Queenright Nests Research Articles

Vitellogenin and vitellogenin-like gene expression patterns in relation to caste and task in the ant Formica fusca

Social insect colonies are characterized by division of labour, and extensive morphological, physiological and behavioural differences between queens and workers. The storage protein vitellogenin (Vg) affects multiple aspects of social insect life histories, and has been suggested as a key player for caste differentiation and maintenance. Recently, three genes homologous to Vg have been described in the ant Formica exsecta. Their role is currently unclear but their structural variation suggests variable functions. We examined the expression patterns of the conventional Vg and the three Vg-like genes using qRT-PCR in the common black ant Formica fusca between queens and workers, between nurse and foragers workers, and across social contexts (queenless vs. queenright nests), and sampling time. As expected, we found a significant queen caste and nurse task-related increase for the conventional Vg, while Vg-like-C displayed a consistent forager-biased expression pattern. Task (forager vs. nurse) was the only factor that explained expression variation among workers in any of the studied genes. The removal of the queen did not affect expression, although the proportion of fertile nurses increased in queenless nests. The observed expression biases suggest that in Formica fusca, the ancestral duplication has led to alternative social functions for Vg-like genes across castes and tasks. To get a broader picture of the role of gene duplications in social evolution and the roles of Vg-like genes in caste differentiation and maintenance, how these genes achieve these roles at a molecular level need to be investigated further.

Population structure and sociogenetic organisation in a species with ergatoid queens, the desert ant Ocymyrmex robustior

In ants, reproductive division of labour is typically associated with queen-worker dimorphism. In some species with ergatoid queens (wingless worker-like queens), this polymorphism is drastically reduced and virgin queens may integrate the worker force. While ergatoid queens have been described in several species, their colony and population genetic structure remain largely unstudied. Here, we investigated the population structure and sociogenetic organisation of the desert ant Ocymyrmex robustior. All Ocymyrmex species have only ergatoid queens that are worker-sized. Workers, queens, and males from a large population were genotyped at ten polymorphic microsatellite loci. Our results show that the study population is genetically structured, consistent with dependent colony foundation. Genetic analyses revealed that 17.6% of the males were diploid; diploid males are fertile, siring triploid females. Nests were typically headed by a single queen, and queens were strictly monandrous. However, several nests in the population shared matrilines, indicating polygyny, polydomy, dependent colony foundation, serial polygyny, or a combination of these processes. Dissections reveal that workers lay eggs in both queenright and queenless nests, while virgin ergatoid queens lay eggs in queenright nests only. However, our genetic analyses show that male offspring in queenright nests are all queen-produced, suggesting worker policing and/or trophic egg laying.

Male production by workers in the polygynous ant Prolasius advenus

The ability of workers to produce male individuals is reported here for the first time in a species of the formicine ant genus Prolasius. We show that Prolasius advenus workers possess ovaries and demonstrate that they are able to produce adult males in queenless colonies. We also experimentally tested the influence of queen volatiles on the level of worker reproduction. Workers produced fewer eggs in treatments where they could perceive odors from queens. Some volatile compounds emitted by queens may thus have a signaling or inhibitory effect on worker reproduction. This effect of queen presence did not entirely stop worker reproduction, however, as adult males still emerged under these conditions. Worker-produced males were absent only in treatments with the physical presence of queens. Dissections of workers collected from queenright nests in the field revealed signs of egg-laying activity in more than half of individuals. Together, these results suggest that in nature P. advenus workers produce males at least in orphaned colonies or in situations where the physical presence of queens is limited.

Mating system and population structure in the desert ant Cataglyphis livida

We investigated population genetic structure, mating system, worker reproduction and thelytokous parthenogenesis in the desert ant Cataglyphis livida. Pedigree analyses at polymorphic microsatellite loci show that colonies are headed by a single queen, and that queens are mated with two to eight males. No inbreeding was found in the population sampled. Colonies are genetically differentiated and exhibit no isolation-by-distance pattern, consistent with independent foundation of new colonies. Workers do reproduce and lay haploid (arrhenotokous) eggs in queenless colonies; conversely, we found no evidence of worker reproduction in queenright nests. In contrast with C. cursor, where new queens are produced by thelytokous parthenogenesis, female sexuals and workers of C. livida arise from classical sexual reproduction. We discuss the parallels and contrasts between the mating system and population structure in C. livida and the other Cataglyphis species studied so far.

Seasonal cycle of colony structure in the Ponerine ant Pachycondyla chinensis in western Japan (Hymenoptera, Formicidae)

Investigation of reproductive strategies of ants in the subfamily Ponerinae is important for understanding of the evolution of social structure and of the significance of caste dimorphisms. The biology of species with mated and egg-laying workers (gamergates) has been studied for many species, however, little attention has been paid to species that reproduce via alate queens only. We investigated the seasonal cycle of changes in the colony structure of Pachycondyla chinensis reproduced by alate queens in western Japan, and found the following novel biological characteristics of this species. P. chinensis showed a remarkable caste dimorphism in ovariole numbers: workers had no ovaries while queens had 18 to 36 ovarioles in their ovaries. The nesting system seemed to be polydomous: 266 of 400 nests collected were queenless. The number of queenless nests increased during the reproductive season. Among the 134 queenright nests, 38 had several mated-queens without significant differences in ovary activation and the remaining 96 nests were monogynous. During winter to early spring, most nests were polygynous. After alate production, most of the old queens seemed to die or be expelled and replaced by new queens. Virgin dealated queens were often found and they seemed to have laid eggs.

The economy of worker reproduction in Acromyrmex leafcutter ants

Workers of most eusocial Hymenoptera can produce sons but rarely do so in the presence of the queen, despite the potentially high fitness payoff of direct reproduction. It has often been suggested that colony-level productivity costs may outweigh the fitness advantages of worker reproduction, but this has never been shown. We estimated these costs in two ant species with large complex societies, the leafcutter ants Acromyrmex echinatior and Acromyrmex octospinosus , by splitting 16 laboratory colonies and comparing the dynamics of the fungus garden volume between orphaned and queenright nests (with and without laying workers, respectively). Apart from fungus volume as a longitudinal measure of colony productivity, we estimated sex allocation, the timing of the switch between raising queen offspring and worker sons, and differences in body mass between queen and worker sons. We showed that orphaned nests give priority to raising dispersing queen-derived sexuals (virgin queens, haploid and diploid queen sons) over raising worker sons. Worker sons could be heavier than, equally heavy as, or lighter than queen sons, depending on the condition of the colony. The reduction in colony productivity of orphaned nests compared to their queenright counterparts was negligible (≤1.2%), at least during the first 9 weeks after queen loss when these costs could be estimated. Such low costs could only outweigh the benefit of worker laying if the survival of worker sons is low, for example if workers show efficient worker policing, or discriminate against all male brood outside the brief reproductive season.

Production of sexuals in a fission-performing ant: dual effects of queen pheromones and colony size

Models based on the kin selection theory predict that in social hymenopterans, queens may favor a lower investment in the production of sexuals than workers. However, in perennial colonies, this conflict may be tuned down by colony-level selection because of the trade off between colony survival and reproductive allocation. In this study, we present a survey of sexual production in colonies of Aphaenogaster senilis, a common species of ant in the Iberian Peninsula. Similar to most species that reproduce by fission, males were found in large excess compared to gynes (172:1). Sexuals were more likely to be found in queenless than in queenright (QR) field colonies. However, we also found a few gynes and numerous males in very large QR colonies. We compared these data with those available in the literature for A. rudis, a congeneric species from North America that has independent colony founding. The sex ratio in this species was only five males for each female, and sexuals were mostly found in QR nests, irrespective of colony size. We confirmed queen inhibition of sexual production in A. senilis in laboratory experiments and provide evidence that this inhibition is mediated by a nonvolatile pheromone. To seek the potential source of such a queen pheromone, we analyzed the secretions of two conspicuous exocrine glands, the Dufour’s and postpharyngeal glands (DG and PPG, respectively) in both queens and workers. Both secretions were composed of hydrocarbons, but that of DG also contained small quantities of tetradecanal and hexadecanal. The hydrocarbon profile of the DG and PPG showed notable caste specificity suggesting a role in caste-related behavior. The PPG secretions also differed between colonies suggesting its role in colony-level recognition. We suggest that in A. senilis, there are two modes of colony fission: First, in very large colonies, gynes are produced, probably because of the dilution of the queen pheromone, and consequently one or more gynes leave the mother colony with workers and brood to found a new nest. This is beneficial at the colony level because it avoids the production of costly sexuals in small colonies. However, because the queen and workers have different optima for sexual production, we hypothesize that queens tend to overproduce the pheromone to delay their production. This in turn may drive workers to leave the mother colony during nest relocation and to produce sexuals once they are away from the queen’s influence, creating a second mode of colony fission.

Social parasitism in ants: effects of the inquiline parasite Plagiolepis xene St. on queen distribution and worker production of its host Plagiolepis pygmaea Latr.

The ant Plagiolepis pygmaea forms polygynous (several mated queens per colony) and polydomous (multiple nests per colony) colonies that may be parasitized by the workerless social parasite Plagiolepis xene. Earlier laboratory experiments showed that P. xene strongly restricts host worker production (Passera, 1972); however, this negative effect seems not to occur under natural conditions. Here, we explored the hypothesis that, in infested natural colonies of P. pygmaea, parasite queens affect the distribution of the host queens resulting in the latter inhabiting unparasitized nests of the colony at the period of egg-laying. Our field studies show that the presence of the inquiline ant P. xene influences host queen distribution, with a higher level of dispersion within parasitized colonies. In winter, there is a positive association between the number of host queens and the number of parasites in queenright nests. Such a relationship does not occur in summer, so that when egg-laying reaches its maximum activity most queens of the host are found in unparasitized nests. Moreover, the proportion of queenright nests parasitized was dramatically lower than in the winter. A detailed comparison of worker production between parasitized and unparasitized field colonies reveals no difference. Experiments show that reduced worker production in parasitized nests reared under laboratory conditions results from oophagy by the parasitic queens. Overall, our data suggest that the segregation of the host and parasite queens in different nests at the time of egg-laying results in part from host queens avoiding inhabiting parasitized nests to reduce oophagy by the parasite. We discuss the behavioral and ecological implications of our findings.

Nesting biology of the fungus-growing ant Cyphomyrmex longiscapus Weber (Attini, Formicidae)

The fungus-growing ant Cyphomymrex longiscapus Weber (Attini, Formicidae) exhibits one of the most peculiar nesting habits of all attine ants. It constructs semi-pendant nests resembling miniature swallow nests at vertical embankments or under overhanging logs and fashions entrances with funnel-like “auricles” constructed of clay. The entrance auricle surrounds a large nest opening that exposes the garden and renders it susceptible to desiccation. The distribution of C. longiscapus therefore is limited to wetter habitats. Nests are strictly monogynous. Queenright nests of C. longiscapus can be collected easily because of its conspicuous and shallow nest architecture, making this species an ideal candidate for population surveys of geographic variation in symbiont diversity, garden substrate, or parasitism in a lower attine ant.

Social Control over the Survival and Selection of Winged Virgin Queens in an Ant without Nuptial Flight: Iridomyrmex humilis

AbstractLaboratory and field experiments show that the number of winged virgin queens of the Argentine ant Iridomyrmex humilis allowed to remain alive in the nest is under social control. When reared in their natal nest, survival of virgin queens depends on the presence of males. In the presence of mature alate males or even male pupae only 20% of the gynes are executed before they are 4 days old. By contrast, if pupal and adult males are absent, about 55% of the gynes are executed. The workers tend to kill the lightest ones. Although the Argentine ant is a unicolonial species exhibiting complete acceptance of foreign conspecific workers and mated queens, regardless of whether the recipient nest is queenright or queenless, the acceptance of gynes is variable: they are executed by foreign queenless workers but widely accepted in queenright nests. These results are discussed with regard to possible existence of queen and gyne pheromones allowing recognition and to the lack of nuptial flight in this species.

Polydomy and sexual allocation ratios in the ant Myrmica punctiventris

Colony structure and reproductive investment were studied in a population of Myrmica punctiventris. This species undergoes a seasonal cycle of polydomy. A colony overwinters in entirety but fractionates into two or more nest sites during the active season and then coalesces in the fall. Colony boundaries were determined by integrating data on spatial pattern, behavioral compatability, and genetic relatedness as revealed by protein electrophoresis. Colonies contained at most one queen. Consequently, a colony consisted of one queenright nest and one or more queenless nests. Furthermore, estimates of relatedness were fully consistent, with queens being single mated. M. punctiventris therefore has a colony genetic structure that conforms to the classical explanation of the maintenance of worker sterility by kin selection. Kin selection theory predicts that workers would favor a female-biased allocation ratio while selection on queens would favor equal investment in males and females. We predicted that in polydomous populations, queenless nests would rear more female reproductives from diploid larvae than queenright nests. There was a significant difference between queenright and queenless nests in sexual allocation; queenless nests allocated energy to reproductive females whereas queenright nests did not. At neither the nest nor colony levels did worker number limit sexual production. We also found that nests tended to rear either males or females but when colony reproduction was summed over nests, the sexes were more equally represented. The difference in allocation ratios between queenless and queenright nests was attributed solely to queen presence/absence. Our work shows that polydomy provides an opportunity for workers to evade queen control and thereby to sexualize brood.

Growth and development ofCamponotus (Paramyrmamblys) kiusiuensis Santschi colonies (Hym. Formicidae)

Colonial growth and development of a bamboo carpenter antCamponotus kiusiuensis was studied both by cultivation in the laboratory and by regular sampling in bamboo thickets. The mean mortality of immatures was 44.9±SD 7.5% during claustral colony foundation by a queen: 25.3±8.1% in eggs, 19.4±9.9% in larvae, 3.9±6.3% in prepupae and 4.2±5.6% in pupae. Although the first workers were monomorphic, polymorphism was developed by an increase in larger workers with the growth of colony, achieving the full variation of worker size at population level of about 100 adults. Well-developed colonies were monogynous and polydomous. Queenless satellite nests were most abundant in the autumn to winter, unlike hitherto studied polydomous ants which usually show the peak of nest density in the summer, suggesting that the dead bamboos limited nest space. The scale of polydomy was remarkably dependent on the population size of the colony. Although the alates were more abundant in queenless nests than in queenright nests, the negative effect of queens on the production was not so strong as in the species hitherto reported.