Parasitism and queen presence interactively shape worker behaviour and fertility in an ant host

Long live the host! Proteomic analysis reveals possible strategies for parasitic manipulation of its social host

Parasites with complex life cycles often manipulate the phenotype of their intermediate hosts to increase the probability of transmission to their definitive hosts. Infection with Anomotaenia brevis, a cestode that uses Temnothorax nylanderi ants as intermediate hosts, leads to a multiple-fold extension of host lifespan and to changes in behaviour, morphology and colouration. The mechanisms behind these changes are unknown, as is whether the increased longevity is achieved through parasite manipulation. Here, we demonstrate that the parasite releases proteins into its host with functions that might explain the observed changes. These parasitic proteins make up a substantial portion of the proteome of the hosts' haemolymph, and thioredoxin peroxidase and superoxide dismutase, two antioxidants, exhibited the highest abundances among them. The largest part of the secreted proteins could not be annotated, indicating they are either novel or severely altered during recent coevolution to function in host manipulation. We also detected shifts in the hosts' proteome with infection, in particular an overabundance of vitellogenin-like A in infected ants, a protein that regulates division of labour in Temnothorax ants, which could explain the observed behavioural changes. Our results thus suggest two different strategies that might be employed by this parasite to manipulate its host: secreting proteins with immediate influence on the host's phenotype and altering the host's translational activity. Our findings highlight the intricate molecular interplay required to influence the phenotype of a host and point to potential signalling pathways and genes involved in parasite-host communication.

From Parasitized to Healthy-Looking Ants (Hymenoptera: Formicidae): Morphological Reconstruction Using Algorithmic Processing

Background: Parasites cause predictable alternative phenotypes of host individuals. Investigating these parasitogenic phenotypes may be essential in cases where parasitism is common or taxa is described based on a parasitized individual. Ignoring them could lead to erroneous conclusions in biodiversity-focused research, taxonomy, evolution, and ecology. However, to date, integrating alternative phenotypes into a set of wild-type individuals in morphometric analysis poses extraordinary challenges to experts. This paper presents an approach for reconstructing the putative healthy morphology of parasitized ants using algorithmic processing. Our concept enables the integration of alternative parasitogenic phenotypes in morphometric analyses. Methods: We tested the applicability of our strategy in a large pool of Cestoda-infected and healthy individuals of three Temnothorax ant species (T. nylanderi, T. sordidulus, and T. unifasciatus). We assessed the stability and convergence of morphological changes caused by parasitism across species. We used an artificial neural network-based multiclass classifier model to predict species based on morphological trait values and the presence of parasite infection. Results: Infection causes predictable morphological changes in each species, although these changes proved to be species-specific. Therefore, integrating alternative parasitogenic phenotypes in morphometric analyses can be achieved at the species level, and a prior species hypothesis is required. Conclusion: Despite the above limitation, the concept is appropriate. Beyond parasitogenic phenotypes, our approach can also integrate morphometric data of an array of alternative phenotypes (subcastes in social insects, alternative morphs in polyphenic species, and alternative sexes in sexually dimorphic species) whose integrability had not been resolved before.

Parasite infection impairs the shoaling behaviour of uninfected shoal members under predator attack

Abstract

A key benefit of sociality is a reduction in predation risk. Cohesive group behaviour and rapid collective decision making are essential for reducing predation risk in groups. Parasite infection might reduce an individuals’ grouping behaviours and thereby change the behaviour of the group as a whole. To investigate the relationship between parasite infection and grouping behaviours, we studied groups of three-spined sticklebacks, Gasterosteus aculeatus, varying the number of individuals experimentally infected with the cestode Schistocephalus solidus. We studied groups of six sticklebacks containing 0, 2, 3, 4 or 6 infected individuals before and after a simulated bird attack. We predicted that infected individuals would have reduced shoaling and swimming speed and that the presence of infected individuals within a group would reduce group cohesion and speed. Uninfected fish increased shoaling and reduced swimming speed more than infected fish after the bird attack. In groups containing both infected and uninfected fish, the group behaviours were dominated by the more frequent character (uninfected versus infected). Interestingly, groups with equal numbers of uninfected and infected fish showed the least shoaling and had the lowest swimming speeds, suggesting that these groups failed to generate a majority and therefore displayed signs of indecisiveness by reducing their swimming speed the most. Our results provide evidence for a negative effect of infection on a group’s shoaling behaviour, thereby potentially deteriorating collective decision making. The presence of infected individuals might thus have far-reaching consequences in natural populations under predation risk.

Significance statement

Parasite-infected individuals often show deviating group behaviours. This might reduce the anti-predator benefits of group living. However, it is unknown whether such deviations in group behaviour might influence the shoaling behaviour of uninfected group members and thereby the behaviour of the group as a whole. By experimentally infecting sticklebacks and investigating groups varying in infection rates, we show that infected sticklebacks differ in their shoaling behaviours from uninfected sticklebacks. Additionally, the presence of infected sticklebacks within the group affected the behaviour of uninfected shoal members. We show that shoals of infected fish are less cohesive and move slower compared to shoals of uninfected fish. Furthermore, we show that the infection rate of the shoal is crucial for how the group behaves.

Extreme lifespan extension in tapeworm-infected ant workers

Social insects are hosts of diverse parasites, but the influence of these parasites on phenotypic host traits is not yet well understood. Here, we tracked the survival of tapeworm-infected ant workers, their uninfected nest-mates and of ants from unparasitized colonies. Our multi-year study on the ant Temnothorax nylanderi, the intermediate host of the tapeworm Anomotaenia brevis, revealed a prolonged lifespan of infected workers compared with their uninfected peers. Intriguingly, their survival over 3 years did not differ from those of (uninfected) queens, whose lifespan can reach two decades. By contrast, uninfected workers from parasitized colonies suffered from increased mortality compared with uninfected workers from unparasitized colonies. Infected workers exhibited a metabolic rate and lipid content similar to young workers in this species, and they received more social care than uninfected workers and queens in their colonies. This increased attention could be mediated by their deviant chemical profile, which we determined to elicit more interest from uninfected nest-mates in a separate experiment. In conclusion, our study demonstrates an extreme lifespan extension in a social host following tapeworm infection, which appears to enable host workers to retain traits typical for young workers.

Among the shapeshifters: parasite-induced morphologies in ants (Hymenoptera, Formicidae) and their relevance within the EcoEvoDevo framework

As social insects, ants represent extremely interaction-rich biological systems shaped by tightly integrated social structures and constant mutual exchange with a multitude of internal and external environmental factors. Due to this high level of ecological interconnection, ant colonies can harbour a diverse array of parasites and pathogens, many of which are known to interfere with the delicate processes of ontogeny and caste differentiation and induce phenotypic changes in their hosts. Despite their often striking nature, parasite-induced changes to host development and morphology have hitherto been largely overlooked in the context of ecological evolutionary developmental biology (EcoEvoDevo). Parasitogenic morphologies in ants can, however, serve as “natural experiments” that may shed light on mechanisms and pathways relevant to host development, plasticity or robustness under environmental perturbations, colony-level effects and caste evolution. By assessing case studies of parasites causing morphological changes in their ant hosts, from the eighteenth century to current research, this review article presents a first overview of relevant host and parasite taxa. Hypotheses about the underlying developmental and evolutionary mechanisms, and open questions for further research are discussed. This will contribute towards highlighting the importance of parasites of social insects for both biological theory and empirical research and facilitate future interdisciplinary work at the interface of myrmecology, parasitology, and the EcoEvoDevo framework.

Parasite Presence Induces Gene Expression Changes in an Ant Host Related to Immunity and Longevity

Most species are either parasites or exploited by parasites, making parasite-host interactions a driver of evolution. Parasites with complex life cycles often evolve strategies to facilitate transmission to the definitive host by manipulating their intermediate host. Such manipulations could explain phenotypic changes in the ant Temnothorax nylanderi, the intermediate host of the cestode Anomotaenia brevis. In addition to behavioral and morphological alterations, infected workers exhibit prolonged lifespans, comparable to that of queens, which live up to two decades. We used transcriptomic data from cestodes and ants of different castes and infection status to investigate the molecular underpinnings of phenotypic alterations in infected workers and explored whether the extended lifespan of queens and infected workers has a common molecular basis. Infected workers and queens commonly upregulated only six genes, one of them with a known anti-aging function. Both groups overexpressed immune genes, although not the same ones. Our findings suggest that the lifespan extension of infected workers is not achieved via the expression of queen-specific genes. The analysis of the cestodes' transcriptome revealed dominant expression of genes of the mitochondrial respiratory transport chain, which indicates an active metabolism and shedding light on the physiology of the parasite in its cysticercoid stage.

Abdominal microbial communities in ants depend on colony membership rather than caste and are linked to colony productivity

Gut bacteria aid their host in digestion and pathogen defense, and bacterial communities that differ in diversity or composition may vary in their ability to do so. Typically, the gut microbiomes of animals living in social groups converge as members share a nest environment and frequently interact. Social insect colonies, however, consist of individuals that differ in age, physiology, and behavior, traits that could affect gut communities or that expose the host to different bacteria, potentially leading to variation in the gut microbiome within colonies. Here we asked whether bacterial communities in the abdomen of Temnothorax nylanderi ants, composed largely of the gut microbiome, differ between different reproductive and behavioral castes. We compared microbiomes of queens, newly eclosed workers, brood carers, and foragers by high-throughput 16S rRNA sequencing. Additionally, we sampled individuals from the same colonies twice, in the field and after 2 months of laboratory housing. To disentangle the effects of laboratory environment and season on microbial communities, additional colonies were collected at the same location after 2 months. There were no large differences between ant castes, although queens harbored more diverse microbial communities than workers. Instead, we found effects of colony, environment, and season on the abdominal microbiome. Interestingly, colonies with more diverse communities had produced more brood. Moreover, the queens' microbiome composition was linked to egg production. Although long-term coevolution between social insects and gut bacteria has been repeatedly evidenced, our study is the first to find associations between abdominal microbiome characteristics and colony productivity in social insects.