CONFLICTS OF INTEREST IN SOCIAL INSECTS: MALE PRODUCTION IN TWO SPECIES OF POLISTES

Polistes dominulus (Hymenoptera, Vespidae) larvae show different cuticular patterns according to their sex: workers seem not use this chemical information

During reproductive phase, larvae of male and female are intermingled in nest of social wasps. Workers care for and feed larvae that gives them an opportunity to bias investment with respect to sex, or even to kill some larvae, if they can distinguish between immature males and females. Cuticular hydrocarbon (CHC) mixtures are the most studied cues for species, nestmate, and caste recognition in social Hymenoptera. In this study, we investigate the paper wasp Polistes dominulus to see if male and female larvae show different patterns of CHCs and if workers are able to discriminate between male and female larvae on this basis. We performed gas chromatography-mass spectrometry analysis on cuticular extracts of larvae, and then we genotyped them to assign sex. We found sex-based variation in CHC-profiles sufficient for discrimination. However, our behavioral assays do not support the view that adults discriminate between male and female larvae within nests.

Male parentage does not vary with colony kin structure in a multiple‐queen ant

Kin selection theory predicts that, in social Hymenoptera, the parentage of males should be determined by within-colony relatedness. We present a model showing that, when sex ratios are split (bimodal) as a function of colony kin structure, the predictions of kin selection theory regarding the occurrence of worker reproduction and policing (prevention of worker reproduction) require modification. To test the predictions of kin selection theory and our model, we estimated using microsatellites the frequency of worker-produced male eggs and adults in the facultatively polygynous (multiple-queen) ant Leptothorax acervorum. Analysis of 210 male eggs and 328 adult males from 13 monogynous (single-queen) and nine polygynous colonies demonstrated that the frequency of worker-produced males was low (2.3-4.6% of all males) and did not differ significantly between colony classes or between eggs and adults. This suggested workers' self-restraint as the cause of infrequent worker reproduction in both colony classes. Such an outcome is not predicted either by comparing relatedness values or by our model. Therefore, it appears that factors other than colony kin structure and sex ratio effects determine the pattern of male parentage in the study population. A likely factor is a colony-level cost of worker reproduction.

Male parentage in dependent-lineage populations of the harvester ant Pogonomyrmex barbatus

We investigated the extent to which workers reproduce in a dependent-lineage population of the monogynous harvester ant Pogonomyrmex barbatus. Dependent-lineage populations contain two interbreeding, yet genetically distinct mitochondrial lineages, each associated with specific alleles at nuclear loci. Workers develop from matings between lineages, and queens develop from matings within lineages, so queens must mate with males of both lineages to produce daughter queens and workers. Males develop from unfertilized eggs and are haploid. Worker production of males could lead to male-mediated gene flow between the lineages if worker-produced males were reproductively capable. This could result in the loss of the dependent-lineage system, because its persistence depends on the maintenance of allelic differences between the lineages. To investigate the extent of worker reproduction in P. barbatus, we genotyped 19-20 males and workers from seven colonies, at seven microsatellite loci, and 1239 additional males at two microsatellite loci. Our methods were powerful enough to detect worker reproduction if workers produced more than 0.39% of males in the population. We detected no worker-produced males; all males appeared to be produced by queens. Thus, worker reproduction is sufficiently infrequent to have little impact on the dependent-lineage system. These results are consistent with predictions based on inclusive fitness theory because the effective queen mating frequency calculated from worker genotypes was 4.26, which is sufficiently high for workers to police those that attempt to reproduce.

Direct assessment of queen quality and lack of worker suppression in a paper wasp

Assessing a conspecific's potential is often crucial to increase one's fitness, e.g. in female choice, contests with rivals or reproductive conflicts in animal societies. In the latter, helpers benefit from accurately assessing the fertility of the breeder as an indication of inclusive fitness. There is evidence that this can be achieved using chemical correlates of reproductive activity. Here, we show that queen quality can be assessed by directly monitoring her reproductive output. In the paper wasp Polistes dominulus, we mimicked a decrease in queen fertility by regularly removing brood. This triggered ovarian development and egg-laying by many workers, which strongly suggests that brood abundance is a reliable cue of queen quality. Brood abundance can be monitored when workers perform regular brood care in small size societies where each brood element is kept in a separate cell. Our results also show that although the queen was not manipulated, and thus remained healthy and fully fertile, she did not control worker egg-laying. Nevertheless, when workers laid eggs, the queen secured a near reproductive monopoly by selectively destroying these eggs, a mechanism known as 'queen policing'. By contrast, workers destroyed comparatively few queen-laid eggs, but did destroy each other's eggs.

Mating frequency, within-colony relatedness and male production in a yellow jacket wasp, Dolichovespula arenaria

We examined the mating frequencies of queens in a social wasp, Dolichovespula arenaria (Vespinae) using DNA microsatellites. Five of the seven colonies supported the hypothesis of single mating by queens. The other two colonies supported two and three matings, with effective paternity of 1.48 and 1.91. Mean worker relatedness was 0.77 +/- 0.09. In two of the four male-containing colonies, all were likely progeny of the queen. In the other two colonies workers produced 8 and 14% of the male wasps. Overall, 94.3% of the male wasps were likely progeny of the queen. These patterns are consistent with published studies of vespine wasps.

Conflict over male parentage in social insects

Mutual policing is an important mechanism that maintains social harmony in group-living organisms by suppressing the selfish behavior of individuals. In social insects, workers police one another (worker-policing) by preventing individual workers from laying eggs that would otherwise develop into males. Within the framework of Hamilton's rule there are two explanations for worker-policing behavior. First, if worker reproduction is cost-free, worker-policing should occur only where workers are more closely related to queen- than to worker-produced male eggs (relatedness hypothesis). Second, if there are substantial costs to unchecked worker reproduction, worker-policing may occur to counteract these costs and increase colony efficiency (efficiency hypothesis). The first explanation predicts that patterns of the parentage of males (male parentage) are associated with relatedness, whereas the latter does not. We have investigated how male parentage varies with colony kin structure and colony size in 50 species of ants, bees, and wasps in a phylogenetically controlled comparative analysis. Our survey revealed that queens produced the majority of males in most of the species and that workers produced more than half of the males in less than 10% of species. Moreover, we show that male parentage does not vary with relatedness as predicted by the relatedness hypothesis. This indicates that intra- and interspecific variation in male parentage cannot be accounted for by the relatedness hypothesis alone and that increased colony efficiency is an important factor responsible for the evolution of worker-policing. Our study reveals greater harmony and more complex regulation of reproduction in social insect colonies than that expected from simple theoretical expectations based on relatedness only.

Workers of social insects prevent other workers laying eggs to increase colony efficiency and not -- as traditionally thought - purely because workers are more related to the queen of the colony

The phylogeny of the social wasp subfamily Polistinae: evidence from microsatellite flanking sequences, mitochondrial COI sequence, and morphological characters

BACKGROUND

Social wasps in the subfamily Polistinae (Hymenoptera: Vespidae) have been important in studies of the evolution of sociality, kin selection, and within colony conflicts of interest. These studies have generally been conducted within species, because a resolved phylogeny among species is lacking. We used nuclear DNA microsatellite flanking sequences, mitochondrial COI sequence, and morphological characters to generate a phylogeny for the Polistinae (Hymenoptera) using 69 species.

RESULTS

Our phylogeny is largely concordant with previous phylogenies at higher levels, and is more resolved at the species level. Our results support the monophyly of the New World subgenera of Polistini, while the Old World subgenera are a paraphyletic group. All genera for which we had more than one exemplar were supported as monophyletic except Polybia which is not resolved, and may be paraphyletic.

CONCLUSION

The combination of DNA sequences from flanks of microsatellite repeats with mtCOI sequences and morphological characters proved to be useful characters establishing relationships among the different subgenera and species of the Polistini. This is the first detailed hypothesis for the species of this important group.

Fluorescence in situ hybridization: a new method for determining primary sex ratio in ants

The haplodiploid sex determining system in Hymenoptera, whereby males develop from haploid eggs and females from diploid eggs, allows females to control the primary sex ratio (the proportion of each sex at oviposition) in response to ecological and/or genetic conditions. Surprisingly, primary sex ratio adjustment by queens in eusocial Hymenoptera has been poorly studied, because of difficulties in sexing the eggs laid. Here, we show that fluorescence in situ hybridization (FISH) can be used to accurately determine the sex (haploid or diploid) of eggs, and hence the primary sex ratio, in ants. We first isolated the homologue coding sequences of the abdominal-A gene from 10 species of 8 subfamilies of Formicidae. Our data show that the nucleotide sequence of this gene is highly conserved among the different subfamilies. Second, we used a sequence of 4.5 kbp from this gene as a DNA probe for primary sex ratio determination by FISH. Our results show that this DNA probe hybridizes successfully with its complementary DNA sequence in all ant species tested, and allows reliable determination of the sex of eggs. Our proposed method should greatly facilitate empirical tests of primary sex ratio in ants.

Male production in stingless bees: variable outcomes of queen-worker conflict

The genetic structure of social insect colonies is predicted to affect the balance between cooperation and conflict. Stingless bees are of special interest in this respect because they are singly mated relatives of the multiply mated honeybees. Multiple mating is predicted to lead to workers policing each others' male production with the result that virtually all males are produced by the queen, and this prediction is borne out in honey bees. Single mating by the queen, as in stingless bees, causes workers to be more related to each others' sons than to the queen's sons, so they should not police each other. We used microsatellite markers to confirm single mating in eight species of stingless bees and then tested the prediction that workers would produce males. Using a likelihood method, we found some worker male production in six of the eight species, although queens produced some males in all of them. Thus the predicted contrast with honeybees is observed, but not perfectly, perhaps because workers either lack complete control or because of costs of conflict. The data are consistent with the view that there is ongoing conflict over male production. Our method of estimating worker male production appears to be more accurate than exclusion, which sometimes underestimates the proportion of males that are worker produced.

Paternity frequency and maternity of males in some stingless bee species

In monogynous hymenopteran societies, the number of mates of a queen strongly influences the potential for conflict between workers and queens over the maternity of males. Queens always 'prefer' their own sons to sons of workers, regardless of queen mating frequency. When a queen mates once, workers are more closely related to, and therefore are expected to prefer, their own sons and then sons of sisters to sons of the queen. However, if effective paternity frequency exceeds 2, workers on average should prefer queen-produced males to males produced by their sisters. We studied the queen mating frequency of seven stingless bee species: the Mexican species Scaptotrigona mexicana, S. pectoralis and the Australian species Austroplebeia symei, Trigona clypearis, T. hockingsi, T. mellipes and T. sapiens. We then determined whether males arise from eggs laid by workers or queens in A. symei, T. clypearis, T. hockingsi and T. mellipes. We show that all seven species investigated are most likely singly mated and that the queen dominates reproduction. This indicates that the queen's mating frequency alone does not determine whether workers or the queen produces the males.

High reproductive skew in tropical hover wasps

A plethora of recent models examines how genetic and environmental factors might influence partitioning of reproduction ('skew') in animal societies, but empirical data are sparse. We used three microsatellite loci to estimate skew on 13 nests of the Malaysian hover wasp, Liostenogaster flavolineata. Groups are small in L. flavolineata (1-10 females) and all females are capable of mating and laying eggs. Despite considerable variation between nests in parameters expected to influence skew, skew was uniformly high. On 11 of the 13 nests, all female eggs had been laid by a single dominant female. A second female had laid one to two out of 5-10 eggs respectively on the two remaining nests. A likelihood analysis suggested that on average, 90% of the male eggs had also been laid by the dominant. The slightly lower skew among male eggs might reflect the lower average relatedness of subordinates to male versus female offspring of the dominant. We suggest that high skew in L. flavolineata may result from strong ecological constraints and a relatively high probability that a subordinate will eventually inherit the dominant, egg-laying position.

Conflicts and alliances in insect families

Hamilton's principle of inclusive fitness implies that reproductive altruism can evolve, because individuals can pass on genes not only through their own offspring, but also through the offspring of their relatives. Social insects are spectacular examples of how some individuals may be selected to forgo reproduction and instead help others reproduce. Social Hymenoptera are also special because relatedness patterns within families can be asymmetrical, so that optimal sex-ratios, preferred male parentage or preferred mating frequencies become objects of reproductive conflict. The now extensive inclusive fitness theory provides precise qualitative predictions with respect to the emergence of such conflicts. Recent advances in the power of genetic markers applied to resolve family structure in insect societies have brought about a series of studies that have tested these predictions. In support of kin selection as a major evolutionary force, the results suggest that workers frequently control sex allocation. However, the very establishment of such worker control has made new conflicts come to light, between mothers and fathers and between adult individuals and brood. Evidence for these conflicts is only just beginning to be gathered. Recent studies tend to include issues such as 'information' and 'power' (i.e. the ability to perceive signals and the opportunity to act upon this information), and to address selection for selfishness at the individual level with costs of social disruption at the colony level.

Molecular ecology of social behaviour: analyses of breeding systems and genetic structure

Molecular genetic studies of group kin composition and local genetic structure in social organisms are becoming increasingly common. A conceptual and mathematical framework that links attributes of the breeding system to group composition and genetic structure is presented here, and recent empirical studies are reviewed in the context of this framework. Breeding system properties, including the number of breeders in a social group, their genetic relatedness, and skew in their parentage, determine group composition and the distribution of genetic variation within and between social units. This group genetic structure in turn influences the opportunities for conflict and cooperation to evolve within groups and for selection to occur among groups or clusters of groups. Thus, molecular studies of social groups provide the starting point for analyses of the selective forces involved in social evolution, as well as for analyses of other fundamental evolutionary problems related to sex allocation, reproductive skew, life history evolution, and the nature of selection in hierarchically structured populations. The framework presented here provides a standard system for interpreting and integrating genetic and natural history data from social organisms for application to a broad range of evolutionary questions.

Relatedness and the fraternal major transitions

Many of the major transitions in evolution involved the coalescence of independent lower-level units into a higher organismal level. This paper examines the role of kinship, focusing on the transitions to multicellularity in animals and to coloniality in insects. In both, kin selection based on high relatedness permitted cooperation and a reproductive division of labour. The higher relatedness of haplodiploid females to their sisters than to their offspring might not have been crucial in the origin of insect societies, and the transition to multicellularity shows that such special relationships are not required. When multicellular forms develop from a single cell, selfish conflict is minimal because each selfish mutant obtains only one generation of within-individual advantage in a chimaera. Conditionally expressed traits are particularly immune to within-individual selfishness because such mutations are rarely expressed in chimaeras. Such conditionally expressed altruism genes lead easily to the evolution of the soma, and the germ line might simply be what is left over. In most social insects, differences in relatedness ensure that there will be potential conflicts. Power asymmetries sometimes lead to such decisive settlements of conflicts that social insect colonies can be considered to be fully organismal.

Do hornets have zombie workers?

Colonies of the European hornet, Vespa crabro, are typically founded by a single queen mated to a single male. From the resulting colony relatedness pattern we predicted strong worker-queen conflict over male production where both the workers and the queen attempt to produce the colony's males. To test for this conflict, male production was studied in 15 hornet nests using a combination of DNA microsatellite analysis (282 males), worker ovary dissections (500 workers from eight nests) and 50 h of observation (four nests). In contrast to our prediction, the data show that hornet males are queens' sons, that workers never attempt to lay eggs, rarely have activated ovaries, and that there is no direct aggression between the queen and the workers. This contrasts with other data for vespine wasps, which support relatedness predictions. Dolichovespula arenaria has the same kin structure as V. crabro and workers produce males in many colonies. The similarity between these two species makes it difficult to explain why workers do not reproduce in V. crabro. Self-restraint is expected if worker reproduction significantly reduces colony productivity but there is no obvious reason why this should be important to V. crabro but not to D. arenaria. Alternatively, queen control may be important. The absence of expressed queen-worker conflict rules out physical control. Indirect pheromonal control is a possibility and is supported by the occurrence of royal courts and queen pheromone in Vespa but not Dolichovespula. Pheromonal queen control is considered evolutionarily unstable, but could result from a queen-worker arms race over reproductive control in which the queen is ahead. The genetic data also revealed diploid males in one colony, the first example in the vespine wasps, and two colonies with double matrilines, suggesting that occasional usurpation by spring queens occurs.

The current state of insect molecular systematics: a thriving Tower of Babel

Insect molecular systematics has undergone remarkable recent growth. Advances in methods of data generation and analysis have led to the accumulation of large amounts of DNA sequence data from most major insect groups. In addition to reviewing theoretical and methodological advances, we have compiled information on the taxa and regions sequenced from all available phylogenetic studies of insects. It is evident that investigators have not usually coordinated their efforts. The genes and regions that have been sequenced differ substantially among studies and the whole of our efforts is thus little greater than the sum of its parts. The cytochrome oxidase I, 16S, 18S, and elongation factor-1 alpha genes have been widely used and are informative across a broad range of divergences in insects. We advocate their use as standards for insect phylogenetics. Insect molecular systematics has complemented and enhanced the value of morphological and ecological data, making substantial contributions to evolutionary biology in the process. A more coordinated approach focused on gathering homologous sequence data will greatly facilitate such efforts.