Modification of morphological characters and cuticular compounds in worker ants Leptothorax nylanderi induced by endoparasites Anomotaenia brevis

Non-lethal fungal infection could reduce aggression towards strangers in ants

Many parasites interfere with the behaviour of their hosts. In social animals, such as ants, parasitic interference can cause changes on the level of the individual and also on the level of the society. The ant-parasitic fungus Rickia wasmannii influences the behaviour of Myrmica ants by expanding the host's nestmate recognition template, thereby increasing the chance of the colony accepting infected non-nestmates. Infected ants consistently show an increase of the alkane tricosane (n-C23) in their cuticular hydrocarbon profiles. Although experimental application of single compounds often elicits aggression towards manipulated ants, we hypothesized that the increase of n-C23 might underlie the facilitated acceptance of infected non-nestmates. To test this, we mimicked fungal infection in M. scabrinodis by applying synthetic n-C23 to fresh ant corpses and observed the reaction of infected and uninfected workers to control and manipulated corpses. Infected ants appeared to be more peaceful towards infected but not uninfected non-nestmates. Adding n-C23 to uninfected corpses resulted in reduced aggression in uninfected ants. This supports the hypothesis that n-C23 acts as a 'pacifying' signal. Our study indicates that parasitic interference with the nestmate discrimination of host ants might eventually change colony structure by increasing genetic heterogeneity in infected colonies.

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.

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.

Extra-genomic instructive influences in morphogenesis: A review of external signals that regulate growth and form

Embryonic development and regeneration accomplish a remarkable feat: individual cells work together to create or repair complex anatomical structures. What is the source of the instructive signals that specify these invariant and robust organ-level outcomes? The most frequently studied source of morphogenetic control is the host genome and its transcriptional circuits. However, it is now apparent that significant information affecting patterning also arrives from outside of the body. Both biotic and physical factors, including temperature and various molecular signals emanating from pathogens, commensals, and conspecific organisms, affect developmental outcomes. Here, we review examples in which anatomical patterning decisions are strongly impacted by lateral signals that originate from outside of the zygotic genome. The endogenous pathways targeted by these influences often show transgenerational effects, enabling them to shape the evolution of anatomies even faster than traditional Baldwin-type assimilation. We also discuss recent advances in the biophysics of morphogenetic controls and speculate on additional sources of important patterning information which could be exploited to better understand the evolution of bodies and to design novel approaches for regenerative medicine.

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.

Destructive disinfection of infected brood prevents systemic disease spread in ant colonies

In social groups, infections have the potential to spread rapidly and cause disease outbreaks. Here, we show that in a social insect, the ant Lasius neglectus, the negative consequences of fungal infections (Metarhizium brunneum) can be mitigated by employing an efficient multicomponent behaviour, termed destructive disinfection, which prevents further spread of the disease through the colony. Ants specifically target infected pupae during the pathogen's non-contagious incubation period, utilising chemical 'sickness cues' emitted by pupae. They then remove the pupal cocoon, perforate its cuticle and administer antimicrobial poison, which enters the body and prevents pathogen replication from the inside out. Like the immune system of a metazoan body that specifically targets and eliminates infected cells, ants destroy infected brood to stop the pathogen completing its lifecycle, thus protecting the rest of the colony. Hence, in an analogous fashion, the same principles of disease defence apply at different levels of biological organisation.

Lock-picks: fungal infection facilitates the intrusion of strangers into ant colonies

Studies investigating host-parasite systems rarely deal with multispecies interactions, and mostly explore impacts on hosts as individuals. Much less is known about the effects at colony level, when parasitism involves host organisms that form societies. We surveyed the effect of an ectoparasitic fungus, Rickia wasmannii, on kin-discrimination abilities of its host ant, Myrmica scabrinodis, identifying potential consequences at social level and subsequent changes in colony infiltration success of other organisms. Analyses of cuticular hydrocarbons (CHCs), known to be involved in insects' discrimination processes, revealed variations in chemical profiles correlated with the infection status of the ants, that could not be explained by genetic variation tested by microsatellites. In behavioural assays, fungus-infected workers were less aggressive towards both non-nestmates and unrelated queens, enhancing the probability of polygyny. Likewise, parasitic larvae of Maculinea butterflies had a higher chance of adoption by infected colonies. Our study indicates that pathogens can modify host recognition abilities, making the society more prone to accept both conspecific and allospecific organisms.

The parasite's long arm: a tapeworm parasite induces behavioural changes in uninfected group members of its social host

Parasites can induce alterations in host phenotypes in order to enhance their own survival and transmission. Parasites of social insects might not only benefit from altering their individual hosts, but also from inducing changes in uninfected group members. Temnothorax nylanderi ant workers infected with the tapeworm Anomotaenia brevis are known to be chemically distinct from nest-mates and do not contribute to colony fitness, but are tolerated in their colonies and well cared for. Here, we investigated how tapeworm- infected workers affect colony aggression by manipulating their presence in ant colonies and analysing whether their absence or presence resulted in behavioural alterations in their nest-mates. We report a parasite-induced shift in colony aggression, shown by lower aggression of uninfected nest-mates from parasitized colonies towards conspecifics, potentially explaining the tolerance towards infected ants. We also demonstrate that tapeworm-infected workers showed a reduced flight response and higher survival, while their presence caused a decrease in survival of uninfected nest-mates. This anomalous behaviour of infected ants, coupled with their increased survival, could facilitate the parasites' transmission to its definitive hosts, woodpeckers. We conclude that parasites exploiting individuals that are part of a society not only induce phenotypic changes within their individual hosts, but in uninfected group members as well.

Ant societies buffer individual-level effects of parasite infections

Parasites decrease host fitness and can induce changes in host behavior, morphology, and physiology. When parasites exploit social insects, they influence not only infected individuals but also the society as a whole. Workers of the ant Temnothorax nylanderi are an intermediate host for the cestode Anomotaenia brevis. We studied a heavily parasitized population and found that, although parasite infection had strong and diverse consequences for individual workers, colony fitness remained unchanged. At the individual level, we uncovered differences among the three worker types, infected and healthy workers from parasitized colonies and healthy workers from nonparasitized colonies. Infected workers were smaller than healthy ones and had smaller heads as parasite load increased. Behavioral changes extended to all workers from parasitized colonies; such workers were less active than workers in nonparasitized colonies but engaged in more grooming. Healthy workers from parasitized colonies showed behavioral patterns intermediate to those of infected workers and healthy workers from nonparasitized colonies. Despite the lower activity level found in ants from parasitized colonies, an important fitness parameter, per-worker productivity, remained unaltered in parasitized colonies. However, the investment strategies of parasitized colonies changed as their sex ratio became male biased and as male body size increased. In short, ant colonies can buffer the drain of resources by the parasite despite strong effects on individual workers.

Exoskeletal thinning in Cephalotes atratus ants (Hymenoptera: Formicidae) parasitized by Myrmeconema neotropicum (Nematoda: Tetradonematidae)

Some parasites modify the color of their arthropod hosts, presumably to facilitate transmission to a new host. Mechanisms for such changes often are unknown, but altered exoskeletal color in adult insects typically occurs via structural modifications or redistribution of pigments. Here, we examine the cuticle structure of workers of the Neotropical canopy ant Cephalotes atratus infected with the nematode Myrmeconema neotropicum. We hypothesized that the conspicuous red color of the gaster (the globular posterior body region) of infected ants results from structural changes, specifically localized exoskeletal thinning. We used scanning electron microscopy to quantify the thickness of gaster cuticle in healthy and infected ants. For comparison, we also measured the cuticle thickness of the head of each ant, which is black in both infected and healthy individuals. The gaster cuticle was 23% thinner in infected ants (average ±SE: 14.8 ± 1.02 µm) versus healthy ants (19.2 ± 0.65 µm) after correcting for body size. In contrast, the thickness of the head exoskeleton was similar among groups. We conclude that parasite-induced thinning of the exoskeleton is associated with the red color of the gaster. Other mechanisms, including translocation or leaching of melanin (by the ant or the parasite, respectively) may operate in concert with thinning to effect the color change, and would be an appropriate extension of this research.

Preliminary assessment of metabolic costs of the nematode Myrmeconema neotropicum on its host, the tropical ant Cephalotes atratus

The parasitic nematode Myrmeconema neotropicum infects workers of the neotropical arboreal ant Cephalotes atratus. Infected ants exhibit altered behavior, e.g., reduced aggression and slower tempo, as well as physical traits, e.g., gaster changes from shiny black to bright red. These changes are thought to induce fruit mimicry and attract frugivorous birds, which are the presumed paratenic hosts for the nematodes. We used respirometry to measure the energetic costs of nematode infection, testing the prediction of higher metabolic rates for infected workers maintaining both ant and nematode biomass. Contrary to this prediction, infected workers had lower mass-specific metabolic rates than uninfected workers. Parasites are limited to the gasters (abdomens) of adult ants, and infected gasters had 57% more mass, but 37% lower metabolic rates, compared to uninfected gasters. These results use a metabolic currency to measure, in vivo, the energetic costs of parasitism, and they shed light on the complex co-evolutionary relationship between host and parasite.

Parasite-induced fruit mimicry in a tropical canopy ant

Some parasites modify characteristics of intermediate hosts to facilitate their consumption by subsequent hosts, but examples of parasite-mediated mimicry are rare. Here we report dramatic changes in the appearance and behavior of nematode-parasitized ants such that they resemble ripe fruits in the tropical rain forest canopy. Unlike healthy ants, which are completely black, infected ants have bright red, berry-like gasters full of parasite eggs. The infected gasters are held in a conspicuous elevated position as the ants are walking, and they are easily detached from living ants, which also exhibit reduced defensive responses. This combination of changes presumably makes the infected ants attractive to frugivorous birds, which ingest the red gasters and pass the parasite eggs in their feces. The feces are collected by ants and fed to the developing brood, thus completing the cycle. This is the first documentation of parasites causing apparent fruit mimicry in an animal host to complete their life cycle.

Behavioral and morphological changes in soldiers of Macrotermes gilvus (Termitidae: Macrotermitinae) parasitized by Misotermes mindeni (Diptera: Phoridae)

Misotermes mindeni Disney and Neoh is a solitary endoparasitoid of Macrotermes gilvus (Hagen) (Termitidae: Macrotermitinae), exclusively parasitizing the head of major soldiers. In this study, behavioral and morphological changes in the parasitized termite host were evaluated. We also observed the larval parasitoid emerging from the host. We found that parasitism mainly occurred in termite mounds overgrown with grass and mounds that had been broken up previously for other experiments. The parasitized soldiers showed a significantly lower level of interspecific aggressiveness compared with healthy soldiers (P < 0.05). Parasitized soldiers also changed in habitat preference to one isolated chamber of the nest. This might be an adaptive strategy that facilitates parasitoid dispersal, provides protection to parasitoids, and reduces the risk of parasitism to host colony. An abnormally rounded head capsule and remarkably short mandibles are characteristics of a parasitized soldier. The older larval fly stages were found only in major soldiers. We suggest that parasitization may first start in fourth or even earlier larval termite instars. The fly larva develops in the termite soldier's head capsule and pupates inside the host's body.

A review of ant cuticular hydrocarbons

We compared the published cuticular hydrocarbon (CHC) profiles of 78 ant species across 5 subfamilies. Almost 1,000 CHCs have been described for these species, composing 187 distinct homologous series and ten hydrocarbon groups. In descending order of occurrence were: n-alkanes > monomethylalkanes > dimethylalkanes > alkenes > dienes>> trimethylalkanes>> methylalkenes > methylalkadienes > trienes > tetramethylalkanes. Odd chain lengths and positions of methyl or double bonds at odd carbon numbers were far more numerous than even chain-length compounds or bond positions. Although each species possess its own unique pattern of CHCs, we found no association between CHC profile and phylogeny. The production of the biosynthetically complex compounds (e.g., methyl branched dienes) by the most primitive living ant suggests that the basic genetic architecture required to produce the rich diversity of CHCs was already present prior to their adaptive radiation. Unlike the ubiquitous n-alkanes and monomethylalkanes, there is a huge diversity of species-specific dimethylalkanes that makes them likely candidates for species and nest-mate discrimination signals.

Analogies in the evolution of individual and social immunity

We compare anti-parasite defences at the level of multicellular organisms and insect societies, and find that selection by parasites at these two organisational levels is often very similar and has created a number of parallel evolutionary solutions in the host's immune response. The defence mechanisms of both individuals and insect colonies start with border defences to prevent parasite intake and are followed by soma defences that prevent the establishment and spread of the parasite between the body's cells or the social insect workers. Lastly, germ line defences are employed to inhibit infection of the reproductive tissue of organisms or the reproductive individuals in colonies. We further find sophisticated self/non-self-recognition systems operating at both levels, which appear to be vital in maintaining the integrity of the body or colony as a reproductive entity. We then expand on the regulation of immune responses and end with a contemplation of how evolution may shape the different immune components, both within and between levels. The aim of this review is to highlight common evolutionary principles acting in disease defence at the level of both individual organisms and societies, thereby linking the fields of physiological and ecological immunology.

Chemical correlates of reproduction and worker policing in a myrmicine ant

In a number of wasps, bees, and in particular ponerine ants, quantitative and qualitative variation in the profile of cuticular hydrocarbons is associated with variation in fecundity and is likely to serve for communicating the reproductive status of an individual. Here, we demonstrate that the chemical profile on the cuticle of fertile workers and queens of the myrmicine ant Temnothorax unifasciatus is different from that of non-reproductive workers. Fertility and apparently also cuticular signatures are reversible under the influence of policing by worker aggression. Though no policing by egg-eating occurs in this species, queen and worker laid eggs also differed in their chemical profile.

Avian cestodes affect the behaviour of their intermediate host Artemia parthenogenetica: An experimental study

The brine shrimp Artemia parthenogenetica (Crustacea, Branchiopoda) is intermediate host for several cestode species whose final hosts are waterbirds. Previous field studies have shown that brine shrimps infected with cestodes have a bright red colour and are spatially segregated in the water column. However, the ethological mechanisms explaining such field observations are unknown. Changes in appearance and behaviour induced by trophically transmitted parasites have been shown to increase the risk of predation by the final host. In this experimental study, we compared the behaviour of uninfected Artemia and those infected by avian cestodes. We found that parasitised individuals behave differently from unparasitised ones in several ways. In contrast to uninfected individuals, infected brine shrimps were photophilous and showed increased surface-swimming behaviour. These observations suggest that the modified behaviour (in addition to the bright red colour of the majority of the infected individuals) results in infected brine shrimps becoming more vulnerable to avian final hosts, which facilitates parasite transmission. We discuss our results in terms of the adaptive nature of behavioural changes and their potential implications for the hypersaline ecosystem.

Cuticular hydrocarbons of Tetramorium ants from central Europe: analysis of GC-MS data with self-organizing maps (SOM) and implications for systematics

Cuticular hydrocarbons were extracted from workers of 63 different nests of five species of Tetramorium ants (Hymenoptera: Formicidae) from Austria, Hungary, and Spain. The GC-MS data were classified (data mining) by self-organizing maps (SOM). SOM neurons derived from primary neuron separation were subjected to hierarchical SOM (HSOM) and were grouped to neuron areas on the basis of vicinity in the hexagonal output grid. While primary neuron separation and HSOM resulted in classifications on a level more sensitive than species differences, neuron areas resulted in chemical phenotypes apparently of the order of species. These chemical phenotypes have implications for systematics: while the chemical phenotypes for T. ferox and T. moravicum correspond to morphological determination, in T. caespitum and T. impurum a total of six chemical phenotypes is found. Three hypotheses are discussed to explain this disparity between morphological and chemical classifications, including in particular the possibility of hybridization and the existence of cryptic species. Overall, the GC-MS profiles classified by SOM prove to be a practical alternative to morphological determination (T. ferox, T. moravicum) and indicate the need to revisit systematics (T. caespitum, T. impurum).

Qualitative and quantitative differences in cuticular hydrocarbons between laboratory and field colonies of Pogonomyrmex barbatus

Ants held in the laboratory and field ants of the species Pogonomyrmex barbatus have quantitative differences in their cuticular hydrocarbons and a qualitative difference in their methyl-branched hydrocarbons. Laboratory-held workers showed twice the hydrocarbon content as field ants. This difference was mainly due to higher amounts of straight-chain alkanes and methyl-branched alkanes in laboratory ants, whereas the proportion of the alkenes remained the same for both groups. In addition to the absence of some hydrocarbons in the field colonies, one of the methyl-branched hydrocarbons differed in amount and branching pattern between the two groups of ants. Whereas, notable peaks of 2-methylalkanes were identified in ants kept in the laboratory, these compounds could not be identified in ants living in their natural habitat. However, a trace amount of 4-methyltriacontane was found in lieu of the 2-methyltriacontane counterpart in field ants. Possible explanations for both qualitative and quantitative differences are discussed.

Chemical ecology and social parasitism in ants

The chemical strategies by which parasites manage to break into the social fortresses of ants offer a fascinating theme in chemical ecology. Semiochemicals used for interindividual nestmate recognition are also involved in the mechanisms of tolerance and association between the species, and social parasites exploit these mechanisms. The obligate parasites are odorless ("chemical insignificance") at the time of usurpation, like all other callow ants, and this "invisibility" enables their entry into the host colony. By chemical mimicry (sensu lato), they later integrate the gestalt odor of this colony ("chemical integration"). We hypothesize that host and parasite are likely to be related chemically, thereby facilitating the necessary mimicry to permit bypassing the colony odor barrier. We also review the plethora of chemical weapons used by social parasites (propaganda, appeasement, and/or repellent substances), particularly during the usurpation period, when the young mated parasite queen synthesizes these chemicals before usurpation and ceases such biosynthesis afterwards. We discuss evolutionary trends that may have led to social parasitism, focusing on the question of whether slave-making ants and their host species are expected to engage in a coevolutionary arms race.