Michael J. Holmes

Maternity of emergency queens in the Cape honey bee, Apis mellifera capensis

During reproductive swarming, some workers of the Cape honey bee, Apis mellifera capensis, lay eggs in queen cells, many of which are reared to maturity. However, it is unknown if workers are able to lay in queen cells immediately after queen loss during an episode of emergency queen rearing. In this study we experimentally de‐queened colonies and determined the maternity of larvae and pupae that were reared as queens. This allowed us to determine how soon after queen loss workers contribute to the production of new queens. We were further interested to see if workers would preferentially raise new queens from queen‐laid brood if this was introduced later. We performed our manipulations in two different settings: an apiary setting where colonies were situated close together and a more natural situation in which the colonies were well separated. This allowed us to determine how the vicinity of other colonies affects the presence of parasites. We found that workers do indeed contribute to queen cell production immediately after the loss of their queen, thus demonstrating that some workers either have activated ovaries even when their colony has a queen or are able to activate their ovaries extremely rapidly. Queen‐laid brood introduced days after queen loss was ignored, showing that workers do not prefer to raise new queens from queen brood when given a choice. We also detected non‐natal parasitism of queen cells in both settings. We therefore conclude that some A. m. capensis genotypes specialize in parasitizing queen cells.

Cheaters sometimes prosper: targeted worker reproduction in honeybee (Apis mellifera) colonies during swarming

Kin selection theory predicts that honeybee (Apis mellifera) workers should largely refrain from producing their own offspring, as the workers collectively have higher inclusive fitness if they rear the sons of their mother, the queen. Studies that have quantified levels of ovary activation and reproduction among workers have largely supported this prediction. We sampled pre‐emergent male pupae and adult workers from seven colonies at regular intervals throughout the reproductive part of the season. We show that the overall contribution of workers to male (drone) production is 4.2%, nearly 40 times higher than is generally reported, and is highest during reproductive swarming, when an average of 6.2% of the males genotyped are worker‐produced. Similarly, workers in our samples were 100 times more likely to have active ovaries than previously assumed. Worker reproduction is seasonally influenced and peaks when colonies are rearing new queens. Not all worker subfamilies contribute equally to reproduction. Instead, certain subfamilies are massively over‐represented in drone brood. By laying eggs within the period in which many colonies produce virgin queens, these rare worker subfamilies increase their direct fitness via their well‐timed sons.

Racial mixing in South African honeybees: the effects of genotype mixing on reproductive traits of workers

To test the hypothesis that the honeybee hybrid zone in South Africa is a tension zone due to increased reproductive conflict in colonies that contain both Apis mellifera capensis and Apis mellifera scutellata worker genotypes, we constructed mixed subspecies and hybrid colonies via a combination of artificial and natural matings. We measured emergence weight, ovary activation, and the presence/absence of a spermatheca on workers of different genotypes. We show that the measured characteristics were all affected by genotype with some traits also affected by the social environment in which the worker was reared. Workers with both an A. m. capensis mother and father had the highest emergence weight. When workers had an A. m. capensis mother, paternity affected emergence weight with A. m. capensis fathers producing heavier workers. When the queen was A. m. scutellata, paternity had less effect on weight. Presence of spermatheca was highest in mixed colonies irrespective of maternity and colonies containing pure A. m. capensis workers only. Paternity had a significant effect on the presence of a spermatheca within mixed colonies, with workers that had an A. m. capensis father being more likely to possess a spermatheca. Rates of ovary activation were highest in colonies with an A. m. scutellata queen mated to drones of both genotypes, suggesting that mixed subspecies colonies likely suffer increased reproductive strife among workers. Our results provide support for the hypothesis that the South African honeybee hybrid zone is a tension zone arising from reduced fitness of genetically mixed colonies.

Genetic reincarnation of workers as queens in the Eastern honeybee Apis cerana

Thelytokous parthenogenesis, or the asexual production of female offspring, is rare in the animal kingdom, but relatively common in social Hymenoptera. However, in honeybees, it is only known to be ubiquitous in one subspecies of Apis mellifera, the Cape honeybee, A. mellifera capensis. Here we report the appearance of queen cells in two colonies of the Eastern honeybee Apis cerana that no longer contained a queen or queen-produced brood to rear queens from. A combination of microsatellite genotyping and the timing of the appearance of these individuals excluded the possibility that they had been laid by the original queen. Based on the genotypes of these individuals, thelytokous production by natal workers is the most parsimonious explanation for their existence. Thus, we present the first example of thelytoky in a honeybee outside A. mellifera. We discuss the evolutionary and ecological consequences of thelytoky in A. cerana, in particular the role thelytoky may play in the recent invasions by populations of this species.

Why acquiesce? Worker reproductive parasitism in the Eastern honeybee (Apis cerana)

Most societies are vulnerable to rogue individuals that pursue their own interests at the expense of the collective entity. Societies often protect themselves from selfish behaviour by ‘policing’, thereby enforcing the interests of the collective over those of individuals. In insect societies, for example, selfish workers can activate their ovaries and lay eggs, exploiting the collective brood rearing system for individual benefit. Policing, usually in the form of oophagy of worker‐laid eggs, controls selfish behaviour. Importantly, once an effective system of policing has evolved, the incentive for personal reproduction is lost, and ‘reproductive acquiescence’ in which ovary activation is rare or absent is predicted to evolve. Studies of social Hymenoptera have largely supported the prediction of worker ‘acquiescence’; workers of most species where policing is well developed have inactive ovaries. However, the eastern honeybee Apis cerana appears to be an exception. A. cerana colonies are characterized by highly efficient policing, yet about 5% of workers have active ovaries, even when a queen is present. This suggests that the evolution of acquiescence is incomplete in A. cerana. We regularly sampled male eggs and pupae from four A. cerana colonies. Workers had high levels of ovary activation overall (11.7%), and 3.8% of assignable male eggs and 1.1% of assignable male pupae were worker‐laid. We conclude that workers with active ovaries lay their eggs, but these rarely survive to pupation because of intense policing. We then used our findings as well as previously published data on A. cerana and A. mellifera to redo the meta‐analysis on which reproductive acquiescence theory is based. Including data on both species did not affect the relationship between effectiveness of policing and levels of worker reproduction. Their inclusion did, however, seriously weaken the relationship between relatedness among workers and levels of worker reproduction. Our work thus suggests that relatedness among workers does not affect the probability that workers will attempt to reproduce, but that it is coercion by peers that limits worker reproduction.

Argentine ants (Linepithema humile) use adaptable transportation networks to track changes in resource quality

ABSTRACT Transportation networks play a crucial role in human and animal societies. For a transportation network to be efficient, it must have adequate capacity to meet traffic demand. Network design becomes increasingly difficult in situations where traffic demand can change unexpectedly. In humans, network design is often constrained by path dependency because it is difficult to move a road once it is built. A similar issue theoretically faces pheromone-trail-laying social insects; once a trail has been laid, positive feedback makes re-routing difficult because new trails cannot compete with continually reinforced pre-existing trails. In the present study, we examined the response of Argentine ant colonies and their trail networks to variable environments where resources differ in quality and change unexpectedly. We found that Argentine ant colonies effectively tracked changes in food quality such that colonies allocated the highest proportion of foragers to the most rewarding feeder. Ant colonies maximised access to high concentration feeders by building additional trails and routes connecting the nest to the feeder. Trail networks appeared to form via a pruning process in which lower traffic trails were gradually removed from the network. At the same time, we observed several instances where new trails appear to have been built to accommodate a surge in demand. The combination of trail building when traffic demand is high and trail pruning when traffic demand is low results in a demand-driven network formation system that allows ants to monopolise multiple dynamic resources. Highlighted Article: Ants track changes in food quality by building flexible transportation networks.

Honeybee (Apis cerana) guards do not discriminate between robbers and reproductive parasites

A hopelessly queenless honeybee colony has only one reproductive option: some workers must produce sons before the colony dies. This requires the workers to curtail egg policing (removal of worker-produced eggs), rendering the colony vulnerable to non-natal reproductive parasitism. In the Western honeybee, Apis mellifera, guarding (prevention of foreign workers from entering a colony) increases in queenless colonies, providing a defence against non-natal parasitism. However, in the closely related Eastern honeybee A. cerana, queenless colonies appear to be more tolerant of bees from other colonies. We presented guards of four A. cerana colonies with three types of workers: nestmate returning foragers, non-nestmate returning foragers and non-nestmates from a laying-worker colony. The latter are likely to have active ovaries, allowing us to test whether guard bees can detect which potential invaders are more likely to be reproductive parasites. After assessing guards’ reactions, we recaptured test bees and dissected them to determine levels of ovary activation. We found that nestmates were accepted significantly more frequently than the other two types of workers. However, there was no difference in the overall acceptance rates of non-nestmate returning foragers and bees from within laying-worker colonies. In addition, ovary-activated workers were no less likely to be accepted than those with inactive ovaries. Interestingly, colonies were more accepting of all three types of test bee after being made queenless. We conclude that, as has been previously suggested, guarding has no specific role in the prevention of non-natal parasitism in A. cerana.

Sperm utilization in honeybees (Apis mellifera scutellata and A. m. capensis) in South Africa

We artificially inseminated queens of Apis mellifera scutellata and A. m. capensis with equal numbers of drones of both subspecies to determine the effects of sperm genotype on rates of sperm utilization. Contrary to a previous study we did not find a consistent overrepresentation of workers sired by A. m. scutellata males in the first four months after insemination. Interestingly, our study does suggest that there is a significant interaction between drone and queen genotype in both subspecies, with queens of each subspecies producing more workers sired by drones of the same subspecies.

The convergent evolution of caste in ants and honey bees is based on a shared core of ancient reproductive genes and many plastic genes

Eusociality has convergently evolved multiple times, but the genomic basis of caste-based division of labor and degree to which independent origins of eusociality have utilized common genes remains largely unknown. We characterized caste-specific transcriptomic profiles across development and adult body segments from pharaoh ants (Monomorium pharaonis) and honey bees (Apis mellifera), representing two independent origins of eusociality. We identified a substantial shared core of genes upregulated in the abdomens of queen ants and honey bees that also tends to be upregulated in mated female flies, suggesting that these genes are part of a conserved insect reproductive groundplan. Outside of this shared groundplan, few genes are differentially expressed in common. Instead, the majority of the thousands of caste-associated genes are plastically-expressed, rapidly evolving, and relatively evolutionarily young. These results emphasize that the recruitment of both highly conserved and lineage-specific genes underlie the convergent evolution of novel traits such as eusociality.Eusociality, characterized by caste-based division of labor, has convergently evolved multiple times. However, the genomic basis of caste and degree to which independent origins of eusociality have utilized common genes is largely unknown. To elucidate these issues, we characterized caste-specific transcriptomic profiles across development and adult body segments from honey bees (Apis mellifera) and pharaoh ants (Monomorium pharaonis), representing two independent origins of eusociality. We identified a shared core of genes upregulated in the abdomens of queen honey bees and ants that is also upregulated in female flies. Outside of this shared core, few genes are differentially expressed in common. Instead, the majority of genes underlying the caste system show plastic expression, are rapidly evolving, and are relatively evolutionary young. Altogether our results show that the convergent evolution of eusociality involves the recruitment of a core reproductive groundplan along with many plastically-expressed and rapidly evolving genes.

When does cheating pay? Worker reproductive parasitism in honeybees

The notion that honeybee colonies are harmonious, isolated societies in which workers selflessly sacrifice their reproductive opportunities to serve their queen has long been debunked. Like any society, honeybee colonies contain selfish individuals that pursue their own interests, or cheat, at the expense of the colony, and these individuals need to be controlled. There are numerous studies detailing the myriad ways in which selfish workers may subvert the typical dominance hierarchy of a haplo-diploid insect society. Recent studies have focussed on thelytoky, the ability to produce diploid offspring without mating, as the most significant attribute of a successful reproductive parasite. However, we argue that thelytoky is not necessary for successful parasitism, and that even arrhenotokous societies contain specialized reproductive parasites. Using A. mellifera and A. cerana as examples, we show that the most important aspects of a would-be reproductive cheat’s success are an ability to escape policing and the timing of the reproductive attempt. Finally, we show that thelytoky, while not necessary for the evolution of successful reproductive parasites, can give rise to specialized parasitic lineages and that such lineages are likely to be far more common than previously assumed.