The genetic population structure of two socially parasitic ants: “the active slave-maker” Myrmoxenus ravouxi and the “degenerate slave-maker” M. kraussei

Slave-making ants exploit the societies of host ant species and are typically rare and patchily distributed. IUCN considers almost all slave-making ants as vulnerable, but solid data on their actual abundance are uncommon. Here we examine the genetic structure of populations of two species of the socially parasitic genus Myrmoxenus, which differ strongly in dispersal behavior and the occurrence of slave-raids. Microsatellite genotypes suggest strong differentiation even among neighboring populations of both species. F_ST-, G″_ST-, and D-values were considerably higher in the “degenerate slave-maker” M. kraussei from Northern Italy than in the active slavemaker M. ravouxi from Southern Germany. This matches observations that sexuals of M. kraussei mate in their natal nest and queens disperse on foot while sexuals of M. ravouxi engage in mating and dispersal flights. Allelic richness was surprisingly high in both species and did not suggest recent bottlenecks, indicating that populations are larger and less vulnerable than expected from the difficulties of locating nests. Despite of considerable inbreeding, only very few diploid males were detected, supporting the view that at least in M. kraussei sex is not determined by single locus complementary sex determination unlike most other social Hymenoptera. The mismatch between the genetic and social structure of M. ravouxi colonies indicates occasionally fusion of slave-maker colonies in the field, mutual raids, or queen replacement. Complementary analyses of the host species of the two social parasites, Temnothorax unifasciatus and T. recedens, revealed low levels of population differentiation and confirmed the colony structure with a single, singly-mated queen.     

Imprinting Effects on Host-selection Behavior of Slave-raiding Chalepoxenus muellerianus (Finzi) Workers (Hymenoptera: Formicidae)

Early experience is known to influence brood care behavior and to induce nesting preferences of several ant species. Scanty information is available with respect to imprinting effects on host selection of dulotic ants. In this paper we investigate host choice behavior of workers of the slave-making ant, Chalepoxenus muellerianus. As in a corresponding paper (Schumann & Buschinger 1994) on host specificity of C. muellerianus young queens during colony foundation, we demonstrate that a blend of innate factors and effects of early experience also influences host species selection of slave-raiding workers.     

Production of winged and wingless males in the ant, <Emphasis Type="Italic">Cardiocondyla minutior</Emphasis>

Summary Wingless (ergatoid) males of the tramp ant Cardiocondyla minutior attack and kill their young ergatoid rivals and thus attempt to monopolize mating with female sexuals reared in the colony. Because of the different strength of local mate competition in colonies with one or several reproductive queens, we expected the production of new ergatoid males to vary with queen number. Sex ratios were mostly female-biased, but in contrast to the sympatric species C. obscurior (Cremer and Heinze, 2002) neither the percentage of ergatoid males nor of female sexuals among the first 20 sexuals produced varied considerably with queen number. As in C. obscurior, experimental colony fragmentation led to the production of winged males, whereas in unfragmented control colonies only ergatoid males eclosed.Received 3 December 2003; revised 20 February 2004; accepted 23 February 2004.     

Phylogeny and phylogeography of the Mediterranean species of the parasitic ant genus <Emphasis Type="Italic">Chalepoxenus</Emphasis> and its <Emphasis Type="Italic">Temnothorax</Emphasis> hosts

We analysed the phylogenetic and phylogeographic relationships of four Mediterranean species of the rare slave-making ant genus Chalepoxenus and eleven of its about 20 Temnothorax host species by sequencing the mitochondrial Cytochrome Oxidase I and II genes. Neighbour-Joining, Maximum Parsimony and Bayesian analyses based on 1320 bp indicate that the genus Chalepoxenus constitutes a monophylum. In all three analyses, C. kutteri from Southwest Europe and the workerless, “degenerate slavemaker” C. brunneus from North Africa form a monophyletic group. C. muellerianus and C. tauricus, distributed in Southern Europe and Ukraine, respectively, form a monophylum in the Neighbour-Joining and the Maximum Parsimony analysis. In our limited set of only 11 of several hundred Temnothorax species, T. flavicornis forms the sister group of Chalepoxenus. Our study further indicates paraphyly of the genus Temnothorax with respect to Chalepoxenus. Moreover, the results suggest that speciation in this slave-making genus is possibly caused by the formation of host races as different Chalepoxenus species use different hosts, and some samples seem to cluster by host species rather than by geographical distance. Received 5 September 2006; revised 17 March 2007; accepted 23 March 2007.     

Immune response to sympatric and allopatric parasites in a snail-trematode interaction

Background

Ants use the odour of the colony to discriminate nestmates. In some species, this odour is learned during the first days following emergence, and thus early experience has a strong influence on nestmate discrimination. Slave-making ants are social parasites that capture brood of other ant species to increase the worker force of their colony. After emerging in the slave-maker nest, slave workers work as if they were in their own colony. We tested the hypothesis that early experience allows the deception of commonly enslaved species, while non-host species use a different mechanism, which does not involve learning.

Results

Pupae of a host species, Temnothorax unifasciatus, and a non-host species, T. parvulus, were allowed to emerge in the presence of workers of one of two slave-maker species, Chalepoxenus muellerianus or Myrmoxenus ravouxi. When T. unifasciatus was exposed to slave-makers for 10 days following emergence, they were more aggressive towards their own sisters and groomed the slave-maker more. T. parvulus gave a less clear result: while workers behaved more aggressively towards their sisters when exposed early to C. muellerianus workers, this was not the case when exposed early to M. ravouxi workers. Moreover, T. parvulus workers allogroomed conspecific nestmates less than T. unifasciatus. Allogrooming activity might be very important for the slave-makers because they are tended by their slaves.

Conclusion

Our findings show that early experience influences nestmate discrimination in the ant T. unifasciatus and can account for the successful enslavement of this species. However, the non-host species T. parvulus is less influenced by the early environment. This might help to explain why this species is never used by social parasites.     

The influence of hybridization on colony structure in the ant species Temnothorax nylanderi and T. crassispinus

The parapatric sibling ant species Temnothorax nylanderi and T. crassispinus hybridize in the contact zone in the Franconian Jura, Southern Germany. Aim of our study was to investigate the impact of hybridization on colony composition and fitness. We classified colonies as either ‘pure’ or containing hybrids by determining their allozyme pattern at GPI, an enzyme that is fixed for different alleles in the two parental species, and quantified their reproductive output. Most colonies with hybrid workers had a T. crassispinus queen. Colonies with heterozygous, hybrid workers produced more young workers than colonies of the parental species but similar numbers of male and female sexuals. Female sexuals from colonies with heterozygous workers had a significantly lower weight than female sexuals from pure colonies. Only a single reproductive queen was found to be heterozygous, suggesting reduced fitness of hybrid queens. As in the parental species, hybrid colonies appear to be frequently taken over by alien queens, which obscures the genetic colony structure. Received 6 April 2006; revised 10 June 2006; accepted 15 June 2006.     

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.     

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.     

Mating is associated with immediate changes of the hydrocarbon profile of Leptothorax gredleri ant queens

While sexual communication is often characterized by attempted manipulation, both sexes agree about females reliably signalling their receptivity. Female sexuals of the ant Leptothorax gredleri quickly became unattractive to males after their first copulation. This loss of attractiveness coincided with almost immediate changes in their cuticular hydrocarbon (CHC) profiles. Already 30 min after mating, the CHC profiles of female sexuals had significantly lower relative amounts of branched alkanes and higher amounts of linear alkanes than those of unmated and freshly mated female sexuals. Discriminant analysis did not distinguish between the profiles of freshly mated and unmated female sexuals, suggesting that the extremely rapid modification of CHC profiles is not caused by males marking females with anti-aphrodisiac CHCs. Instead, the new profile is produced by the female sexuals themselves. In addition to making them unattractive to males, this change may also help mated female sexuals when seeking adoption into established colonies.     

Mating system and population structure in the desert ant Cataglyphis livida

We investigated population genetic structure, mating system, worker reproduction and thelytokous parthenogenesis in the desert ant Cataglyphis livida. Pedigree analyses at polymorphic microsatellite loci show that colonies are headed by a single queen, and that queens are mated with two to eight males. No inbreeding was found in the population sampled. Colonies are genetically differentiated and exhibit no isolation-by-distance pattern, consistent with independent foundation of new colonies. Workers do reproduce and lay haploid (arrhenotokous) eggs in queenless colonies; conversely, we found no evidence of worker reproduction in queenright nests. In contrast with C. cursor, where new queens are produced by thelytokous parthenogenesis, female sexuals and workers of C. livida arise from classical sexual reproduction. We discuss the parallels and contrasts between the mating system and population structure in C. livida and the other Cataglyphis species studied so far.     

Intraspecific nestmate recognition in two parabiotic ant species: acquired recognition cues and low inter-colony discrimination

Parabiotic ants—ants that share their nest with another ant species—need to tolerate not only conspecific nestmates, but also nestmates of a foreign species. The parabiotic ants Camponotus rufifemur and Crematogaster modiglianii display high interspecific tolerance, which exceeds their respective partner colony and extends to alien colonies of the partner species. The tolerance appears to be related to unusual cuticular substances in both species. Both species possess hydrocarbons of unusually high chain lengths. In addition, Cr. modiglianii carries high quantities of hereto unknown compounds on its cuticle. These unusual features of the cuticular profiles may affect nestmate recognition within both respective species as well. In the present study, we therefore examined inter-colony discrimination within the two parabiotic species in relation to chemical differentiation. Cr. modiglianii was highly aggressive against workers from alien conspecific colonies in experimental confrontations. In spite of high inter-colony variation in the unknown compounds, however, Cr. modiglianii failed to differentiate between intracolonial and allocolonial unknown compounds. Instead, the cuticular hydrocarbons functioned as recognition cues despite low variation across colonies. Moreover, inter-colony aggression within Cr. modiglianii was significantly influenced by the presence of two methylbranched alkenes acquired from its Ca. rufifemur partner. Ca. rufifemur occurs in two varieties (‘red’ and ‘black’) with almost no overlap in their cuticular hydrocarbons. Workers of this species showed low aggression against conspecifics from foreign colonies of the same variety, but attacked workers from the respective other variety. The low inter-colony discrimination within a variety may be related to low chemical differentiation between the colonies. Ca. rufifemur majors elicited significantly more inter-colony aggression than medium-sized workers. This may be explained by the density of recognition cues: majors carried significantly higher quantities of cuticular hydrocarbons per body surface.     

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.     

Worker dominance and policing in the ant Temnothorax unifasciatus

In many species of eusocial Hymenoptera, conflict about the production of males is resolved through “policing.” Recent studies in wasps and the ant Temnothorax unifasciatus suggest that in these species policing workers are dominant themselves and selfishly increase their own chances of later becoming fertile. Policing may therefore to some extent be associated with dominance and selfishness, and dominance and policing behaviour are indeed difficult to distinguish and often not mutually exclusive. Moreover, selfish policing requires that workers form rank orders already in the presence of the queen. Here, we try to allocate aggressiveness by workers towards policing and/or dominance behaviour and investigate whether hierarchies based on subtle, non-aggressive interactions exist in queenright colonies of the ant T. unifasciatus. We either split colonies into a queenright and queenless halve or temporarily removed the queen from complete colonies, which in both cases allows a few dominant workers to lay eggs in the queenless colony. Reunification of colony halves and return of the queen to orphaned colonies led to aggression against those workers that had become fertile during the absence of the queen. Dominant workers in reunited, split colonies were more severely attacked than those in orphaned colonies after return of the queen. Furthermore, we observed that workers, which later became dominant egg layers under queenless conditions, have more contact with the queen than other workers. Both results corroborate the existence of rank relationships among workers in queenright colonies and show that results from policing experiments may be affected by the disturbance of pre-existing hierarchies through colony splitting.     

Evolutionary transition to a semelparous life history in the socially parasitic ant Acromyrmex insinuator

The recently discovered social parasite Acromyrmex insinuator ( 25 ) exploits colonies of the leafcutter ant A. echinatior. We document that A. insinuator represents a rare early stage in the evolution of social parasitism, because a worker caste is still partially present and mating phenology has remained at least partially similar to that of the host. A. insinuator is tolerant of host queens, and sexual offspring produced in parasitized colonies can be either exclusively A. insinuator or a mix of A. insinuator and A. echinatior. The remarkably high abundance of A. insinuator in nests of the investigated Panamanian host population and the fact that A. insinuator colonies readily reproduce under laboratory conditions allowed us to test evolutionary predictions on reproductive life history evolution that are not possible in most other socially parasitic ants. We show that (1) A. insinuator has a semelparous ‘big bang’ reproductive life history which exploits host colonies without leaving reserves for survival; (2) social parasite sexuals are significantly smaller than A. echinatior host sexuals, but still large compared to host workers, confirming an evolutionary scenario of gradual size reduction and loss of the worker caste after transition towards a socially parasitic life history; (3) major changes in the life history of ants can evolve relatively quickly compared to adaptations in morphology, caste differentiation and mating phenology.     

The ecological success of a social parasite increases with manipulation of collective host behaviour

Many parasites alter the behaviour of their host to their own advantage, yet hosts often vary in their susceptibility to manipulation. The ecological and evolutionary implications of such variation can be profound, as resistant host populations may suffer lower parasite pressures than those susceptible to manipulation. To test this prediction, we assessed parasite‐induced aggressive behaviours across 16 populations of two Temnothorax ant species, many of which harbour the slavemaker ant Protomognathus americanus. This social parasite uses its Dufour's gland secretions to manipulate its hosts into attacking nestmates, which may deter defenders away from itself during invasion. We indeed find that colonies that were manipulated into attacking their Dufour‐treated nestmates were less aggressive towards the slavemaker than those that did not show slavemaker‐induced nestmate attack. Slavemakers benefited from altering their hosts’ aggression, as both the likelihood that slavemakers survived host encounters and slavemaker prevalence in ant communities increased with slavemaker‐induced nestmate attack. Finally, we show that Temnothorax longispinosus colonies were more susceptible to manipulation than Temnothorax curvispinosus colonies. This explains why T. curvispinosus colonies responded with more aggression towards invading slavemakers, why they were less likely to let slavemakers escape and why they were less frequently parasitized by the slavemaker than T. longispinosus. Our findings highlight that large‐scale geographic variation in resistance to manipulation can have important implications for the prevalence and host preference of parasites.     

Colony growth in <Emphasis Type="Italic">Myrmica rubra</Emphasis> with supplementation of myrmecochorous seeds

Myrmecochory is an important ant–plant relationship, which presumably has benefits for the reproductive success of ant colonies through the nutritional value of elaiosomes. In a feeding experiment, we provided elaiosomes of Scilla bifolia and Corydalis cava to colonies of Myrmica rubra. Seeds were transported by ant workers to their nests and the elaiosomes were removed afterwards. After 3 months, elaiosome-supplemented colonies contained significantly more worker pupae than control colonies, whereas the number of new female sexuals was reduced. This result is discussed as a possible long-term benefit for M. rubra colonies through myrmecochory by colony growth.     

Colony structure and sex allocation ratios in the ant <Emphasis Type="Italic">Temnothorax crassispinus</Emphasis>

Summary. We analyzed the impact of ecological parameters, such as nest density and nest site availability, on colony organization and investment patterns in two populations of the ant Temnothorax crassispinus, a parapatric sibling species of the well-studied T. nylanderi (Temnothorax was until recently referred to as Leptothorax (Myrafant); Bolton, 1993). As in T. nylanderi, sex allocation ratios were strongly associated with total sexual reproduction, i. e., nests with large sexual investment produced mainly female sexuals. Furthermore, nest site quality affected sex allocation ratios, with colonies from ephemeral nest sites producing a more male-biased sex allocation ratio than colonies from sturdy nest sites. In contrast to T. nylanderi, workers in colonies of T. crassispinus were mostly fullsisters both in a dense and a sparsely populated area, suggesting that colony fusion and colony usurpation are rare in this species. In addition, the presence of a queen in a local nest unit strongly influenced sex ratio decisions, in that these nests raised a more male biased allocation ratio compared to queenless nests. This also suggests that colony structure is more stable in T. crassispinus than in T. nylanderi. We conclude that sibling species, though often very similar in their morphology and ecological requirements, may nevertheless react very differently to ecological variation.Received 11 December 2003; revised 4 March 2004; accepted 19 April 2004.     

Selective Acceptance of Alien Host Species Pupae by Slaves of the Dulotic Ant,Harpagoxenus sublaevis (Hymenoptera,Formicidae, Myrmicinae)

The European slave-making ant, Harpagoxenus sublaevis, has up to three host species which occasionally are found all together in one colony. Colonies with only Leptothorax acervorum slaves, or with both L. acervorum and L. muscorum, are frequent, whereas colonies with only L. muscorum slaves are rare, although young H. sublaevis queens found their colonies about equally frequently with either of the two main host species. We demonstrate here, that the two host species exhibit an asymmetric acceptance behavior towards each other's pupae: L. muscorum workers accept L. acervorum pupae, both in non-enslaved colonies and as slaves in H. sublaevis nests, whereas L. acervorum never accepted L. muscorum pupae. Origin and meaning of this behavioral difference are unknown. The findings, however, might be very important with respect to host specificity and host-parasite interactions also in other slave-making ants.     

Queen-queen competition by precocious male production in multiqueen ant colonies

Arriving earlier in the breeding area than his rivals may be beneficial for a male when females mate only once or during a short time span. The timing of a male's entrance is usually determined by the male himself, e.g., through returning early from his winter quarters or through accelerated larval development . Here, we document a surprisingly simple way of "first come, first served" in a species with local mate competition. In multiqueen colonies of a Cardiocondyla ant, mother queens make sure that their own sons are the first to monopolize mating with a large harem of female sexuals by producing extremely long-lived males early in colony life. Whereas queens in newly founded single-queen colonies started to produce male and female sexuals only several weeks after the eclosion of their first worker offspring, queens in multiqueen colonies precociously reared sons long before the first female sexuals and even before the emergence of their first workers. These early males killed all later emerging males in the nest and mated with all female sexuals subsequently produced. Our data document that the patterns of growth and productivity of insect colonies are surprisingly flexible and can be turned upside down under appropriate selection pressures.     

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.