytes-derived F4/80intermediate macrophages from infected Il-4rα-/-/Il-5-/- mice failed to mature into resident F4/80high large macrophages. Altogether these data emphasize that the presence of both microfilariae and IL-4R/IL-5 signaling are critical in the development of the pathology and in the phenotype of macrophages. In Il-4rα-/-/Il-5-/- mice, the balance is in favor of parasite development while limiting the pathology associated with the host immune response.

Decision-making at the time of parasitism: cowbirds prefer to peck eggs with weaker shells.

Interspecific avian brood parasites, like cuckoos and cowbirds, lay their eggs in nests of other species, the hosts, which take over the entire parental care of the parasite's eggs and chicks. This breeding strategy requires decisions that may affect the parasite's reproductive success. During the breeding season, cowbirds search for host nests and revisit them to monitor its progress and parasitize at the time host laying begins. When visiting hosts nests, they repeatedly peck the nest contents trying to destroy one or more eggs. This behaviour favours parasite's offspring by reducing the competition for food with nestmates. We evaluated if the egg-pecking behaviour of female shiny (Molothrus bonariensis) and screaming (M. rufoaxillaris) cowbirds is affected by the strength and the size of the eggs they find in the nest. We presented to wild-caught females artificial clutches with two natural eggs that differ in size and shell strength. We found that female shiny and screaming cowbirds adjusted egg-pecking behaviour based on the strength but not on the size of the eggs. When differences in strength between eggs were high, both cowbird species pecked more frequently the egg with the weaker shell, increasing the probability of a successful puncture. Our results indicate that female cowbirds can discriminate eggs through the strength of the shell, and by choosing the weaker egg to peck, they increase the probability of puncturing.

Pedigree simulations reveal that maternity assignment is reliable in populations with conspecific brood parasitism, incomplete parental sampling and kin structure

Modern genetic parentage methods reveal that alternative reproductive strategies are common in both males and females. Under ideal conditions, genetic methods accurately connect the parents to offspring produced by extra-pair matings or conspecific brood parasitism. However, some breeding systems and sampling scenarios present significant complications for accurate parentage assignment. We used simulated genetic pedigrees to assess the reliability of parentage assignment for a series of challenging sampling regimes that reflect realistic conditions for many brood-parasitic birds: absence of genetic samples from sires, absence of samples from brood parasites and female kin-structured populations. Using 18microsatellite markers and empirical allele frequencies from two populations of a conspecific brood parasite, the wood duck (Aix sponsa), we simulated brood parasitism and determined maternity using two widely used programs, cervus and colony. Errors in assignment were generally modest for most sampling scenarios but differed by program: cervus suffered from false assignment of parasitic offspring, whereas colony sometimes failed to assign offspring to their known mothers. Notably, colony was able to accurately infer unsampled parents. Reducing the number of markers (nine loci rather than 18) caused the assignment error to slightly worsen with colony but balloon with cervus. One potential error with important biological implications was rare in all cases-few nesting females were incorrectly excluded as the mother of their own offspring, an error that could falsely indicate brood parasitism. We consider the implications of our findings for both a retrospective assessment of previous studies and suggestions for best practices for future studies.

Adjustment of egg laying by both hosts and intraspecific brood parasites in a beetle

AbstractBrood parasites lay their eggs in the nests of other females, thereby shifting the costs of offspring care onto others. Given that care is costly, potential hosts should evolve mechanisms to avoid brood parasitism. Meanwhile, brood parasites should evolve mechanisms to circumvent host defences. Here we investigate whether hosts or intraspecific brood parasites adjust their egg laying behaviour as a mechanism to reduce or increase the effectiveness of brood parasitism. We use the burying beetle Nicrophorus vespilloides as our study system, in which hosts and brood parasites lay their eggs in the soil around a carcass controlled by the host. To test whether females adjust their egg laying behaviour when breeding as a host or brood parasite, we used an experimental design with three treatments: hosts, where focal females bred alongside a smaller female; brood parasites, where focal females bred alongside a larger female; and controls, where focal females bred alone. We used focal females from a narrowly defined size range to control for potential effects of body size. We found that hosts delayed the start of egg laying, which may allow them to recognise brood parasitic offspring that arrive too early. Meanwhile, brood parasites laid their eggs over an extended period, which may increase the chances that their egg laying overlapped with the host. Our results suggest that adjusting egg laying behaviour is a mechanism used by both hosts and brood parasites that may contribute to the differences in reproductive success shown in prior studies.

The blunt pole is not a source of more salient recognition cues than the sharp pole for the rejection of model eggs by American robins (Turdus migratorius)

Hosts of obligate avian brood parasites can reduce the costs of raising parasitic offspring by rejecting foreign eggs from their nests. Rejecter hosts use various visual and tactile cues to discriminate between own and foreign eggs. The blunt pole hypothesis specifically states that avian-perceivable visual information at and around the broader pole of the eggshell contains more salient recognition cues than does the sharp pole of the same egg. The directional prediction is, therefore, that eggs painted non-mimetically on their blunt pole should more likely be rejected relative to those similarly painted on their sharp pole. This hypothesis had been experimentally tested and its predictions supported solely in mimetic avian host-parasite systems, with hosts producing denser and more variable eggshell maculation patterns at the blunt pole, and in one species with immaculate eggs but still with distinctly discernible blunt-pole specific colouration. Here we aimed to expand upon these previous works and assessed whether the blunt pole of model eggs contains more salient egg rejection cues, relative to the sharp pole, for the American robin (Turdus migratorius), a robust rejecter of non-mimetic brown-headed cowbird (Molothrus ater) eggs. In this system host eggs are uniformly immaculate whereas the brood parasitic shell is maculated. We painted model cowbird-sized eggs on either the blunt or the sharp half to mimic the immaculate robin egg colours and the other half to resemble non-mimetic egg colours and patterns. There was no statistical support for the predicted outcomes of the blunt pole hypothesis in our trials as rejection rates were similar regardless of whether eggs were painted with non-mimetic colours on the blunt or sharp poles. Future work should test the role of asymmetrical signalling content for anti-parasitic rejection of eggs in additional host species, especially those with both immaculate own and mimetic parasitic eggs.

Genetic patterns of repeat and multiple parasitism by screaming cowbirds, a specialist brood parasite

Avian brood parasites lay their eggs in the nests of other species, leaving the hosts to care for the parasitic offspring. The bookkeeping hypothesis predicts that, in order to reduce competition between parasitic nestlings, female parasites should keep a mental inventory of host nests that they have already parasitized and avoid laying multiple eggs in the same host nest. However, selection against repeat parasitism should be weaker when host nests are limited, or when hosts are able to rear more than one parasitic nestling. Here we use microsatellite genotyping of parasitic eggs to test whether female screaming cowbirds, Molothrus rufoaxillaris, avoid repeatedly parasitizing nests of their primary host, the greyish baywing, Agelaioides badius, in Argentina. Parasitism rates were extremely high (96.5% of 57 host clutches were parasitized with an average of 5.7 cowbird eggs each), indicating that host nests are limited. Although eggs laid by the same female showed moderate spatiotemporal clustering, individual females rarely laid more than one egg in the same host clutch (2 of 57 clutches, 26 of which contained multiple genotyped cowbird eggs). Females were much more likely to lay subsequent eggs in different host nests than to return to the same host nest. We found no evidence for kin structure among female cowbirds parasitizing the same host nest, which were no more closely related than chance would predict. These results suggest that female screaming cowbirds frequently lay eggs in host nests that have already been parasitized by unrelated females. However, they typically lay just one egg per host clutch, even though greyish baywings are capable of rearing several nestlings. Since screaming cowbird laying is often poorly synchronized with that of their host, avoidance of repeat parasitism may be adaptive if it allows females to spread the risk of failure among multiple host nests.

How to learn to recognize conspecific brood parasitic offspring.

Recognition systems evolve to reduce the risk and costs of making recognition errors. Two main sources of recognition error include perceptual error (error arising from inability to discriminate between objects) and template error (error arising from using the wrong recognition template). We focus on how template error shapes host defence against avian brood parasites. Prior experiments in American coots (Fulica americana), a conspecific brood parasite, demonstrated how hosts learn to recognize brood parasitic chicks using predictable patterns of hatching order of host and parasite eggs. Here, we use these results to quantify the benefit of chick rejection as well as the cost of template error, and we then use mathematical models to explore fitness payoffs of chick recognition from different template acquisition mechanisms. We find that fitness differences between mechanisms do not fully explain aspects of the learning mechanism, such as why coots reacquire their recognition template each year. Other constraints arising from mating systems and genetic mechanisms likely influence which learning mechanism for parasitic chick recognition is optimal. Our approach highlights how mechanisms of template acquisition influence other recognition systems, including parasitic chick recognition in other brood parasite hosts. This article is part of the theme issue 'Signal detection theory in recognition systems: from evolving models to experimental tests'.

Great spotted cuckoos show dynamic patterns of host selection during the breeding season. The importance of laying stage and parasitism status of magpie nests

AbstractAvian brood parasites depend entirely on their hosts to raise their nestlings until independence. Thus, parasite females should select suitable host nests for egg laying according to traits that enhance offspring survival. The availability of nests of certain characteristics influencing the survival of parasitic offspring is, however, temporally dynamic and, thus, patterns of host selection should be evaluated considering characteristics of available host nests the day of parasitism. This allows detecting possible seasonal changes and, therefore, a more realistic picture of host selection by brood parasites. In this paper, we adopt such a new approach and consider daily availability of magpie (Pica pica) host nests at different breeding stage that were or were not parasitized by the great spotted cuckoo (Clamator glandarius). Theory predicts that cuckoos should select host nests at the laying stage. Accordingly, we detected that cuckoos preferred to parasitize magpie nests at the laying stage but, mainly, those that already harbored one or two cuckoo eggs, which may seem counterintuitive. We also showed that patterns of host selection by cuckoos varied during the breeding season, which implies that brood parasite–host interaction is dynamic depending on phenology. These patterns are hidden when not considering the temporally dynamic nature of the availability of host nests of characteristics of interest. We discuss the importance of such patterns and considering diary hosts nests availability for detecting them.

Alloparental care in the sea: Brood parasitism and adoption within and between two species of coral reef Altrichthys damselfish?

The evolution of parental care opens the door for the evolution of brood parasitic strategies that allow individuals to gain the benefits of parental care without paying the costs. Here we provide the first documentation for alloparental care in coral reef fish and we discuss why these patterns may reflect conspecific and interspecific brood parasitism. Species-specific barcodes revealed the existence of low levels (3.5% of all offspring) of mixed interspecific broods, mostly juvenile Amblyglyphidodon batunai and Pomacentrus smithi damselfish in Altrichthys broods. A separate analysis of conspecific parentage based on microsatellite markers revealed that mixed parentage broods are common in both species, and the genetic patterns are consistent with two different modes of conspecific brood parasitism, although further studies are required to determine the specific mechanisms responsible for these mixed parentage broods. While many broods had offspring from multiple parasites, in many cases a given brood contained only a single foreign offspring, perhaps a consequence of the movement of lone juveniles between nests. In other cases, broods contained large numbers of putative parasitic offspring from the same parents and we propose that these are more likely to be cases where parasitic adults laid a large number of eggs in the host nest than the result of movements of large numbers of offspring from a single brood after hatching. The evidence that these genetic patterns reflect adaptive brood parasitism, as well as possible costs and benefits of parasitism to hosts and parasites, are discussed.

Sex-specific responses to simulated territorial intrusions in the common cuckoo: a dual function of female acoustic signaling

The two-note call of the male common cuckoo (Cuculus canorus), the so-called “cu-coo”, is well known to people as a natural and cultural signal. However, the so-called “bubbling” call of the female cuckoo is almost unknown to most, and its function in the social organization of cuckoos remains understudied. We carried out a study of a possible intraspecific communication function of female bubbling calls, using playbacks to female cuckoos in their natural environment. Regarding vocal responses, both female and male cuckoos paid attention to the bubbling calls as they consistently responded acoustically by calling but did not so during control playbacks of collared dove (Streptopelia decaocto) calls. Accordingly, in about 63% of trials, females approached the loudspeaker closely and 81% uttered bubbling calls themselves during the experiment. These results are consistent with a function that the bubbling call plays a role in territorial signaling and defense among females. Male cuckoos also showed strong responses to playbacks of bubbling calls, as they approached the speaker and themselves called in 94% of playbacks; this is consistent with a scenario that they are interested in unfamiliar, new females in the area. Specifically, males approached the speaker repeatedly by flight, often flew around it and then perched on a tree, and uttered different call types beside the general “cu-coo” (e.g., quick “cu-cu-coo”, “gowk” call, and “guo” call). Our results represent an illustrative example that a simple female call may have multiple functions, as the cuckoo bubbling call advertises territory need for female cuckoos and attracts males.Significance statement Avian brood parasites lay their eggs in nests of other bird species, causing hosts to incubate, feed, and rear the parasitic offspring. Parasitic adult common cuckoos maintain a complex acoustic communication system, but female cuckoo calls are only beginning to be studied. The basic intraspecific functions of females’ sparrowhawk-like “bubbling calls” have not yet been characterized, whereas interspecifically, they use it for reducing antiparasitic attacks by their hosts. Our playback experiments with bubbling calls revealed that both female and male cuckoos responded acoustically to unfamiliar bubbling calls and more males than females approached the speaker, relative to control playbacks. We conclude that bubbling call has dual basic intraspecific functions: mate attraction, and territorial spacing.

Crozier’s Effect and the Acceptance of Intraspecific Brood Parasites

Organisms can often benefit by distinguishing between different classes of individuals. An example is kin recognition, whereby individuals preferentially associate with or aid genetic relatives that bear matching recognition cues but reject others. Despite its potential benefits, however, kin recognition using genetically based cues is often weak or absent [1-4]. A general explanation, termed "Crozier's effect," is that when individuals interact randomly, rarer cue alleles less often match cues of other individuals, and so are involved predominantly in "reject"-type interactions. If such interactions are more costly, positive frequency-dependent selection will erode the cue diversity upon which discrimination depends [4,5]. Although widely cited [1, 2, 4, 6-9], this idea lacks rigorous testing in the field. Here, we show how Crozier's effect applies to interactions between hosts and conspecific parasites, and measure it using field data. In the wasp we studied, conspecific parasitism fits a key assumption of Crozier's model: the same females act as both hosts and parasites. By exchanging offspring between nests experimentally, we find no evidence that females respond to genetically based cues associated with foreign offspring. Through measuring costs and benefits, however, we demonstrate a strong Crozier effect: because more parental investment is wasted when foreign offspring are rejected, interactions involving rejection have substantially lower payoffs than interactions involving acceptance. Costly rejection can thus eliminate cue diversity by causing selection against rare cue alleles, consistent with the absence of genetically based recognition that we observe. Females instead appear to rely on non-genetic cues that enable them to detect less than half of parasitic offspring.

Virulence in the three-spined stickleback specific parasite Schistocephalus solidus is inherited additively

Parasite virulence is a key trait in host-parasite interactions and plays a crucial role in infection dynamics. Our study system offers the rare opportunity to study the virulence of an individual macroparasite (Schistocephalus solidus) in its vertebrate fish host (Gasterosteus aculeatus). The size of the tapeworm in the fish can be regarded as a good proxy for individual parasite virulence, as parasite size correlates negatively with fitness traits of the stickleback host (i.e. the bigger the parasite, the lower the host's reproductive success) as well as directly with the number of parasite offspring to be expected.To investigate how virulence is inherited, laboratory bred, parasite-naïve stickleback were infected with a cross of two S. solidus populations of either high or low virulence, as well as one hybrid cross between the two. The relative weight of the parasite as expressed in the parasite index served as a measure of virulence. Furthermore, we measured several condition and immune related traits in the fish host to assess parasite impact on the stickleback. We hypothesized that parasite virulence is to a large extent genetically determined and correlated with several fitness traits in the stickleback host.We found that virulence is inherited additively in S. solidus, with hybrids of high and low virulence parasites displaying intermediate levels. However, contrary to expectation, infection rate of S. solidus in three-spined stickleback is not related to virulence. Even though the presence of the parasite caused differences in host condition, these were indistinguishable between the different levels of virulence in this experiment. Fish immune traits also showed a response to infection but had no correlation with level of parasite virulence.With this experiment we have taken the first step towards understanding how virulence is inherited and how it is driven in the Schistocephalus-stickleback system, even though virulence, as measured here, does not directly translate into cost for the host. A better understanding of the costs inflicted on the host by S. solidus infection is needed to understand this interaction in greater detail.

Kin-biased conspecific brood parasitism in a native Mandarin duck population

Conspecific brood parasitism (CBP) is a widespread female alternative reproductive tactic, where clutches tended by a single female contain eggs laid by different females. CBP is disproportionately common in waterfowl, where females, not males, exhibit natal philopatry. This raises the possibility that closely related females cooperate with each other for inclusive fitness benefits and may explain why females expend effort raising offspring that are not their own. Some studies show higher host–parasite relatedness than expected, while a similar number of studies fail to do so. Here we test the hypothesis that host and parasite are closely related in Mandarin ducks (Aix galericulata) to further our understanding of the role of host–parasite relatedness in the evolution of CBP. Using microsatellite markers, we found that more than 33.7 % of ducklings were parasites and that more than 70 % of nests contained parasitic offspring. Relatedness between host mother and parasitic ducklings was significantly higher than that of host mothers and ducklings in other nests, showing hosts were, on average, more related to their parasites than to other breeding females. There was a weak negative relationship between pairwise relatedness and distance between nests. However, distance did not contribute to the higher relatedness of host and parasite. Thus, host–parasite relatedness likely involved some form of kin recognition. Based on our results, relatedness plays a role in CBP in Mandarin ducks, though the explicit fitness benefits to both the host and the parasite still need to be determined.

Regulation of host colony activity by the social parasite Polistes semenowi

The productivity of social groups depends critically on effective regulation of work effort among group members. In social insect colonies, regulation of work may be decentralised or alternatively may be controlled by one or a few individuals (‘pacemakers’) within the colony. Social parasites, which usurp host colonies and replace the dominant as the principal reproductive, similarly depend on efficient regulation of work by hosts to rear parasite offspring, but few studies have explored the strategies used by parasites to achieve this. We compared the role of the social parasite Polistes semenowi in regulating host activity with that of the dominant individual on unparasitized nests of the host species, P. dominula. Dominant foundresses acted as pacemakers within unparasitized colonies, interacting frequently with colony members to initiate activity bursts and foraging trips, whereas parasites did not initiate more activity than the average colony member. Nonetheless, overall activity levels were similar in parasitized and unparasitized colonies, indicating that parasites may use other, indirect means to control the host activity. Colony activity did not change significantly following the removal of parasites or dominant host foundresses, perhaps because other individuals rapidly assumed the dominant position, or because of persistent indirect effects on colony activity. The role of P. semenowi in regulating the host activity differs strikingly from that reported for a second Polistes social parasite, P. atrimandibularis, suggesting that different Polistes social parasites may have fundamentally different social roles within host colonies, despite being closely phylogenetically related to one another.