Age polyethism,idiosyncrasy and behavioural flexibility in the queenless ponerine ant,Diacamma sp.

Age-related behavioural change of individually marked and known-age workers of a colony ofDiacamma sp. was observed through entire ant lives in the laboratory for more than a year. Although typical age polyethism was observed, activity of inside and outside tasks, life span, or time schedule of behavioural change varied considerably among workers. Idiosyncrasies in age-related behavioural change was detected among workers that eclosed on the same day. Cluster analyses revealed that individual patterns of behavioural change tended to be different when eclosion dates were far apart. This suggests that most workers change their behaviour flexibly. Thus, there is little effect of idiosyncrasy on colony response to changes in colony condition.     

Identification of polymorphic microsatellite loci in the queenless,ponerine ant Diacamma ceylonense

Diacamma ceylonense is a queenless, ponerine ant whose colonies are headed by a single, mated, egg‐laying worker referred to as the gamergate. Thus, new colonies are a result of dispersal by wingless gamergates. This is expected to influence patterns of colony dispersal and spatial distribution of genetic variablity. In order to facilitate the study of population genetic structure we have identified six unique, polymorphic, microsatellite loci. We have used fluorescence tagged primers to detect polymorphism at these loci.     

Low intraspecific aggression level in the polydomous and facultative polygynous ant Ectatomma tuberculatum

Nestmate recognition is a key feature of social insects, as it preserves colony integrity. However, discrimination of non‐nestmates and nestmate recognition mechanisms are highly variable according to species and social systems. Here, we investigated the intraspecific level of aggression in the facultative polygynous and polydomous ant, Ectatomma tuberculatum Olivier (Hymenoptera: Formicidae: Ectatomminae), in a population with a strong genetic structure. We found that the intraspecific level of aggression was generally low in this population of E. tuberculatum. However, the level of aggression was significantly correlated with the geographical distance, suggesting that both genetic and environmental cues could be involved in nestmate recognition and discrimination mechanisms. Moreover, polydomy was confirmed by the absence of aggression between workers from nests at a distance of 3 m, while the level of aggression was significantly higher between workers from nests separated by a distance of 10 or 1300 m. Field experiments showed that the low level of aggression between neighbouring colonies was associated with shared foraging areas, which could suggest that familiarization processes may occur in this species. We propose that the particular social organization of this species, with secondary polygyny, polydomy, and budding, may have favoured a high acceptance threshold, because of the low probability of interactions with unrelated conspecifics, the high cost of erroneously rejecting nestmates, and the low cost of accepting non‐nestmate workers. The resulting open recognition system can thus allow privileged relationships between neighbouring colonies and promote the ecological dominance of E. tuberculatum in the mosaic of arboreal ants.     

Thief ants have reduced quantities of cuticular compounds in a ponerine ant, Ectatomma ruidum

Abstract. Workers of a tropical ponerine ant, Ectatomma ruidum (Roger), exhibit a system of intercolonial thievery for food; thief ants enter conspecific colonies, intercept food, and carry it to their own colony. We analysed thief ants, ants from the thief's colony, and ants from the colony being robbed to determine if changes in cuticular profiles could facilitate entry by thieves into a target colony. Thief ants have significantly lower total quantities of dichloromethane-soluble cuticular compounds than non-thieves. The reduction in cuticular compounds on thieves suggests that thievery is facilitated by inhibition of either synthesis or acquisition of its home colony's recognition cues. Five of the fifteen compounds differed significantly in their relative proportion among colonies. Principal components analysis lends support to the hypothesis that both inhibition of a thief's home colony cues and acquisition of its target colony's cues play a role in successful thievery.     

Reproductive conflicts affect labor and immune defense in the queenless ant Diacamma sp. "nilgiri"

In many species of social Hymenoptera, totipotency of workers induces potential conflicts over reproduction. However, actual conflicts remain rare despite the existence of a high reproductive skew. One of the current hypotheses assumes that conflicts are costly and thus selected against. We studied the costs of conflicts in 20 colonies of the queenless ant Diacamma sp. "nilgiri" by testing the effects of conflicts on labor and worker immunocompetence, two parameters closely linked to the indirect fitness of workers. In this species, the dominant female is the only mated worker (gamergate) and monopolizes reproduction. We experimentally induced conflicts by splitting each colony into two groups, a control group containing the gamergate and an orphaned group displaying aggressions until a new dominant worker arises. Immunocompetence was assessed by the clearance of Escherichia coli bacteria that we injected into the ants. Time budget analysis revealed a lower rate of labor and especially brood care in orphaned groups, supporting the existence of a cost of conflicts on labor. Fifteen days after splitting, a lower immunocompetence was also found in orphaned groups, which concerned workers involved and not involved in conflicts. We propose that this immunosuppression induced by conflicts could stem from stress and not directly from aggression.     

Differentiation of the reproductive tract between dominant and subordinate workers in the Japanese queenless ant Diacamma sp.

In queenless ants, gamergates (mated egg‐laying workers) fulfil the reproductive task normally reserved for the queen. Every worker is a potential gamergate, thus we expect pronounced conflicts over sexual reproduction within their colonies. In the queenless ant genus Diacamma, gamergates inhibit nest mates from mating by aggressively removing (‘mutilating’) a pair of small appendages on the thorax, termed gemmae, shortly after eclosion. Dissection and serial sectioning of the reproductive tracts of both mutilated and unmutilated individuals of Diacamma sp. from Japan at different ages revealed that mutilation inhibits the development of the bursa copulatrix and the spermatheca, two structures fundamental for sexual reproduction. The precursor of the bursa copulatrix develops into a fully functional structure in unmutilated individuals, whereas it degenerates irreversibly in mutilated callows. Experimental manipulations showed that the removal of the gemmae is not the sole factor regulating this development. The spermathecal epithelium and accessory spermathecal gland of unmutilated individuals are thicker than that of mutilated individuals, indicating a higher degree of activity in the former. Mutilated females are therefore left incapable of copulating and less competent for long‐time sperm storage.     

Durability of information concerning the presence of a gamergate in Diacamma sp. from Japan

In social insects, information on the presence of a queen is known to affect the reproductive behaviour of workers. In the queenless ant Diacamma sp., information of the presence of the gamergate, the functional queen, is transmitted exclusively by direct physical contact between the gamergate and workers. Periodic contacts between the gamergate and each worker are therefore necessary to maintain worker sterility. However, how the difference in the contact interval influences reproductive behaviour of workers is unclear. In the present study, by artificially controlling the exact interval of physical contact between workers with the gamergate, it is shown that the inter‐contact interval influences the worker's reproductive condition (i.e. a longer interval leads to more developed ovaries). This system provides a good opportunity to study the mechanisms underling the colony size dependence of individual as well as colony characteristics.     

Interspecific variation in neighbour–stranger discrimination in ants of the Neoponera apicalis complex

1. The ecological success of social insects lies in their ability to prevent the exploitation of colony resources by competitors or parasites. Nestmate recognition is therefore of crucial importance in maintaining the integrity of the colony. Furthermore, inter‐colony competitive relationships are often complex, as many species discriminate between neighbours and strangers, with reduced (the dear enemy phenomenon) or increased levels of aggression towards nearby colonies, depending on the species. In this context, between‐species comparisons could be particularly helpful to investigate the proximate causes underlying this context‐dependent phenomenon, but these are notoriously lacking. 2. Here an attempt was made to circumvent this drawback by studying three closely related sympatric ant species with very similar life histories that belong to the Neoponera apicalis complex. The present study investigated how nestmate recognition and inter‐colony competitive relationships were influenced by spatial, chemical and genetic distances between colonies. 3. It was found that one species, N. apicalis morph 7, showed a clear dear enemy phenomenon with no influence of chemical and genetic distances, suggesting the existence of a learning process. In contrast, N. apicalis morph 4 and Neoponera verenae morph 1 failed to show any strong discrimination between close and distant non‐nestmates. 4. These results are discussed in the light of the observed interspecific variation in nesting preferences, possibly constraining the opportunities of familiarisation between nearby nests, and modulating the competition for resources between colonies. 5. It is argued that this study further reinforces the relative threat level hypothesis as an ultimate explanation for neighbour–stranger discrimination processes.     

Variation in ant aggression and kin discrimination ability within and between colonies

Significant variation in aggressiveness and kin discrimination ability occurs between different laboratory colonies of the ant Rhytidoponera confusa.Different colonies show consistently high (or low) levels of aggression toward nonnestmates over 4–19 weeks. Earlier studies excluded colony size and the natural presence or absence of the queen in colonies and differences in hunger as possible sources of variation. The present study excluded the number of larvae in colonies and the time of the light period of the light cycle when recognition tests were carried out. Highly significant variation occurs between the kin discrimination ability of individual workers in any particular colony. Approximately 28% of the workers in colonies of R. confusashowed very poor kin discrimination. Much of the colony's kin discrimination is carried out by a small number of highly aggressive workers.     

Morphological variations in the pre-imaginal development of the ponerine ant Diacamma ceylonense

In the present paper, the larval development of the queenless ponerine ant Diacamma ceylonense is studied. Four instars were identified on the basis of cuticular processes – tubercles and spinules – which show discontinuous variation during growth and provide precise and reliable external morphological criteria for instar discrimination. In the first three instars, the larva possesses a striking proboscis, which disappears between the third and fourth instars. Larval weight increased around 50‐fold in the 21 days of larval life between eclosion and pupation (mean weight for first instar = 0.37 mg; for fourth instar = 20 mg). In addition, the morphology of the gemmae, structures resembling vestigial wings in workers, is described in nymphae.     

Estimating the rate of gamergate turnover in the queenless ant Diacamma cyaneiventre using a maximum likelihood model

In monogynous ants, the death of the queen generally implies the death of the colony. However, queen replacement by either unrelated or related queen has been described in few species. In queenless ants from the genus Diacamma, the single reproductive worker (gamergate) is replaced by one of her daughters (or occasionally by a sister). From a long-term genetic survey of nests of D. cyaneiventre, we estimated the rate of gamergate turnover as well as the lifespan of workers and gamergate tenure using a maximum likelihood model developed for this purpose. We specifically compared the genotypes of two cohorts of workers sampled at 2 and 16 months interval from the same nests, using five microsatellite markers. To improve the accuracy of the estimates, we also used in the model the nests from the same population sampled only once and analysed by André et al. (2001). The model indicates that the possibility of the same nest not sheltering the same colony at two different sampling dates (colony turnover) was not significantly different from zero in our sample. The likelihood of the model was maximal for a probability of gamergate change p_γ = 0.005 per day (i.e. a gamergate tenure of 200 days) and a worker lifespan w=60 days, indicating that the gamergate ‘s tenure is about 3 times longer than workers’ expected lifespan in the population studied. Moreover, the genetic analysis of the gamergate and brood in three colonies excavated completely, reveals that colony fission can occur just after a gamergate replacement with the sister of the new gamergate reproducing in the new propagule. Received 12 October 2005; revised 7 December 2005; accepted 5 January 2006.     

Very low genetic variability in the Indian queenless ant Diacamma indicum

Abstract We developed microsatellite markers and combined them with mitochondrial markers to analyse the population genetic structure of the queenless ant Diacamma indicum. This species, lacking winged queens, is likely to have a restricted female dispersal but exhibits various life history traits suggesting higher dispersal abilities than the other Diacamma species. Only 4 of 11 microsatellites were polymorphic and only 1 had more than 4 alleles over 166 individuals originating from 7 populations from the south of India. Only one mitochondrial DNA (mtDNA) haplotype was detected throughout India (including one population in the north) and Sri Lanka. Such a level of polymorphism is particularly low compared with other Diacamma species having much smaller ranges in the south of India. A strong genetic differentiation was observed between populations separated by more than a few kilometres. We also analysed the genetic differentiation between the Indian populations and two populations from the Japanese island of Okinawa, which are morphologically similar and might belong to the same species. The genetic differentiation was high for both markers, suggesting an absence of ongoing gene flow between these populations.     

Intraspecific aggression and the colony structure of the invasive ant Myrmica rubra

1. Patterns of aggression between ants from different nests influence colony and population structure. Several species of invasive ants lack colony boundaries over large expanses, forming ‘supercolonies’ with many nests among which workers can move without encountering aggression. 2. Bioassays of aggression were used to determine the colony structure of the invasive ant Myrmica rubra (L.) at eight sites in Massachusetts, the state where the species was first discovered in North America. To improve the ability to distinguish systematic patterns from background variability in aggressiveness, a repeated‐measures design was used and replicate assays for each pair of nests were conducted. 3. Aggressive responses showed that populations at all sites consisted of multiple distinct colonies. Patterns of aggression were repeatable and transitive, with few exceptions. Colonies were identified as clusters of nests whose workers showed little to no aggression towards one another but were aggressive towards conspecifics from more distant nests. 4. The degree of aggression varied considerably among different colony pairs but did not depend in any consistent way on the distance of separation or on whether colonies were neighbours. 5. Territories of neighbouring colonies abutted, indicating that they were restricted by intraspecific competition. Mapped territories ranged in size from 0.03 to 1.2 ha, but colonies at the study sites have not undergone the enormous expansions seen in introduced populations of some other species of invasive ants, and neighbouring colonies compete locally.     

Ovarian development and insulin-signaling pathways during reproductive differentiation in the queenless ponerine ant Diacamma sp.

In many social hymenopteran species, workers possess functional ovaries that are physiologically inactive in the presence of queens. We investigated the ovarian regulatory mechanism of workers and reproductives in a queenless ponerine ant, Diacamma sp., using histological and molecular techniques. In this ant, clear reproductive differentiation occurs via a highly sophisticated dominance behavioral interaction called “gemmae mutilation”. This clear and rapid bifurcation of reproductive physiology allows us to elucidate the detailed ovarian differentiation process. Histological characteristics of functional ovaries (fusomes and ring canals) were found in both workers and reproductives, suggesting that early oogenesis is not blocked in workers. Since insulin/insulin-like growth factor signaling (IIS) is known to control insect reproduction, orthologs of 2 positive IIS regulators, insulin receptor and serine-threonine kinase Akt (protein kinase B), were cloned in Diacamma (DiaInR, DiaAkt); their expression patterns during reproductive differentiation were examined by real-time quantitative polymerase chain reaction; DiaInR and DiaAkt were strongly expressed in the gasters of reproductives. Whole-mount in situ hybridization of ovaries indicated that DiaInR and DiaAkt were expressed in nurse cells, oocytes, and upper germarial regions of reproductives but not of workers. Our data suggest that the IIS pathway accounts for reproductive differentiation in late oogenesis.     

Population genetic structure and male-biased dispersal in the queenless ant Diacamma cyaneiventre

In this study we investigated the population genetic structure of the queenless ant Diacamma cyaneiventre. This species, lacking winged queens, is likely to have a restricted female dispersal. We used both mitochondrial and microsatellite markers to assess the consequence of such restricted female dispersal at three geographical scales: within a given locality (< 1 km), between localities within a given region (< 10 km) and between regions (> 36 km). Within a locality, a strong population structure was observed for mitochondrial DNA (mtDNA) whereas weak or nonexistent population genetic structure was observed for the microsatellites (around 5% of the value for mtDNA). Male gene flow was estimated to be about 20-30 times higher than female gene flow at this scale. At a larger spatial scale, very strong genetic differentiation for both markers was observed between localities - even within a single region. Female dispersal is nonexistent at these scales and male dispersal is very restricted, especially between regions. The phylogeographical structure of the mtDNA haplotypes as well as the very low genetic diversity of mtDNA within localities indicate that new sites are colonized by a single migration event from adjacent localities, followed by successive colony fissions. These patterns of genetic variability and differentiation agree with what is theoretically expected when colonization events are kin-structured and when, following colonization, dispersion is mainly performed by males.     

The policing behavior ‘immobilization’ towards ovary-developed workers in the ant, Diacamma sp. from Japan

Summary. In social Hymenoptera worker policing that inhibits direct reproduction of workers occurs mainly in the two ways: (1) destruction of worker progenies (postovipositional policing), and (2) aggression towards ovary-developed workers (preovipositional policing). In the queenless ponerine ant, Diacamma sp. from Japan, the existence of the former type of worker policing has been reported, whereas previous studies have failed to find the latter type. We examine the presence of the latter type of worker policing in this species using more careful observational methods. By a series of experiments reuniting previously separated orphaned and non-orphaned subcolonies we found the following facts. Immediately after the colony reunification, aggression frequently took place. Unlike the one-on-one dominance interaction that occurs in non-manipulated colonies, aggression was often directed towards a single victim from multiple attackers, grasping and pulling the victim. The duration of each aggressive interaction was on average far longer in this situation than that of usual dominance interactions. Most victims consisted of ex-orphan workers, while the majority of the attackers were ex-non-orphan workers. Dissection after the above behavioral observation revealed that the ovaries of the victims were on average more active, often containing mature oocytes, than those of non-victims, while the ovaries of attackers were always inactive. The above findings indicate that worker policing via immobilization, which has been reported in some other Ponerinae, also exists in Diacamma sp. from Japan.Received 22 April 2003; revised 28 June and 19 August 2004; accepted 1 September 2004.