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

Parentage exclusion of close relatives in haplodiploid species

Parentage exclusion probability is usually calculated to evaluate the informativeness of a set of markers for, and the statistical power of, a parentage analysis. Equations for parentage exclusion probability have been derived in various scenarios such as paternity exclusion when maternity is known or unknown or when candidate males are unrelated or loosely related (being from the same subpopulation) to the father. All previous work assumes a diploid species. Although marker-based parentage analyses have been conducted in haploidiploid species (such as ants, bees and wasps) for diploid offspring at the individual level or haploid offspring at the class level, rigorously derived formulations of parentage exclusion probability for haploid offspring at the individual level are lacking, which prevents the precise evaluation of the informativeness for and the statistical power of a parentage analysis. In this study we derive equations for the exclusion probability of maternity of a haploid male when multiple mother candidates (workers or queens) are unrelated or fullsibs to the mother. The usefulness of the equations is exemplified by numerical examples, and the results are discussed in the context of the study of worker reproductivity in eusocial haplodiploid species. The results are especially valuable for an optimal experimental design in determining sampling intensities (e.g. number of markers and number of individuals) to achieve satisfactory statistical power of a parentage analysis in investigating workers' reproductivity in eusocial haplodiploid species.

Worker reproduction of the invasive yellow crazy ant Anoplolepis gracilipes

Background

Reproductive division of labor is one of the key features of social insects. Queens are adapted for reproduction while workers are adapted for foraging and colony maintenance. In many species, however, workers retain functional ovaries and can lay unfertilized male eggs or trophic eggs. Here we report for the first time on the occurrence of physogastric workers and apparent worker reproduction in the invasive yellow crazy ant Anoplolepis gracilipes (Fr. Smith). We further examined the reproductive potential and nutritional role of physogastric workers through multidisciplinary approaches including morphological characterization, laboratory manipulation, genetic analysis and behavioral observation.

Results

Egg production with two types of eggs, namely reproductive and trophic eggs, by physogastric workers was found. The reproductive egg was confirmed to be haploid and male-destined, suggesting that the workers produced males via arrhenotokous parthenogenesis as no spermatheca was discovered. Detailed observations suggested that larvae were mainly fed with trophic eggs. Along with consumption of trophic eggs by queens and other castes as part of their diet, the vital role of physogastric workers as “trophic specialist” is confirmed.

Conclusion

We propose that adaptive advantages derived from worker reproduction for A. gracilipes may include 1) trophic eggs provisioned by physogastric workers likely assist colonies of A. gracilipes in overcoming unfavorable conditions such as paucity of food during critical founding stage; 2) worker-produced males are fertile and thus might offer an inclusive fitness advantage for the doomed orphaned colony.

Electronic supplementary material

The online version of this article (doi:10.1186/s12983-017-0210-4) contains supplementary material, which is available to authorized users.

Queen Killing Is Linked to High Worker-Worker Relatedness in a Social Wasp

Social insect colonies are pinnacles of evolved altruism but also exhibit dramatic conflict among relatives. In many species, a colony's workers compete with the queen and each other over the production of males. Interspecific comparisons demonstrate the importance of within-colony relatedness in determining the outcome of such conflicts, but facultative responses to within-colony relatedness are rarely reported. Here, I report facultative matricide (worker killing of a colony's queen) in the social wasp Dolichovespula arenaria. Matricide is strongly associated with high worker-worker relatedness, as predicted by theory, because closely related workers value nephews more than brothers. This pattern is the result of variation in both paternity frequency and the paternity skew of colonies with multiple patrilines, implicating worker-worker relatedness rather than a direct effect of multiple mating on queen survival. Furthermore, occasional inbreeding can explain why some multiple-patriline colonies exhibit high paternity skew associated with matricide. In general, these results support the hypothesis that workers can facultatively respond to intracolony relatedness determined by queen mating behavior and demonstrate a novel benefit of polyandry in annual social insects. Facultative matricide shows dramatically how workers are evolutionary actors with interests that can diverge from the queen's, rather than being "extrasomatic projections of her personal genome".

No facultative worker policing in the honey bee (Apis mellifera L.)

Kin selection theory predicts that in colonies of social Hymenoptera with multiply mated queens, workers should mutually inhibit ("police") worker reproduction, but that in colonies with singly mated queens, workers should favor rearing workers' sons instead of queens' sons. In line with these predictions, Mattila et al. (Curr Biol 22:2027-2031, 2012) documented increased ovary development among workers in colonies of honey bees with singly mated queens, suggesting that workers can detect and respond adaptively to queen mating frequency and raising the possibility that they facultative police. In a follow-up experiment, we test and reject the hypothesis that workers in single-patriline colonies prefer worker-derived males and are able to reproduce directly; we show that their eggs are policed as strongly as those of workers in colonies with multiply mated queens. Evidently, workers do not respond facultatively to a kin structure that favors relaxed policing and increased direct reproduction. These workers may instead be responding to a poor queen or preparing for possible queen loss.

Worker reproduction in Formica ants

A potential tragedy of the commons arises in social-insect colonies where workers are fertile if egg-laying workers decrease their contribution to other tasks. We studied worker ovary development and egg laying in relation to kin structure, colony size, and the presence of a queen in nine species (11 populations) of Formica ants. Workers were highly fertile and laid eggs in the presence of a queen in five out of the seven species where egg samples were obtained. Worker fertility correlated neither with colony size nor with kin structure, which suggests that colony-level costs and efficiency of policing precede relatedness as the most important conflict determinant. We conclude that careful quantification of the costs of worker reproduction and policing is essential for inferences about the tragedy of the commons.

Worker reproduction in the ant Formica fusca

We studied the kin conflict over male parentage in the ant Formica fusca. The conflict arises because each worker and queen is most related to her own sons and is thus predicted to lay eggs. Microsatellite analysis of eggs revealed that workers laid eggs in more than half the queenright experimental nests. Nevertheless, almost exclusively diploid offspring were reared in the presence of a queen. This also occurred when worker-laid haploid male eggs were experimentally introduced in to the nests. Because our experimental setup allowed us to exclude the possibility of queen policing, we conclude that worker laid eggs are removed by other workers, either as a response to their parentage or gender. Our results suggest that worker reproduction in F. fusca is ultimately an interplay of conflicts over male parentage and sex allocation and that both worker and self policing have roles as proximate mechanisms of resolution.

Worker reproduction and policing in insect societies: an ESS analysis

Insect societies are vulnerable to exploitation by workers who reproduce selfishly rather than help to rear the queen's offspring. In most species, however, only a small proportion of the workers reproduce. Here, we develop an evolutionarily stable strategy (ESS) model to investigate factors that could explain these observed low levels of reproductive exploitation. Two key factors are identified: relatedness and policing. Relatedness affects the ESS proportion of reproductive workers because laying workers generally work less, leading to greater inclusive fitness costs when within-colony relatedness is higher. The second key factor is policing. In many species, worker-laid eggs are selectively removed or 'policed' by other workers or the queen. We show that policing not only prevents the rearing of worker-laid eggs but can also make it unprofitable for workers to lay eggs in the first place. This can explain why almost no workers reproduce in species with efficient policing, such as honeybees, Apis, and the common wasp, Vespula vulgaris, despite relatively low relatedness caused by multiple mating of the mother queen. Although our analyses focus on social insects, the conclusion that both relatedness and policing can reduce the incentive for cheating applies to other biological systems as well.

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

Social parasitism by male-producing reproductive workers in a eusocial insect

The evolution of extreme cooperation, as found in eusocial insects (those with a worker caste), is potentially undermined by selfish reproduction among group members. In some eusocial Hymenoptera (ants, bees and wasps), workers can produce male offspring from unfertilized eggs. Kin selection theory predicts levels of worker reproduction as a function of the relatedness structure of the workers' natal colony and the colony-level costs of worker reproduction. However, the theory has been only partially successful in explaining levels of worker reproduction. Here we show that workers of a eusocial bumble bee (Bombus terrestris) enter unrelated, conspecific colonies in which they then produce adult male offspring, and that such socially parasitic workers reproduce earlier and are significantly more reproductive and aggressive than resident workers that reproduce within their own colonies. Explaining levels of worker reproduction, and hence the potential of worker selfishness to undermine the evolution of cooperation, will therefore require more than simply a consideration of the kin-selected interests of resident workers. It will also require knowledge of the full set of reproductive options available to workers, including intraspecific social parasitism.

Egg viability and worker policing in honey bees

In many species of social Hymenoptera, unmated workers can lay eggs that will produce males by parthenogenesis. Nevertheless, in queenright honey bee colonies (Apis mellifera), worker reproduction is low. One possible mechanism for this difference is worker policing, the removal of worker-laid eggs by other workers. This behavior can evolve in species in which queens are multiply mated, where workers are more closely related to the sons of their mother than those of their sisters. Another possible mechanism of the low level of worker reproduction is worker-laid eggs being less viable than queen-laid eggs. We show that this difference in quality is the case for honey bees.

Multilevel selection and social evolution of insect societies

How sterile, altruistic worker castes have evolved in social insects and how they are maintained have long been central topics in evolutionary biology. With the advance of kin selection theory, insect societies, in particular those of haplodiploid bees, ants, and wasps, have become highly suitable model systems for investigating the details of social evolution and recently also how within-group conflicts are resolved. Because insect societies typically do not consist of clones, conflicts among nestmates arise, for example about the partitioning of reproduction and the allocation of resources towards male and female sexuals. Variation in relatedness among group members therefore appears to have a profound influence on the social structure of groups. However, insect societies appear to be remarkably robust against such variation: division of labor and task allocation are often organized in more or less the same way in societies with high as in those with very low nestmate relatedness. To explain the discrepancy between predictions from kin structure and empirical data, it was suggested that constraints-such as the lack of power or information-prevent individuals from pursuing their own selfish interests. Applying a multilevel selection approach shows that these constraints are in fact group-level adaptation preventing or resolving intracolonial conflict. The mechanisms of conflict resolution in insect societies are similar to those at other levels in the biological hierarchy (e.g., in the genome or multicellular organisms): alignment of interests, fair lottery, and social control. Insect societies can thus be regarded as a level of selection with novelties that provide benefits beyond the scope of a solitary life. Therefore, relatedness is less important for the maintenance of insect societies, although it played a fundamental role in their evolution.