Intranest relatedness and nestmate recognition in the meadow ant Formica pratensis (R.)

The impact of intranest relatedness on nestmate recognition was tested in a population of polydomous and monodomous nests of the mound-building ant Formica pratensis. Nestmate recognition was evaluated by testing aggression levels between 37 pairs of nests (n=206 tests). Workers from donor colonies were placed on the mounds of recipient nests to score aggressive interactions among workers. A total of 555 workers from 27 nests were genotyped using four DNA microsatellites. The genetic and spatial distances of nests were positively correlated, indicating budding and/or fissioning as spread mechanisms. Monodomous and polydomous nests did not show different aggression levels. Aggression behavior between nests was positively correlated with both spatial distance and intranest relatedness of recipient colonies, but not with genetic distance or intranest relatedness of donor colonies. Multiple regression analysis revealed a stronger effect of spatial distance than of genetics on aggression behavior in this study, indicating that the relative importance of environment and genetics can be variable in F. pratensis. Nevertheless, the positive regression between intranest relatedness of recipient colonies and aggression in the multiple analysis supports earlier results that nestmate recognition is genetically influenced in F. pratensis and further indicates that foreign label rejection most likely explains our data.     

Seasonal nestmate recognition in the ant Formica exsecta

Under favorable conditions, the mound-building ant Formica exsecta may form polydomous colonies and can establish large nest aggregations. The lack of worker aggression towards nonnestmate conspecifics is a typical behavioral feature in such social organization, allowing for a free flux of individuals among nests. However, this mutual worker toleration may vary over the seasons and on spatial scales. We studied spatio-temporal variation of worker–worker aggression within and among nests of a polydomous F. exsecta population. In addition, we determined inter- and intracolony genetic relatedness by microsatellite DNA genotyping and assessed its effect on nestmate recognition. We found significant differences in the frequency of worker exchange among nests between spring, summer, and autumn. Moreover, we found significant seasonal variation in the level of aggression among workers of different nests. Aggression levels significantly correlated with spatial distance between nests in spring, but neither in summer nor in autumn. Multiple regression analysis revealed a stronger effect of spatial distances rather than genetic relatedness on aggressive behavior. Because nestmate discrimination disappeared over the season, the higher aggression in spring is most plausibly explained by cue intermixing during hibernation.     

Kin discriminators in the eusocial sweat bee <Emphasis Type="Italic">Lasioglossum malachurum</Emphasis>: the reliability of cuticular and Dufour’s gland odours

The ability to discriminate degrees of relatedness may be expected to evolve if it allows unreciprocated altruism to be preferentially directed towards kin (Hamilton in J Theor Biol 7:1–16, 1964). We explored the possibility of kin recognition in the primitively eusocial halictid bee Lasioglossum malachurum by investigating the reliability of worker odour cues that can be perceived by workers to act as indicators of either nest membership or kinship. Cuticular and Dufour’s gland compounds varied significantly among colonies of L. malachurum, providing the potential for nestmate discrimination. A significant, though weak, negative correlation between chemical distance and genetic relatedness (r = −0.055, p < 0.001) suggests a genetic component to variation in cuticular bouquet, but odour cues were not informative enough to discriminate between different degrees of relatedness within nests. This pattern of variation was similar for Dufour’s gland bouquets. The presence of unrelated individuals within nests that are not chemically different from their nestmates suggests that the discrimination system of L. malachurum is prone to acceptance errors. Compounds produced by colony members are likely combined to generate a gestalt colony chemical signature such that all nestmates have a similar smell. The correlation between odour cues and nest membership was greater for perceived compounds than for non-perceived compounds, suggesting that variability in perceived compounds is a result of positive selection for nestmate recognition despite potentially stabilising selection to reduce variability in odour differences and thereby to reduce costs derived from excessive intracolony nepotistic behaviour.     

Transient nestmate recognition cues contribute to a multicolonial population structure in the ant,Leptothorax curvispinosus

Summary Individual nests of the facultatively polygynous and polydomous ant, Leptothorax curvispinosus, were mapped at two sites, collected, and maintained under uniform laboratory conditions. Tests of worker acceptability between nests were conducted 2–4 weeks and 13–17 weeks after collection. Nests collected near to one another (0.09–1.87 m) were sometimes nonaggressive, and were significantly less aggressive than those from different sites (7 km apart); and there was no significant difference in aggressiveness between tests for these distance categories. However, aggression between nests collected farther away from one another at the same site (1.52–4.65 m) decreased significantly between tests: initially, the level of aggressiveness was equivalent to that between nests from different sites but later it was reduced to that between near nests. These results indicate that polydomous colonies of this species occur within multicolonial populations; and that colony segregation within local populations is largely maintained by transient environmentally-based nestmate recognition cues. More stable cues of genetic or environmental origin (or both) are also present and contribute to discrimination even after extended periods of culture under uniform conditions. These results suggest that the maintenance of colony autonomy within genetically highly interrelated populations may be the prime function of environmentally-based nestmate recognition cues. Colony autonomy under such circumstances may be important to maintain a relatively small but optimal colony size, or because the mechanisms which regulate colony growth, development, etc., require a limited colony size.     

Relatedness,recognition errors,and colony fusion in the termite <Emphasis Type="Italic">Nasutitermes corniger</Emphasis>

Loss of aggression between social groups can have far-reaching effects on the structure of societies and populations. We tested whether variation in the genetic structure of colonies of the termite Nasutitermes corniger affects the probability of aggression toward non-nestmates and the ability of unrelated colonies to fuse. We determined the genotypes of workers and soldiers from 120 colonies at seven polymorphic microsatellite loci. Twenty-seven colonies contained offspring of multiple founding queens or kings, yielding an average within-colony relatedness of 0.33. Genotypes in the remaining 93 colonies were consistent with reproduction by a single queen and king or their progeny, with an average within-colony relatedness of 0.51. In standardized assays, the probability of aggression between workers and soldiers from different colonies was an increasing function of within-colony relatedness. The probability of aggression was not affected significantly by the degree of relatedness between colonies, which was near zero in all cases, or by whether the colonies were neighbors. To test whether these assays of aggression predict the potential for colony fusion in the field, we transplanted selected nests to new locations. Workers and soldiers from colonies that were mutually tolerant in laboratory assays joined their nests without fighting, but workers and soldiers that were mutually aggressive in the assays initiated massive battles. These results suggest that the presence of multiple unrelated queens or kings promotes recognition errors, which can lead to the formation of more complex colony structures.     

Queen acceptance and the complexity of nestmate discrimination in the Argentine ant

In most social insect species, individuals recognize and behave aggressively towards non-nestmate conspecifics to maintain colony integrity. However, introduced populations of the invasive Argentine ant, Linepithema humile, exhibit pronounced variation in intraspecific aggression denoting diversity in nestmate recognition behavior, which possibly shapes their social structure and the varying levels of unicoloniality observed among these populations. One approach to better understand differential aggression behaviors towards conspecifics and recognition cue perception and response in L. humile is to examine variation in nestmate discrimination capability among genetically distinct colonies under different social contexts. Consequently, we investigated the dynamics of queen and worker recognition in southeastern US L. humile queenless and queenright colonies by measuring rates of non-nestmate worker and queen adoption and intercolony genetic similarity. Aggression levels between colony pairs differed and were associated with non-nestmate worker, but not queen adoption. Adoption of queens and workers was a function of host colony origin, while colony queen number affected adoption of queens, but not workers, with queens more readily accepted by queenless hosts. Fecundity of adopted non-nestmate queens was comparable to that of rejected non-nestmate and host colony queens, suggesting that queen fecundity did not affect adoption decisions. Genetic similarity between colonies ranged from 30 to 77% alleles shared, with more genetically similar colonies showing lower levels of intraspecific aggression. Non-nestmate queens and workers that were more genetically similar to host colony workers were more likely to be adopted. We provide the first evidence for the role of L. humile colony queen number on queen discrimination and suggest an effect of resident queens on worker conspecific acceptance thresholds. Our findings indicate a role for genetically based cues in L. humile nestmate recognition. However, subtle discrimination capability seems to be influenced by the social context, as demonstrated by more frequent recognition errors in queenless colonies.     

Nestmate recognition for eggs in the honeybee (<Emphasis Type="Italic">Apis mellifera</Emphasis> L.)

Colony integrity is fundamental to social insects and is threatened by the reproduction of non-nestmates. Therefore, discrimination between eggs derived from nestmates and non-nestmates would constitute an adaptation to prevent exploitation of the entire cooperative group by unrelated individuals. The removal of nestmate and non-nestmate queen and worker-laid eggs was evaluated in honeybees using colonies of Apis mellifera capensis to test female and of A. m. scutellata to test male eggs. The data show that honeybees can distinguish between nestmate and non-nestmate eggs of both sexes. Moreover, non-nestmate female queen-laid eggs were removed significantly faster than nestmate female worker-laid eggs in A. m. capensis, indicating that nestmate recognition cues can override caste-specific ones. While the experimental manipulation accounts for 37.2% (A. m. scutellata) or 1.6% (A. m. capensis) of variance in relation to egg removal, nestmate recognition explains 33.3% for male eggs (A. m. scutellata) and 60.6% for female eggs (A. m. capensis), which is almost twice as high as the impact of caste (16.7% A. m. scutellata; 25% A. m. capensis). Our data show a stronger effect of nestmate recognition on egg removal in the honeybee, suggesting that cues other than caste-specific ones (viability/kin) can dominate egg removal behavior. In light of intraspecific social parasitism, preventing the reproduction of unrelated individuals (group selection) rather than preferring queens’ eggs (kin selection) appears to be the driving force behind the evolution of egg removal behavior in honeybees.     

Nestmate recognition in three species of stenogastrine wasps (Hymenoptera,Vespidae)

The capacity to recognise a conspecific intruder was investigated in Parischnogaster jacobsoni, Liostenogaster flavolineata and L. vechti, three species of primitively social wasps of the subfamily Stenogastrinae. Results of behavioural experiments carried out in the field showed that females of all three species react pacifically if presented with female nestmates, but aggressively reject an intruder from a conspecific colony. As L. flavolineata and L. vechti both build large clusters of nests, often very close to each other, the recognition capacity among females from different nests, but in the same conspecific cluster, was also investigated. Females of both species were more aggressive towards females from a different colony in the same cluster than towards their female nestmates. Additional experiments on L. flavolineata showed that there was no difference in reaction towards females from colonies nearer or further from the tested colony but within the same cluster, nor towards females from a different cluster. The capacity to recognise an alien conspecific nest containing immature brood was investigated in P. jacobsoni. Adult females of this species, invited to land on an alien nest which had experimentally been exchanged for their own, accepted the new nest and partially destroyed the immature brood. The behaviour of the females when they land on an alien nest, however, suggests that they do recognise the nest as foreign. Acceptance of foreign nests coupled with low immature brood destruction is probably due to the high energetic costs of egg-deposition and larval rearing in stenogastrine wasps. These results suggest that nestmate recognition in these wasps is very efficient, even though they belong to the most primitive subfamily of social wasps. Received: 16 April 1996/Accepted after revision: 9 August 1996     

Genetic population structure and sociogenetic organisation in Formica truncorum (Hymenoptera; Formicidae)

Summary The genetic population structure and the sociogenetic organization of the red wood ant Formica truncorum were compared in two populations with monogynous colonies and two populations with polygynous colonies. The genetic population structure was analysed by measuring allele frequency differences among local subsets of the main study populations. The analysis of sociogenetic organisation included estimates of nestmate queen and nestmate worker relatedness, effective number of queens, effective number of matings per queen, relatedness among male mates of nestmate queens and relatedness between queens and their male mates. The monogynous populations showed no differentiation between subpopulations, whereas there were significant allele frequency differences among the subpopulations in the polygynous population. Workers, queens and males showed the same genetical population structure. The relatedness among nestmate workers and among nestmate queens was identical in the polygynous societies. In three of the four populations there was a significant heterozygote excess among queens. The queens were related to their male mates in the polygynous population analysed, but not in the monogynous ones. The data suggest limited dispersal and partial intranidal mating in the populations with polygynous colonies and outbreeding in the populations having monogynous colonies. Polyandry was common in both population types; about 50% of the females had mated at least twice. The males contributed unequally to the progeny, one male fathering on average 75% of the offspring with double mating and 45–80% with three or more matings. Correspondence to: L. Sundström     

Relatedness and sex ratio in a primitively eusocial halictine bee

Lasioglossum laevissimum was studied in Calgary, Alberta, where it is eusocial with one worker brood. Estimates of relatedness were obtained among various categories of nestmate based upon four polymorphic enzyme loci, two of which exhibited significant levels of linkage disequilibrium. Relatedness estimates among workers and among reproductive brood females were very close to the expected 0.75 value that obtains when nests are headed by one, singly mated queen. However, relatedness between workers and the reproductive brood females they reared was significantly lower than 0.75. A low frequency of orphaning with subsequent monopolisation of oviposition by one worker brood female in orphaned nests may explain these results. Workers were significantly more and queens significantly less closely related to male reproductives than expected if all males were to have resulted from queen-laid eggs. Orphaning and worker-produced males contribute to this result. The sex investment ratio was 1:2.2 in favour of females, in excellent agreement with the predictions based upon relative relatednesses between workers and reproductive brood males and females. Adaptive intercolony variation in investment ratios was detected: the sex ratio was more heavily female-biased in nests in which the relative relatedness asymmetry between workers and reproductive brood was more female-biased. The study species is the most weakly eusocial hymenopteran for which relatedness estimates and sex ratio data are available. With high relatedness among nestmates and a strongly female-biased sex ratio, this study suggests the importance of indirect fitness contributions in the early stages of social evolution. Correspondence to: L. Packer     

Lack of detectable nepotism in multiple-queen colonies of the fire ant Solenopsis invicta (Hymenoptera: Formicidae)

Multiple-queen (polygyne) colonies of the introduced fire ant Solenopsis invicta present a paradox for kin selection theory. Egg-laying queens within these societies are, on average, unrelated to one another, and the numbers of queens per colony are high, so that workers appear to raise new sexuals that are no more closely related to them than are random individuals in the population. This paradox could be resolved if workers discriminate between related and unrelated nestmate sexuals in important fitness-related contexts. This study examines the possibility of such nepotism using methods that combine the following features: (1) multiple relevant behavioral assays, (2) colonies with an unmanipulated family structure, (3) multiple genetic markers with no known phenotypic effects, and (4) a statistical technique for distinguishing between nepotism and potentially confounding phenomena. We estimated relatedness between interactants in polygyne S. invicta colonies in two situations, workers tending egg-laying queens and workers feeding maturing winged queens. In neither case did we detect a significant positive value of relatedness that would implicate nepotism. We argue that the non-nepotistic strategies displayed by these ants reflect historical selection pressures experienced by native populations, in which nestmate queens are highly related to one another. The markedly different genetic structure in native populations may favor the operation of stronger higher-level selection that effectively opposes weaker individual-level selection for nepotistic interactions within nests. Received: 28 June 1996 / Accepted after revision: 6 October 1996     

Extended family structure in the ant Formica paralugubris: the role of the breeding system

In populations of various ant species, many queens reproduce in the same nest (polygyny), and colony boundaries appear to be absent with individuals able to move freely between nests (unicoloniality). Such societies depart strongly from a simple family structure and pose a potential challenge to kin selection theory, because high queen number coupled with unrestricted gene flow among nests should result in levels of relatedness among nestmates close to zero. This study investigated the breeding system and genetic structure of a highly polygynous and largely unicolonial population of the wood ant Formica paralugubris. A microsatellite analysis revealed that nestmate workers, reproductive queens and reproductive males (the queens' mates) are all equally related to each other, with relatedness estimates centring around 0.14. This suggests that most of the queens and males reproducing in the study population had mated within or close to their natal nest, and that the queens did not disperse far after mating. We developed a theoretical model to investigate how the breeding system affects the relatedness structure of polygynous colonies. By combining the model and our empirical data, it was estimated that about 99.8% of the reproducing queens and males originated from within the nest, or from a nearby nest. This high rate of local mating and the rarity of long-distance dispersal maintain significant relatedness among nestmates, and contrast with the common view that unicoloniality is coupled with unrestricted gene flow among nests. Received: 8 February 1999 / Received in revised form: 15 June 1999 / Accepted: 19 June 1999     

Comparative study of genetic and social structure in two forms of the fire ant Solenopsis invicta (Hymenoptera: Formicidae)

Summary The genetic and social structures of polygyne and monogyne forms of the fire ant, Solenopsis invicta, are investigated in a comparative manner using allozyme data from two polymorphic loci. Foundress queens of the monogyne form are signly inseminated and appear to produce all males present in the colony during the major summer mating flights. The average regression coefficient of relatedness (b) among female nestmates of the monogyne form is 0.714 (Fig. 2), statistically indistinguishable from the pedigree coefficient of relatedness (G) of 0.75. We suggest that the evolution of obligate worker sterility in Solenopsis is associated with this high relatedness between workers and the queens they rear. Functional queens in polygyne nests also are singly inseminated and are no more closely related to nestmate queens than to other queens (within-nest b=0). Within-nest relatedness of workers in the polygyne population is similarly low (Fig. 2). Both the monogyne and polygyne populations from northcentral Georgia are in Hardy-Weinberg equilibrium at both allozyme loci and we found no evidence of significant population subdividion or inbreeding in the polygyne population. These results do not support the view that kin selection has promoted the evolution of polygyny in North American S. invicta. Rather, mutualism appears to be the most likely selective factor mediating queen associations inthis ant.     

The origin of the chemical profiles of fungal symbionts and their significance for nestmate recognition in <Emphasis Type="Italic">Acromyrmex</Emphasis> leaf-cutting ants

Cuticular hydrocarbon profiles are essential for nestmate recognition in insect societies, and quantitative variation in these recognition cues is both environmentally and genetically determined. Environmental cues are normally derived from food or nest material, but an exceptional situation may exist in the fungus-growing ants where the symbiotic fungus garden may be an independent source of recognition compounds. To investigate this hypothesis, we quantified the chemical profiles of the fungal symbionts of 18 sympatric colonies of Acromyrmex echinatior and Acromyrmex octospinosus and evaluated the quantitative variation of the 47 compounds in a multivariate analysis. Colony-specific chemical profiles of fungal symbionts were highly distinct and significantly different between the two ant species. We also estimated the relative genetic distances between the fungal symbionts using amplified fragment length polymorphism (AFLP) and correlated these with the overall (Mahalanobis) chemical distances between the colony-specific profiles. Despite the standardized laboratory conditions, the correlations were generally weak, but a statistically significant portion of the total variation in chemical profiles could be explained by genetic differences between the fungal symbionts. However, there was no significant effect of ant species in partial analyses because genetic differences between symbionts tend to coincide with being reared by different ant species. However, compound groups differed significantly with amides, aldehydes, and methyl esters contributing to the correlations, but acetates, alkanes, and formates being unrelated to genetic variation among symbionts. We show experimentally that workers that are previously exposed to and fed with the fungal symbiont of another colony are met with less aggression when they are later introduced into that colony. It appears, therefore, that fungus gardens are an independent and significant source of chemical compounds, potentially contributing a richer and more abundant blend of recognition cues to the colony “gestalt” than the innate chemical profile of the ants alone. Freddie-Jeanne Richard and Michael Poulsen contributed equally to this work.     

Differential aggressiveness between fire salamander (<Emphasis Type="Italic">Salamandra infraimmaculata</Emphasis>) larvae covaries with their genetic similarity

Responding differentially to kin and non-kin is known to be adaptive in many species. One example is the inclusive fitness benefits of reducing aggression toward closer relatives. Little is known, however, about the ability of animals to assess differential degrees of genetic relatedness and to respond accordingly with differential levels of aggression. In the present study, we tested whether aggressiveness between body mass-matched pairs of fire salamander (Salamandra infraimmaculata) larvae covaried with the genetic similarity between them. We quantified aggressiveness at three levels of genetic similarity by selecting pairs within and across pools from recently genotyped populations. We also assessed aggression between pairs of siblings. Aggression and associated injuries decreased as genetic similarity increased across the groups. These findings suggest that cannibalistic salamanders can assess their degree of genetic relatedness to conspecifics and vary their behavioral responses depending on the degree of similarity between them along a genetic relatedness continuum.     

Variation in colony structure in the subterranean termite Reticulitermes flavipes

The genetic organization of colonies of the subterranean termite Reticulitermes flavipes in two subpopulations in Massachusetts was explored using five polymorphic allozymes and double-strand conformation polymorphism (DSCP) analysis of the mitochondrial control region. Empirically obtained estimates of worker relatedness and F-statistics were compared with values generated by computer simulations of breeding schemes to make inferences about colony organization. In one study site (G), worker genotypes indicated the presence of a mixture of colonies headed by monogamous outbred primary reproductives and colonies headed by inbreeding neotenic reproductives, both colony types having limited spatial ranges. A second site (S) was dominated by several large colonies with low relatedness among nestmates. Mixed DSCP haplotypes in three colonies indicated that nestmates had descended from two or three unrelated female reproductives. Computer simulations of breeding schemes suggested that positive colony inbreeding coefficients at site S resulted from either commingling of workers from different nests or different colonies. Such an exchange of workers between nests corresponds to the multiple-site nesting lifetype of many subterranean termites and resembles colony structure in polycalic Formica ants. Our study demonstrates considerable variation in R. flavipes colony structure over a small spatial scale, including colonies headed by monogamous outbred primary reproductives, colonies containing multiple inbred neotenic reproductives and large polydomous colonies containing the progeny of two or more unrelated queens, and suggests that the number of reproductives and nestmate relatedness change with colony age and size.     

Ant gardens: interspecific recognition in parabiotic ant species

 In French Guiana, parabiotic societies (natural mixed colonies) are frequently found in ant gardens. Crematogaster limata parabiotica (Myrmicinae), often associated with Camponotus femoratus (Formicinae), was found for the first time in parabiosis with ponerine ants: Pachycondyla goeldii and Odontomachus mayi. A detailed study of the relationships between Cr. l. parabiotica and O. mayi showed that each species is aggressive towards allospecific or conspecific individuals belonging to another colony, but tolerates allospecific individuals from the multi-species society. Studies of cuticular substances of the four ant species were made using gas chromatography. The results showed that each species, living alone or in parabiosis, possesses a specific chemical profile. Thus, the ants are able to recognise nestmate and non-nestmate individuals of the associated species, even though their cuticular profiles are different. The hypothesis that the nestmate allospecific profile is learned is suggested to explain this pattern of recognition. Received: 5 June 1996 / Accepted after revision: 17 October 1996     

Hierarchical perception of fertility signals and nestmate recognition cues in two dolichoderine ants

In social insects, queens are likely to “honestly” inform their nestmates of their presence and fertility status through pheromonal communication. Cuticular hydrocarbons (CHCs) have been reported to be effective nestmate discriminators and strongly suspected to act as fertility signals, at least in some species. The use of the same chemical bouquet (i.e., the CHC profile) to convey two fundamentally different information seems puzzling. However, a recent threshold model proposes a hierarchy in the discriminating processes, i.e., fertility signals can only be perceived if nestmate recognition has been reached (Le Conte and Hefetz, Annu Rev Entomol 53:523–542, 2008). Here, we developed a simple behavioral bioassay based on chemical recruitment toward a queen placed outside the nest in two dolichoderine ants (Linepithema humile and Tapinoma erraticum), which allowed us to investigate the interplay between fertility signaling and colonial recognition. Using queen corpses of various origins (nestmates or aliens) and physiological states (fertile or infertile; mated or unmated), we demonstrated that nestmate recognition cues clearly override fertility signals under our experimental conditions. Indeed, while nestmate infertile queens were largely ignored by the workers, nestmate fertile queens (mated or not) induced worker recruitment, whereas alien fertile queens did not and were aggressed by the workers.     

Co-evolution-driven cuticular hydrocarbon variation between the slave-making ant Rossomyrmex minuchae and its host Proformica longiseta (Hymenoptera: Formicidae)

Summary. Proformica longiseta exists as two populations in the Sierra Nevada Mountains in Spain, only one of which is parasitized by the slave-maker ant Rossomyrmex minuchae. To investigate the possible effect of co-evolutionary pressures on cuticular hydrocarbon (CHC) profiles (the presumed nestmate recognition cues), we performed a comparative analysis of the CHC of R. minuchae and P. longiseta colonies from both the allopatric and sympatric populations; the latter includes samples of enslaved as well as free-living workers. Discriminant analyses based on these chemical profiles showed two clear profile groups: the first comprised R. minuchae and both enslaved and free-living P. longiseta from the sympatric population; and the second the allopatric P. longiseta workers. As expected, the profiles of the two sympatric P. longiseta groups (enslaved and free-living) were distinct; but, interestingly, those of the enslaved P. longiseta and its parasite R. minuchae were also distinguishable. This indicates that despite their cohabitation each species maintains its own chemical identity. Profile similarity between the sympatric free-living P. longiseta and its parasite may explain the lower than expected aggression observed during raids. We further speculate that in view of the differences between the sympatric and allopatric population of P. longiseta, co-evolutionary pressures have driven changes in the profile of the former to better match that of its parasite R. minuchae. Such an adjustment may have enabled nests of the sympatric P. longiseta to endure multiple raids by the parasite (due to the reduced aggression) and thus to continue to reproduce despite the damage inflicted by the raids.     

An indirect test of inbreeding depression in the termites Reticulitermes flavipes and Reticulitermes virginicus

We analyzed tandem-running pairs of the termites Reticulitermes flavipes and Reticulitermes virginicus utilizing 13 and 12 microsatellite loci, respectively. Newly formed pairs in both species were significantly related to one another, but this average relatedness was considerably higher in R. flavipes (0.130 vs 0.060). These average relatedness levels resulted from some tandem pairs forming between nestmate termites: more than one quarter of all R. flavipes pairs (26.1%) met this criterion, while this was the case for only about one of every 20 R. virginicus pairs (5.1%). The likelihood that termites paired with siblings was inversely related to the inferred dispersal ability of the two species. F _ST, measured over identical spatial scales, was significantly higher in R. flavipes (0.034) than in R. virginicus (0.008). A comparison in R. flavipes of the observed proportion of nestmate pairs observed during tandem running vs the proportion found in established colonies revealed a significant excess of close relatives when pairs were first formed. There are two possible causes of this discrepancy: inbreeding depression (ID) may eliminate inbred colonies early in development, or related pairs may part late in the tandem-running phase or after it is completed. The latter explanation of inbreeding avoidance implies either historical or contemporary ID, and these results therefore suggest that, either directly or indirectly, ID could be a more potent force in the evolution of termite mating systems than is generally appreciated. This work was funded by grants from the United States Department of Agriculture National Research Initiative Competitive Grants Program (nos. 00-35302-9377 and 2002-35302-12490).