Opportunist slave-making ants Myrmoxenus ravouxi discriminate different host species from a non-host species

Slave-making ants exploit the worker force of host colonies permanently and have to make recurrent raids in order to replenish the slave’s stock. Some of these parasite species exploit different host species and few studies so far have been devoted to host species recognition mechanisms. Here, we tried to determine if opportunist slave-making ants using different host species rely on innate or experience-induced preferences to discriminate host from non-host species. We show that Myrmoxenus ravouxi slave-making workers are not only more aggressive toward heterocolonial host and potential host species workers when compared with non-host species workers, but also toward heterocolonial host workers than toward heterocolonial conspecifics. Moreover, M. ravouxi workers display more antennations and contacts toward the heterocolonial host species when compared with the non-host species. We also show that they do not discriminate between homocolonial and heterocolonial conspecifics. Together, our results suggest that this opportunistic slave-making ant species may have a complex social recognition template based on both innate and experience-based mechanisms.     

FIRST EVIDENCE FOR SLAVE REBELLION: ENSLAVED ANT WORKERS SYSTEMATICALLY KILL THE BROOD OF THEIR SOCIAL PARASITE PROTOMOGNATHUS AMERICANUS

During the process of coevolution, social parasites have evolved sophisticated strategies to exploit the brood care behavior of their social hosts. Slave-making ant queens invade host colonies and kill or eject all adult host ants. Host workers, which eclose from the remaining brood, are tricked into caring for the parasite brood. Due to their high prevalence and frequent raids, following which stolen host broods are similarly enslaved, slave-making ants exert substantial selection upon their hosts, leading to the evolution of antiparasite adaptations. However, all host defenses shown to date are active before host workers are parasitized, whereas selection was thought to be unable to act on traits of already enslaved hosts. Yet, here we demonstrate the rebellion of enslaved Temnothorax workers, which kill two-thirds of the female pupae of the slave-making ant Protomognathus americanus . Thereby, slaves decrease the long-term parasite impact on surrounding related host colonies. This novel antiparasite strategy of enslaved workers constitutes a new level in the coevolutionary battle after host colony defense has failed. Our discovery is analogous to recent findings in hosts of avian brood parasites where perfect mimicry of parasite eggs leads to the evolution of chick recognition as a second line of defense.     

Geographic distribution of the anti-parasite trait “slave rebellion”

Social parasites exploit the brood care behavior of other species and can exert strong selection pressures on their hosts. As a consequence, hosts have developed defenses to circumvent or to lower the costs of parasitism. Recently, a novel, indirect defense trait, termed slave rebellion, has been described for hosts of a slave-making ant: Enslaved Temnothorax longispinosus workers reduce local parasite pressure by regularly killing pupae of their obligatory slavemaking parasite Protomognathus americanus. Subsequently, growth of social parasite nests is reduced, which leads to fewer raids and likely increases fitness of neighboring related host colonies. In this study, we investigate the presence and expression the slave rebellion trait in four communities. We report its presence in all parasitized communities, document strong variation in its expression between different geographic sites and discuss potential explanations for this observed variation.     

Increased host aggression as an induced defense against slave-making ants

Slave-making ants reduce the fitness of surrounding host colonies through regular raids, causing the loss of brood and frequently queen and worker death. Consequently, hosts developed defenses against slave raids such as specific recognition and aggression toward social parasites, and indeed, we show that host ants react more aggressively toward slavemakers than toward nonparasitic competitors. Permanent behavioral defenses can be costly, and if social parasite impact varies in time and space, inducible defenses, which are only expressed after slavemaker detection, can be adaptive. We demonstrate for the first time an induced defense against slave-making ants: Cues from the slavemaker Protomognathus americanus caused an unspecific but long-lasting behavioral response in Temnothorax host ants. A 5-min within-nest encounter with a dead slavemaker raised the aggression level in T. longispinosus host colonies. Contrarily, encounters with nonparasitic competitors did not elicit aggressive responses toward non-nestmates. Increased aggression can be adaptive if a slavemaker encounter reliably indicates a forthcoming attack and if aggression increases postraid survival. Host aggression was elevated over 3 days, showing the ability of host ants to remember parasite encounters. The response disappeared after 2 weeks, possibly because by then the benefits of increased aggression counterbalance potential costs associated with it.     

Sympatric speciation and radiative evolution of socially parasitic ants - Heretic hypotheses and their factual background

According to current hypotheses the main types of social parasitism among ants, namely slavery, temporary parasitism, and inquilinism, arose from such features as predation on other ants, or territorial behavior, both presumed precursors of slavemaking, and polygyny, a presumed precursor of temporary parasitism and inquilinism. The latter is believed also to represent a final instar in several evolutionary pathways leading from slavery, temporary parasitism, and xenobiosis to this permanently parasitic, workerless condition. Speciation, the origin of parasitic species from their usually closely related host species, is suggested to occur due to temporary geographic isolation and subsequent transition of one of the newly formed daughter species to parasitism in the nests of the other. Evidence is presented suggesting that the main types of social parasitism originated independently of each other. 15 ant genera are parasitized exclusively by inquilines, Eve other genera exclusively by temporary parasites. Only four groups of non-parasitic ant species (Formica, Tet-ramorium, Leptothorax subgenera Leptothorax and Myrafant) have parasites of several types each. Within these roups, however, there is little evidence of evolutionary transitions from one type to another. The few exceptions, mainly workerless species of the genera Epimyrma and Chalepoxenus, represent parasites which clearly derive from slave-making congeners, but differ from ordinary inquilines in that they eliminate the host colony queens like their actively dulotic ancestors. The new hypothesis suggests that all forms of interspecific true social parasitism (excluding xenobiosis) orginated from a common “preparasitic” stage, a subpopulation of reproductives in polygynous colonies and species, with diverging sexual behavior (near-nest mating vs. swarming) and caste ratios (production of more sexuals vs. workers). Arguments for sympatric speciation are compiled. Various features of the ancestral, and then host species (colony sizes, population density and structure, transition from polygyny to monoyny, etc.), and of the “preparasite” (production of few, or no workers, etc.) may shape the developing parasite to become a slave-maker, inquiline, or temporary parasite. These features usually leave open only one, or in a few genera, several options. The different types of parasitism within one host species group thus may have developed in a radiative manner from the common, preparasitic stage, which explains that independent colony foundation is a common feature of all true social parasites among ants.     

Do host species evolve a specific response to slave-making ants?

Background

Social parasitism is an important selective pressure for social insect species. It is particularly the case for the hosts of dulotic (so called slave-making) ants, which pillage the brood of host colonies to increase the worker force of their own colony. Such raids can have an important impact on the fitness of the host nest. An arms race which can lead to geographic variation in host defenses is thus expected between hosts and parasites. In this study we tested whether the presence of a social parasite (the dulotic ant Myrmoxenus ravouxi) within an ant community correlated with a specific behavioral defense strategy of local host or non-host populations of Temnothorax ants. Social recognition often leads to more or less pronounced agonistic interactions between non-nestmates ants. Here, we monitored agonistic behaviors to assess whether ants discriminate social parasites from other ants. It is now well-known that ants essentially rely on cuticular hydrocarbons to discriminate nestmates from aliens. If host species have evolved a specific recognition mechanism for their parasite, we hypothesize that the differences in behavioral responses would not be fully explained simply by quantitative dissimilarity in cuticular hydrocarbon profiles, but should also involve a qualitative response due to the detection of particular compounds. We scaled the behavioral results according to the quantitative chemical distance between host and parasite colonies to test this hypothesis.

Results

Cuticular hydrocarbon profiles were distinct between species, but host species did not show a clearly higher aggression rate towards the parasite than toward non-parasite intruders, unless the degree of response was scaled by the chemical distance between intruders and recipient colonies. By doing so, we show that workers of the host and of a non-host species in the parasitized site displayed more agonistic behaviors (bites and ejections) towards parasite than toward non-parasite intruders.

Conclusions

We used two different analyses of our behavioral data (standardized with the chemical distance between colonies or not) to test our hypothesis. Standardized data show behavioral differences which could indicate qualitative and specific parasite recognition. We finally stress the importance of considering the whole set of potentially interacting species to understand the coevolution between social parasites and their hosts.

     

Microsatellite analysis reveals strong but differential impact of a social parasite on its two host species

The speed and the dynamics of the co-evolutionary process strongly depend on the relative strengths of reciprocal selection pressures exerted by the interacting species. Here, we investigate the influence of an obligate social parasite, the slave-making ant Harpagoxenus sublaevis, on populations of the two main host species Leptothorax acervorum and Leptothorax muscorum from a German ant community. A combination of genetic and demographic data allowed us to analyse the consequences of raiding pressure on the hosts' life history and possible host preferences of the parasite. We can demonstrate that slave raids during which the social parasite pillages brood from neighbouring host colonies are both frequent and extremely destructive for both host species. Microsatellite analysis showed that, on average, a single slave-maker colony conducts more than three raids per year and that host colonies mostly perish in the aftermath of these parasite attacks. Only in few cases, surviving nests of previously raided host colonies were found in the surroundings of slave-maker colonies. As a consequence of the high prevalence of parasites and their recurrent and devastating slave raids on host colonies, the life expectancy of host colonies was severely reduced. Combining our results on host-specific parasitic colony founding and raiding frequencies with the post-raid survival rate, we can demonstrate an overall higher mortality rate for the smaller host species L. muscorum. This might be caused by a preference of H. sublaevis for this secondary host species as demographic data on host species usage indicate.     

The Effect of Social Parasitism by Polyergus breviceps on the Nestmate Recognition System of Its Host,Formica altipetens

Highly social ants, bees and wasps employ sophisticated recognition systems to identify colony members and deny foreign individuals access to their nest. For ants, cuticular hydrocarbons serve as the labels used to ascertain nest membership. Social parasites, however, are capable of breaking the recognition code so that they can thrive unopposed within the colonies of their hosts. Here we examine the influence of the socially parasitic slave-making ant, Polyergus breviceps on the nestmate recognition system of its slaves, Formica altipetens. We compared the chemical, genetic, and behavioral characteristics of colonies of enslaved and free-living F. altipetens. We found that enslaved Formica colonies were more genetically and chemically diverse than their free-living counterparts. These differences are likely caused by the hallmark of slave-making ant ecology: seasonal raids in which pupa are stolen from several adjacent host colonies. The different social environments of enslaved and free-living Formica appear to affect their recognition behaviors: enslaved Formica workers were less aggressive towards non-nestmates than were free-living Formica. Our findings indicate that parasitism by P. breviceps dramatically alters both the chemical and genetic context in which their kidnapped hosts develop, leading to changes in how they recognize nestmates.     

Evidence for adaptive brain tissue reduction in obligate social parasites (Polyergus mexicanus) relative to their hosts (Formica fusca)

Brain investment is evolutionarily constrained by high costs of neural tissue. Several ecological factors favour the evolution of increased brain investment; we predict reduced brain region investment will accompany the evolution of organismal or social parasitism when parasites rely on host behaviour and cognition to solve ecological problems. To test this idea we investigated whether brain region investments differed between obligate slave‐making Polyergus mexicanus ant workers and their Formica fusca slave workers. Polyergus workers perform little labour for their colonies; enslaved workers of Formica host species forage, excavate nests and tend the brood. We focused on the calyces of the mushroom bodies, central processing brain regions that are larger in social insect workers that perform complex tasks. As predicted we found lower relative investment in mushroom body calyx in P. mexicanus workers than in F. fusca workers; by contrast, enslaved and free F. fusca workers did not differ in mushroom body calyx volume. We then tested whether slave‐makers and hosts differed in brain investment among sensory modalities. Polyergus slave‐makers employ several unique classes of pheromones during raids, and eye size relative to head size was smaller in P. mexicanus workers than in F. fusca workers. The size of antennal brain tissues relative to visual tissues was greater in Polyergus, both in the peripheral sensory lobes and in the mushroom body calyx, suggesting greater relative investment in antennal processing by slave‐makers. © 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 113 , 415–422.     

Convergent Loss of Chemoreceptors across Independent Origins of Slave-Making in Ants

The evolution of an obligate parasitic lifestyle often leads to the reduction of morphological and physiological traits, which may be accompanied by loss of genes and functions. Slave-making ants are social parasites that exploit the work force of closely related ant species for social behaviors such as brood care and foraging. Recent divergence between these social parasites and their hosts enables comparative studies of gene family evolution. We sequenced the genomes of eight ant species, representing three independent origins of ant slavery. During the evolution of eusociality, chemoreceptor genes multiplied due to the importance of chemical communication in insect societies. We investigated the evolutionary fate of these chemoreceptors and found that slave-making ant genomes harbored only half as many gustatory receptors as their hosts’, potentially mirroring the outsourcing of foraging tasks to host workers. In addition, parasites had fewer odorant receptors and their loss shows striking patterns of convergence across independent origins of parasitism, in particular in orthologs often implicated in sociality like the 9-exon odorant receptors. These convergent losses represent a rare case of convergent molecular evolution at the level of individual genes. Thus, evolution can operate in a way that is both repeatable and reversible when independent ant lineages lose important social traits during the transition to a parasitic lifestyle.     

Blending in with the crowd: social parasites integrate into their host colonies using a flexible chemical signature

Social parasites are able to exploit their host's communication code and achieve social integration. For colony foundation, a newly mated slave-making ant queen must usurp a host colony. The parasite's brood is cared for by the hosts and newly eclosed slave-making workers integrate to form a mixed ant colony. To elucidate the social integration strategy of the slave-making workers, Polyergus rufescens, behavioural and chemical analyses were carried out. Cocoons of P. rufescens were introduced into subcolonies of four potential host species: Formica subgenus Serviformica (Formica cunicularia and F. rufibarbis, usual host species; F. gagates, rare host; F. selysi, non-natural host). Slave-making broods were cared for and newly emerged workers showed several social interactions with adult Formica. We recorded the occurrence of abdominal trophallaxis, in which P. rufescens, the parasite, was the donor. Social integration of P. rufescens workers into host colonies appears to rely on the ability of the parasite to modify its cuticular hydrocarbon profile to match that of the rearing species. To study the specific P. rufescens chemical profile, newly emerged callows were reared in isolation from the mother colony (without any contact with adult ants). The isolated P. rufescens workers exhibited a chemical profile closely matching that of the primary host species, indicating the occurrence of local host adaptation in the slave-maker population. However, the high flexibility in the ontogeny of the parasite's chemical signature could allow for host switching.     

Facultative and obligate slavery in formicine ants: frequency of slavery, and proportion and size of slaves

Slave-making ants raid nests of other ant species, capture the developing offspring and rear them to slave workers. Here we compare slave-making of three formicine slave-making ants: the facultative Formica subnuda, the obligate Polyergus breviceps, and F. subintegra which previously has been considered facultative but appears to be an obligate slave-making ant. If F. subintegra is an obligate slavemaker, slave-making of F. subintegra should differ from that of F. subnuda but closely resemble slave-making of P. breviceps in the following aspects: (1) Obligate slavemakers are rarer than facultative slavemakers. (2) Slaveless colonies of facultative slavemakers are found, but obligate slavemakers always have slaves. (3) Because obligate slavemakers depend on their slaves, they should have a higher proportion of slaves than facultative slavemakers. (4) Owing to special adaptations obligate slavemakers are able to raid bigger colonies, and hence have bigger slaves than facultative slavemakers. (5) Dufour's gland of F. subintegra should be larger than that of F. subnuda. Per 100 free F. podzolica colonies, the number of P. breviceps and F. subintegra colonies with F. podzolica slaves were 1.3% and 3.9%, respectively, and the number of F. subnuda colonies with F. podzolica 3.7%, and without F. podzolica 7.5%. The proportion of slaves, when present, varied between 1–30% in the colonies of F. subnuda, and between 70–90% in the colonies of the other species. The slaves of F. subnuda were significantly smaller than those of F. subintegra and P. breviceps. The length of F. subnuda's Dufour's gland was one third of the length of F. subintegra's gland. The results show that slave-making of F. subintegra parallels that of P. breviceps, and contrary to the earlier notion, F. subintegra is an obligate slave-making ant. We suggest that F. subnuda and F. subintegra represent extreme modes of slave-making behaviour in the Formica sanguinea group.     

Nest composition and worker relatedness in three slave-making ants of the genus Rossomyrmex Arnoldi and their Proformica Ruzsky hosts (Hymenoptera,Formicidae)

Abstract In this paper, we analyze and compare nest composition and architecture as well as worker relatedness in three related species of slave-making ants: Rossomyrmex anatolicus, R. minuchae, and R. quandratinodum. Colony structure within nests is an important trait in ants, especially in the case of mixed societies, when host and parasite coexist in the same nest. Data for their respective free-living hosts, Proformica korbi, P. longiseta and P. sp., are also provided. For our study, we integrated a meticulous excavation procedure with a genetic method. We conclude that the average number of parasites, as well as of slaves, is species-specific, whereas nest depth depends on the nest architecture of the host. The genus Rossomyrmex seems to be monogynous and monandrous, whereas Proformica shows differences in the number of queens and mating frequency. R. quandratinodum shows different traits in nest composition (host/parasite ratio: P/R) and architecture. The difference in traits may account for some differences in parasitism: raid process or avoidance of parasitism.     

A meta‐analysis of ant social parasitism: host characteristics of different parasitism types and a test of Emery’s rule

Abstract 1. In ant social parasitism, the process by which parasite–host systems evolved and the types of invasion mechanisms parasites use are being debated. Emery’s rule, for example, states that social parasites are the closest relatives to their hosts. The present study uses previously published data to test whether Emery’s rule applies equally to all parasitism types (i.e. xenobiosis, temporary, dulosis, and inquilinism). In addition, this study also investigates other links between parasite–host relatedness and host biology, which has implications for understanding the invasion mechanisms used by certain parasites. 2. We find that xenobiotic parasites typically use distantly‐related host species that are of at least medium colony size. Temporary parasites often have multiple host species that are very closely related to the parasite and hosts with medium‐size colonies. Dulotic parasites frequently have multiple host species that are slightly less related and of any size. Lastly, inquiline parasites tend to have a single, very closely related, host species with medium‐size colonies. 3. Parasites tend to be more closely related to host species if they have a single host species or when the host has a large colony size. In contrast, parasites with multiple host species or hosts of small colony size tend to be less related to their hosts. 4. This study is the first to examine trends in ant social parasitism across all known parasite species. Our meta‐analysis shows that Emery’s rule applies to inquilinism and temporary parasitism, but not to dulosis and xenobiosis. Our results also suggest that both parasitism type and parasite–host relatedness predict the number of hosts and host colony size. It may be that a chemical mimicry mechanism allows invasion of large host colonies, but requires close relatedness of parasite and host, and concentration on a single host species.     

Polyrhachis lama, a parasitic ant with an exceptional mode of social integration

Social parasitism is a common phenomenon amongst ants that occurs in manifold variations with differing levels of parasite–host integration. Particularly, high levels of social integration occur amongst closely related species (Emery’s rule), which form mixed colonies with their hosts and comprise the vast majority of social parasites. Considerable lower levels of integration are typically found amongst unrelated species that live in clearly separated colonies. The formicine ant Polyrhachis lama, however, parasitises a phylogenetically distant host species, Diacamma sp. of the subfamily Ponerinae, but lives spatially mixed with the host colonies. Studies on integration and communication have indicated that P. lama shows a high degree of host integration. However, the allocation of brood care behaviour, a central aspect of parasite integration, has not been studied. Because all known ant social parasites that are fully mixed with their host colonies are also true brood parasites, we investigated the integration of P. lama brood. Our results demonstrate that the parasite brood has a high degree of spatial integration, although it remains functionally separated regarding nutritive brood care. This can be attributed to behavioural and morphological differences between the phylogenetically distant species. The observed spatial confinement of parasite brood, however, is most likely due to an unusual method of chemical host integration. The parasite brood remains accepted in the Diacamma colonies only under the presence of adult parasites. Altogether, this suggests an active mechanism of chemical integration based on the acceptance allomones originating from P. lama workers. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Parasite scouting and host defence behaviours are influenced by colony size in the slave-making ant Protomognathus americanus

In the slave-making ant Protomognathus americanus, scout workers leave their colony, discover host colonies, and initiate slave raids. Captured host pupae subsequently emerge in the slavemaker colony and replenish the slave workforce. The course of these antagonistic encounters can be influenced by the species, aggressivity, or size of the host colony. We asked how the demography of parasite and host colonies influences the initial raiding phase by observing the scouting behaviour of P. americanus slavemakers during 48 raiding attempts. Experiments were performed under controlled laboratory conditions in a Y-shaped experimental arena. The number of active scouts increased with increasing slavemaker worker numbers, but was unaffected by the slave to slavemaker ratio, showing that slavemaker worker numbers are a good indicator for the scouting workforce. Colonies with fewer slaves discovered host colonies faster (colonies with 15 or less slaves: median 9:53 min, colonies with 42 or more slaves: median 18:55 min), suggesting that small slave workforces lead to intensified scouting behaviour. The more scouts were active, the faster a host colony was discovered, but the time between discovery and trial completion was unaffected by slavemaker colony demography. Host colonies were successfully attacked in 79.2 % of the trials, and they fought off an intruding scout only once. Yet host aggression towards slavemaker scouts increased with host colony size, and higher aggression rates delayed a subsequent attack. Our study demonstrates that colony size influences the behaviour and the course of crucial interspecific interactions of a social parasite and its host.     

Social parasitism by Cape honeybee workers in colonies of their own subspecies (Apis mellifera capensis Esch.)

Social parasitism is widespread in the eusocial insects. Although social parasites often show a reduced worker caste, unmated workers can also parasitize colonies. Cape honeybee workers, Apis mellifera capensis, can establish themselves as social parasites in host colonies of other honeybee subspecies. However, it is unknown whether social parasitism by laying workers also occurs among Cape honeybee colonies. In order to address this question we genotyped worker offspring of six queenless A. m. capensis colonies and determined the maternity of the reproducing workers. We found that three non-nestmate workers dominated reproduction in a host colony and produced 62.5% of the progeny. Our results show that social parasitism by laying workers is a naturally occurring part of the biology of Cape honeybees. However, such social parasitism is not frequently found (6.41% of the total worker offspring) probably due to co-evolutionary processes among A. m. capensis resulting in an equilibrium between selection for reproductive dominance in workers, colony maintenance and queen adaptation. Received 28 July 2005; revised 19 September and 11 November 2005; accepted 16 November 2005.     

The reproductive potential of workers in slave-making ants

Summary Colonies of slave-making ants have been used repeatedly to test sex allocation theory. It was suggested that workers of slave-making ants are more strongly selected to reproduce than workers of related, non-parasitic species, because they are incapable of manipulating sex allocation and the sexualization of larvae in their colonies. I show here that in slave-making Formicoxenini, worker ovaries on average consist of significantly more ovarioles than in non-parasiticLeptothorax species. Similarly, whereas in mostLeptothorax species, workers form reproductive hierarchies and lay eggs only in orphaned colonies, slave-maker workers show antagonistic interactions already in the presence of the queen and at least in some species have been observed ovipositing in queen-right colonies. The significance of these results is discussed.     

The evolution of social parasitism in <Emphasis Type="Italic">Acromyrmex</Emphasis> leaf-cutting ants: a test of Emery’s rule

Summary Emerys rule predicts that social parasites and their hosts share common ancestry and are therefore likely to be close relatives. Within the leaf-cutting ant genus Acromyrmex, two taxa of social parasites have been found, which are thought to occupy opposite grades of permanent social parasitism, based on their contrasting morphologies: Acromyrmex insinuator differs little in morphology from its free-living congeneric host species and produces a worker caste, and is thus thought to represent an early grade of social parasitism. At the other extreme, Pseudoatta spp. exhibit a very specialised morphology and lack a worker caste, both of which are characteristics of an evolutionarily derived grade of social parasitism. Here we present a molecular phylogeny using partial sequences of cytochrome oxidase I and II of about half of the known Acromyrmex species including two social parasites, their hosts and all congeneric species occurring sympatrically. We show that the two inquiline parasites represent two separate origins of social parasitism in the genus Acromyrmex. The early-grade social parasite A. insinuator is highly likely to be the sister species of its host Acromyrmex echinator, but the derived social parasite Pseudoatta sp. is not the sister species of its extant host Acromyrmex rugosus.Received 18 November 2002; revised 16 July 2003; accepted 24 July 2003.