Evolution of wingless reproductives in ants: weakly specialized ergatoid queen instead of gamergates in Platythyrea conradti

Platythyrea conradti is the only species in this genus with ergatoid (= permanently wingless) queens. Colonies lack gamergates (unlike other species in this genus), yet aggressive interactions among queen and workers define a hierarchy. A single fertile queen has the top rank and highranking workers do not lay eggs, except when the queen dies. Colonial reproduction by both alate queens (independent foundation) and gamergates (fission) seems the ancestral state in Platythyrea. Independent foundation can be selected against in some species, causing the loss of alate queens for economic reasons. Thus gamergates become the only reproductives, except in P. conradti in which queens became ergatoid. Gamergates and ergatoid queens are two mutually redundant reproductive phenotypes that allow colonial reproduction by fission. Received 1 July 2005; revised 10 November 2005; accepted 16 November 2005.     

Absence of individual sterility in thelytokous colonies of the ant <Emphasis Type="Italic">Cerapachys biroi</Emphasis> Forel (Formicidae,Cerapachyinae)

Summary The reproductive partitioning generates a persistent conflict within insects societies and a sustained theoretical and empirical attention is devoted to understand its resolution. In that context, thelytokous parthenogenesis by workers is an intriguing phenomenon where each individual is virtually reproducing. This reproductive strategy, scarce among Formicidae, was studied in C. biroi, an obligatory thelytokous cerapachyine ant. Particularly, we searched for a reproductive division of labour in colonies assumed to be clonal. The results revealed that no sterile caste was present in the colonies. However, reproduction was linked both to a temporal polyethism, in which older workers ceased to lay as they became foragers, and to a morphological polyethism, illustrated by two morphological types of individuals displaying different task allocations and ovary capacities. Evolutionary implication of this uncommon social structure, seemingly free of traditional conflict and characterised by a reproduction evenly distributed among nestmates, is discussed from comparisons to other ant species with extreme kin structure.Received 18 June 2003; revised 6 August 2003; accepted 7 August 2003.     

Shift in the behaviours regulating monogyny is associated with highgenetic differentiation in the queenless ant Diacamma ceylonense

Summary In the queenless ant genus Diacamma, one mated worker (called gamergate) maintains reproductive mono-poly in a colony by mutilating newly emerged workers. However, in several populations from south India, referred to as nilgiri, gamergates do not mutilate their nestmates but monopolize reproduction using dominance interactions . Various lines of evidence indicate that nilgiri populations are closely related to the neighboring species D. ceylonense. To determine whether this important behavioural difference between nilgiri and D. ceylonense is associated with signi-ficant genetic differentiation, we have used microsatellite and mitochondrial markers to examine genetic variation within and between nilgiri and D. ceylonense. We found a very high genetic differentiation between the two forms, which suggests a lack of gene flow. There was an unexpected pattern of mitochondrial variation, because all nilgiri populations show identical or very closely related COII sequences except one population with a very different haplotype. This divergent haplotype is genetically much more distant from the other nilgiri haplotypes than are D. ceylonense haplotypes. This pattern is not observed at the nuclear level, which suggests that introgression of mitochondrial DNA probably occurred in some nilgiri populations.     

Ovarian activity correlates with extreme changes in cuticular hydrocarbon profile in the highly polygynous ant, Linepithema humile

In social insects, cuticular hydrocarbons of adults may vary with ovarian activity. Such variations are suggested to function in the regulation of reproduction within colonies. The modification of the CH profile with ovarian activity is usually interpreted as a signal of fertility causing workers to refrain from reproducing in the presence of the queen. We examined the effect of ovarian activity on the CH profiles in the Argentine ant Linepithema humile, a species where workers lack ovaries and are completely sterile. Our data show considerable differences in the CH profiles between fertile and infertile individuals. These differences are mainly qualitative. The CH profile strongly changes at the start of egg laying, both in mated and unmated queens. These results show that variation in cuticular hydrocarbons with ovarian activity in ants is not restricted to species with worker reproduction. We propose that in the Argentine ant, the cuticular hydrocarbons of laying queens correspond to a signal of fertility involved in the regulation of various aspects of reproduction, such as the rearing of new sexuals.     

Flexible task allocation and the organization of work in ants

Flexibility in task performance is essential for a robust system of division of labour. We investigated what factors determine which social insect workers respond to colony-level changes in task demand. We used radio-frequency identification technology to compare the roles of corpulence, age, spatial location and previous activity (intra-nest/extra-nest) in determining whether worker ants (Temnothorax albipennis) respond to an increase in demand for foraging or brood care. The less corpulent ants took on the extra foraging, irrespective of their age, previous activity or location in the nest, supporting a physiological threshold model. We found no relationship between ants that tended the extra brood and corpulence, age, spatial location or previous activity, but ants that transported the extra brood to the main brood pile were less corpulent and had high previous intra-nest activity. This supports spatial task-encounter and physiological threshold models for brood transport. Our data suggest a flexible task-allocation system allowing the colony to respond rapidly to changing needs, using a simple task-encounter system for generalized tasks, combined with physiologically based response thresholds for more specialized tasks. This could provide a social insect colony with a robust division of labour, flexibly allocating the workforce in response to current needs.     

Genome-wide analysis of alternative reproductive phenotypes in honeybee workers

A defining feature of social insects is the reproductive division of labour, in which workers usually forego all reproduction to help their mother queen to reproduce. However, little is known about the molecular basis of this spectacular form of altruism. Here, we compared gene expression patterns between nonreproductive, altruistic workers and reproductive, non-altruistic workers in queenless honeybee colonies using a whole-genome microarray analysis. Our results demonstrate massive differences in gene expression patterns between these two sets of workers, with a total of 1292 genes being differentially expressed. In nonreproductive workers, genes associated with energy metabolism and respiration, flight and foraging behaviour, detection of visible light, flight and heart muscle contraction and synaptic transmission were overexpressed relative to reproductive workers. This implies they probably had a higher whole-body energy metabolism and activity rate and were most likely actively foraging, whereas same-aged reproductive workers were not. This pattern is predicted from evolutionary theory, given that reproductive workers should be less willing to compromise their reproductive futures by carrying out high-risk tasks such as foraging or other energetically expensive tasks. By contrast, reproductive workers mainly overexpressed oogenesis-related genes compared to nonreproductive ones. With respect to key switches for ovary activation, several genes involved in steroid biosynthesis were upregulated in reproductive workers, as well as genes known to respond to queen and brood pheromones, genes involved in TOR and insulin signalling pathways and genes located within quantitative trait loci associated with reproductive capacity in honeybees. Overall, our results provide unique insight into the molecular mechanisms underlying alternative reproductive phenotypes in honeybee workers.     

Specialization in policing behaviour among workers in the ant Pachycondyla inversa

Most animal societies are non-clonal and thus subject to conflicts. In social insects, conflict over male production can be resolved by worker policing, i.e. eating of worker-laid eggs (WLE) or aggression towards reproductive workers. All workers in a colony have an interest in policing behaviour being expressed, but there can be asymmetries among workers in performing the actual behaviour. Here, we show that workers of the ant Pachycondyla inversa specialize in policing behaviour. In two types of behavioural assays, workers developed their ovaries and laid eggs. In the first experiment, reproductive workers were introduced into queenright colonies. In the second experiment, WLE were introduced. By observing which individuals policed, we found that aggressive policing was highly skewed among workers that had opportunity to police, and that a similar tendency occurred in egg policing. None of the policing workers had active ovaries, so that policing did not incur a direct selfish benefit to the policer. This suggests that policing is subject to polyethism, just like other tasks in the colony. We discuss several hypotheses on the possible causes of this skew in policing tasks. This is the first non-primate example of specialization in policing tasks without direct selfish interests.     

Alternative splicing associated with phenotypic plasticity in the bumble bee Bombus terrestris

Phenotypic plasticity is when one genome can produce more than one phenotype. The caste system found in many social insects is an important example of plasticity. Several studies have examined gene expression in social insect developmental and caste differences. Changes in gene expression, however, are not the only source of phenotypic plasticity. Here, we investigate the role of alternative splicing in the buff‐tailed bumble bee Bombus terrestris. We found that 5,458 genes in B. terrestris (40%) express more than one isoform. Larvae have the lowest level of splicing events, followed by adults and then pupae. We found that when an isoform is expressed in a given caste in the larval stage, it tends to be expressed in all castes at the larval stage. The same is true at the pupal stage. However, we see more complicated interactions between the adult castes with reproductive females showing different isoform expression compared to nonreproductive females and male adults showing the most distinct patterns. We found 455 isoform switching genes, that is genes, where one developmental stage, sex or caste uses a specific isoform and another type uses a different isoform. Among genes displaying isoform switching are some involved in the ecdysteriod pathway, an important system in insect behaviour.     

Use of artificial arenas to predict the social organisation of halictine bees: Data for fourteen species from Chile

I studied the behavioural profiles of fourteen species of Chilean halictine bee using the circle tube apparatus. Interactions were classified as aggressive, avoidance or cooperative. One species, Corynura chloris, is believed to be semisocial and its behaviour was largely aggressive, Ruizantheda mutabilis is known to be communal and its behaviour was predominantly cooperative whereas Penapis toroi is a member of a subfamily from which only solitary behaviour is known and it primarily exhibited avoidance behaviours. As a result, I suggest that the relative frequency of these different behaviours may be useful in establishing the social organisation of species. Based upon comparisons of behavioural profiles, I predict the social organizations of the remaining species and suggest that at least some of the bees in the populations of Ruizantheda proxima, Caenohalictus dolator, Ca. species A and Pseudagapostemon pississi investigated exhibit communal behaviour; those of Co. patagonica and Co. herbsti likely have a reproductive division of labour, probably within semisocial societies, and that the populations of Co. corinogaster, Co. melanocladius, Ca. pygosinuatum, Ca. cuprellus and Lasioglossum aricense studied are probably solitary. Phylogenies suggest that solitary behaviour in Ca. pygosinuatum and Ca. cuprellus is reversed from communal behaviour and solitary behaviour in L. aricense may be a reversal from eusociality. The response “back and follow” is not indicative of dominance or cooperation as it correlated only with the frequency of avoidance interactions among the taxa studied. I discuss the utility of the circle tube apparatus in the identification of populations worthy of more detailed sociobiological investigation. Received 10 August 2005; revised 8 March 2006; accepted 21 March 2006.     

Production of trophic eggs by virgin workers in the ponerine ant Gnamptogenys menadensis

Abstract. Most colonies (thirty-five out of thirty-seven) of the ant Gnamptogenys menadensis (Mayr) (Hymenoptera: Formicidae: Ponerinae) lack queens. Mated workers produce reproductive eggs, whereas virgin workers can lay only smaller trophic eggs (350 ovipositions observed). These two egg types are morphologically distinct (e.g. in the pattern of oogenesis and ultrastructure of membranes and micropyle) and relate to different ovarian characteristics (ovariole length, number of yolky oocytes and yellow bodies). When reproductives are removed, a small number of virgin workers switch to producing reproductive eggs, although only 3% of these develop into larvae. Once workers are mated, up to 50% of their eggs develop further. Trophic eggs are generally absent in social insects lacking physical castes, and we review adaptive explanations of its occurrence in G. menadensis .     

Sex ratio and maternal investment in the multivoltine large carpenter bee Xylocopa sulcatipes (Apoideh: Anthophoridae)

Abstract. 1. Females of the multivoltine carpenter bee Xylocopa sulcutipes (Maa) (Hymenoptera: Anthophoridae) usually excavate a straight tunnel in dead twigs and mass provision a linear array of up to ten brood cells with pollen and nectar. An egg is deposited upon each food mass within one cell.
2. Female offspring generally receive a higher provisioning mass (0.180 ± 0.048 g) than males, a significant difference ( P > 0.001). There are, however, male larvae that receive as much food or more as their sisters or female larvae reared in another nest.
3. There is a close positive association between the size of a mother and the weight of provisions for individual daughters, but not for sons.
4. Female offspring are positioned in the innermost brood cells (Gositions 1, 2 and 3). The sex ratio of the outer cells is either significantly male biased (positions 4–6) or skewed towards males (positions 8 and 9). Positions 7 and 10 are in equilibrium.
5. Solitary females produce a significantly female biased sex ratio ( P < 0.01). Sex ratio in social nests is skewed toward females, but not significantly so ( P < 0.2). There is no significant difference between the sex ratio of solitary and social nests ( P = 0.361). The population sex ratio (pooled sex ratio of all broods produced) is significantly female biased ( P = 0.003).
6. Females kept in the laboratory produced female biased sex ratios whilst unmated females produced all-male broods indicating that insemination and ovarian development are not causally related.
7. The expected sex ratio (ESR) under equal investment, calculated as 1/CR (CR = mean male provision weight/mean female provision weight), is 137.5:117.5 (males:females), and differs significantly from that observed, 104:151 (males:females) ( P < 0.001). The 'Local Resource Enhlancement' hypothesis best explains the female biased sex ratio found in X.sulcatipes and its maintenance in the population.     

Host propagation permits extreme local adaptation in a social parasite of ants

The Red Data Book hoverfly species Microdon mutabilis is an extreme specialist that parasitises ant societies. The flies are locally adapted to a single host, Formica lemani , more intimately than was thought possible in host–parasite systems. Microdon egg survival plummeted in F. lemani colonies > 3 km away from the natal nest, from c. 96% to 0% to < 50%, depending on the hoverfly population. This is reflected in the life-time dispersal of females, measured at < 2 m, resulting in oviposition back into the same ant nests for generation after generation. To counter destabilizing effects on the host, Microdon manipulates the social dynamics of F. lemani by feeding selectively on ant eggs and small larvae, which causes surviving larvae to switch development into queens. Infested colonies rear double the number of new queens, thus propagating the vulnerable local genotype and compensating for damage to the host colonies. The consequences of such extreme host specificity for insect conservation are discussed.     

Differential gene expression between adult queens and workers in the ant Lasius niger

Ants and other social insects forming large societies are generally characterized by marked reproductive division of labour. Queens largely monopolize reproduction whereas workers have little reproductive potential. In addition, some social insect species show tremendous lifespan differences between the queen and worker caste. Remarkably, queens and workers are usually genotypically identical, meaning that any phenotypic differences between the two castes arise from caste-specific gene expression. Using a combination of differential display, microarrays and reverse Northern blots, we found 16 genes that were differentially expressed between adult queens and workers in the ant Lasius niger, a species with highly pronounced reproductive division of labour and a several-fold lifespan difference between queens and workers. RNA ligase mediated rapid amplification of cDNA ends (RLM-RACE) and gene walking were used to further characterize these genes. On the basis of the molecular function of their nearest homologues, three genes appear to be involved in reproductive division of labour. Another three genes, which were exclusively overexpressed in queens, are possibly involved in the maintenance and repair of the soma, a candidate mechanism for lifespan determination. In-depth functional analyses of these genes are now needed to reveal their exact role.     

Fitness measures in selection analyses: sensitivity to the overall number of offspring produced in a lifetime

Age at first (α) and last (ω) breeding are important life‐history traits; however, the direction and strength of selection detected on traits may vary depending on the fitness measure used. We provide the first estimates of lifetime breeding success (LBS) and λ_ind (the population growth rate of an individual) of European badgers Meles meles, by genotyping 915 individuals, sampled over 18 years, for 22 microsatellites. Males are slightly larger than females, and the opportunity for selection was slightly greater for males, as predicted. λ_ind and LBS both performed well in predicting the number of grand‐offspring, and both detected selection for a late ω, until the age of eight. Differential selection (S′_α) for an early α, however, was only detected using LBS, not with λ_ind. In declining populations (λ_ind < 1) selection favours reproduction later in life, whereas early reproduction is selected in increasing populations (λ_ind > 1). As 41% of badgers were assigned only one offspring (λ_ind < 1), whereas 40% were assigned more than two (λ_ind > 1), this cancelled out S′_α measured by λ_ind.     

Culture at work: Polish migrants in the ethnic division of labour on Norwegian construction sites

Following large-scale labour migration from Poland to the Norwegian construction sector since 2004, new ethnic divisions of labour have been established between the usually native core workforces of construction firms, and Polish migrant workers hired through temporary subcontracting and staffing agencies. Survey data suggest that there is very little mobility between these segments of the labour market. The establishment and reproduction of this ethnic division of labour is analysed through qualitative interviews with Norwegian employers and Polish migrant workers. Polish migrants and their particular ‘work culture’ are perceived by Norwegian employers as well-suited for work in the firms’ temporary external workforces but unfit for permanent positions unless they assimilate to a ‘Norwegian work culture’. These stereotyped employment practices are reinforced by the migrants’ own tactical use of the cultural capital available to them when negotiating the conflicting expectations in different job segments.     

Organism size promotes the evolution of specialized cells in multicellular digital organisms

Specialized cells are the essence of complex multicellular life. Fossils allow us to study the modification of specialized, multicellular features such as jaws, scales, and muscular appendages. But it is still unclear what organismal properties contributed to the transition from undifferentiated organisms, which contain only a single cell type, to multicellular organisms with specialized cells. Using digital organisms I studied this transition. My simulations show that the transition to specialized cells happens faster in organism composed of many cells than in organisms composed of few cells. Large organisms suffer less from temporarily unsuccessful evolutionary experiments with individual cells, allowing them to evolve specialized cells via evolutionary trajectories that are unavailable to smaller organisms. This demonstrates that the evolution of simple multicellular organisms which are composed of many functionally identical cells accelerates the evolution of more complex organisms with specialized cells.     

Polymorphism and division of labour in a socially complex ant: neuromodulation of aggression in the Australian weaver ant,Oecophylla smaragdina

Complex social structure in eusocial insects can involve worker morphological and behavioural differentiation. Neuroanatomical variation may underscore worker division of labour, but the regulatory mechanisms of size-based task specialization in polymorphic species are unknown. The Australian weaver ant, Oecophylla smaragdina, exhibits worker polyphenism: larger major workers aggressively defend arboreal territories, whereas smaller minors nurse brood. Here, we demonstrate that octopamine (OA) modulates worker size-related aggression in O. smaragdina. We found that the brains of majors had significantly higher titres of OA than those of minors and that OA was positively and specifically correlated with the frequency of aggressive responses to non-nestmates, a key component of territorial defence. Pharmacological manipulations that effectively switched OA action in major and minor worker brains reversed levels of aggression characteristic of each worker size class. Results suggest that altering OA action is sufficient to produce differences in aggression characteristic of size-related social roles. Neuromodulators therefore may generate variation in responsiveness to task-related stimuli associated with worker size differentiation and collateral behavioural specializations, a significant component of division of labour in complex social systems.     

Performance of Myrmica ant colonies is correlated with the presence of social parasites

1. The performance of ant colonies depends on different factors such as nest site, colony structure or the presence of pathogens and social parasites. Myrmica ants host various types of social parasites, including the larvae of Maculinea butterflies and Microdonmyrmicae (Schönrogge) hoverfly. How these social parasites affect host colony performance is still unexplored. 2. It was examined how the presence of Maculinea teleius Bergsträsser, Maculinea alcon (Denis & Schiffermüller), and M. myrmicae larvae, representing different feeding and growth strategies inside host colonies, is associated with worker survival, the number of foragers, and colony productivity parameters such as growth and reproduction. 3. It was found that the presence of social parasites is negatively associated with total colony production and the production of ant larvae and gynes. Male production was lower only in nests infested by M. teleius, whereas the number of worker pupae was significantly higher in all types of infested colonies than in uninfested colonies. Laboratory observations indicated that nests infested by Maculinea larvae are characterised by a higher number of foragers compared to uninfested nests but we did not find differences in worker survival among nest types. 4. The observed pattern of social parasite influence on colony productivity can be explained by the feeding strategies of parasitic larvae. The most negative effect was found for M. teleius, which feeds on the largest host brood and eliminates a high number of sexual forms. The strong, adverse influence of all studied parasite species on gyne production may result in low queen production in Myrmica populations exposed to these social parasites.