Queen execution in the Argentine ant, Iridomyrmex humilis

ABSTRACT. Field censuses and laboratory experiments show that in the Argentine ant, Iridomyrmex humilis (Mayr), c. 90% of the queens are executed by workers in May, at the beginning of the reproductive season. The reduction in the number of queens probably decreases the inhibition exerted by queens on the differentiation of sexuals and thus allows the production of new queens and males shortly thereafter. In the laboratory, there was no correlation between the percentage of queens executed and their weight or fecundity. At the time of execution of queens, nearly all queens were of the same age; less than 1 year. Therefore it is not likely that the age of queens plays any role in the choice that workers make in the queens they executed. Execution of these queens results in a heavy energetic cost for the colony which amounts c. 8% of the total biomass. This behaviour of workers executing nestmate queens is discussed with regard to possible evolutionary significance at the queen and worker level.     

Effect of queen number on the production of sexuals in natural populations of the fire ant, Solenopsis invicta

ABSTRACT. To determine whether the production of sexuals in the fire ant, Solenopsis invicta Buren, is related to queen number in the field, the numbers of sexuals in monogyne (M) and polygyne (P) colonies of this species were compared. Large colonies ( n =25 M and 25 P) were collected at random during spring, summer and fall when sexuals are actively produced, and the numbers of sexuals present were counted. The numbers of alates plus sexual pupae and larvae in M colonies far exceeded those in P colonies in each season. Significant differences between M and P colonies were also found when the numbers of alates and sexual immatures (pupae + larvae) were compared separately in each season. In addition, M colonies contained significantly more female sexuals (alates + pupae) than did P colonies in the summer and fall, and significantly more males in all three seasons. The negative relationship between queen number and number of sexuals provides evidence that queen control over the production of sexuals, previously established in laboratory experiments, also occurs under natural conditions.     

对蜜蜂性别决定和蜂王级型确立理论的新诠释

从全新视角概貌性解读蜜蜂Apis诸多生殖机制,依据的就是蜂王或产卵工蜂的卵子发生、性别决定假说、性位点研究以及蜂王级型确立机制。这种不同于单倍-二倍性性别决定机制的新诠释,适用于明确性别决定机制、简化定向育种方法、减少比较性实验的难度和时间、实施亲子鉴定和抽象并量化种群的概念。     

Elimination of sexual brood in the Argentine antLinepithema humile: queen effect and brood recognition

Mated queens of the antLinepithema humile (Iridomyrmex humilis Mayr) introduced into dequeened colony fragments rearing sexual brood elicited worker aggression resulting in queen larvae being bitten and eliminated. By contrast, male larvae were spared. Regarding queen brood, killing mainly concerned small and medium sized larvae. A large proportion of the large larvae escaped extermination, and prepupae and pupae were spared. These data suggest that workers were able to discriminate sex, caste and age of the brood. That a queen pheromone may be involved was shown by experiments using whole or cut corpses that were either rinsed or not rinsed in pentane. The pheromone eliciting worker aggressive behaviour was shown to act over a short distance, suggesting that it is somewhat volatile. Similarities and differences between this new queen pheromone and other known queen pheromones acting on queen production or worker attraction are discussed as well as the origin of the signals underlying the recognition of the larval classes.     

On the relationship between queen number and fecundity in polygyne colonies of the fire ant Solenopsis invicta

ABSTRACT. The reproductive status of individual queens in relation to the number of queens in polygyne colonies of Solenopsis invicta Buren was investigated under both field and laboratory conditions. In field colonies, the weight of individual queens is inversely related to the number of resident queens present. The weight of a queen is positively correlated with fecundity, as measured by the number of eggs laid in 5 h in isolation. Thus increasing queen number has a strong negative effect on the fecundity of individual queens in these colonies. A similar relationship was established in standardized laboratory colonies using queens from a single experimental population. Colony size by itself had no significant effect on queen weight in either the field or laboratory colonies, suggesting that the inverse relationship between queen number and fecundity results from mutual inhibition among queens, possibly involving pheromones, rather than reduced nutrition through lower worker/queen ratios. While uninseminated queens can comprise up to 50% of the functional (i.e. egglaying) queens in polygyne colonies of this ant, they tend to be less fecund than their inseminated nestmates; uninseminated queens within a given colony generally weigh 5–25% less.     

Cold hardiness of the overwintering black carpenter ant

Abstract Cold hardiness was investigated in overwintering field nests of the black carpenter ant Camponotus pennsylvanicus (De Geer) in the Commonwealth of Virginia. No active nest thermoregulation was observed: temperatures of galleries, worker bodies, worker clusters, and larval clusters were within 3°C of ambient temperature. Nest temperatures generally fluctuated less rapidly and severely than did ambient temperature; thus, the nest afforded protection from potentially fatal sudden temperature drops. Glycerol, the only polyol cryoprotectant detected, was found in all castes and larvae. Supercooling points were low and ranged from ? 17°C in major workers to ?22°C in larvae. A second heat release peak, occurring around ? 8°C, was seen in all adults, but it was not observed in larvae. This higher temperature peak in adults probably represents the freezing of the gut contents, as adults were found to overwinter with the crop full or partially full. Larvae did not overwinter with liquid food in the gut.     

Queen-worker conflict over sex ratio: A comparison of primary and secondary sex ratios in the Argentine ant,Iridomyrmex humilis

We compare the primary sex ratio (proportion of haploid eggs laid by queens) and the secondary sex ratio (proportion of male pupae produced) in the Argentine ant Iridomyrmex humilis with the aim of investigating whether workers control the secondary sex ratio by selectively eliminating male brood. The proportion of haploid eggs produced by queens was close to 0.5 in late winter, decreased to less than 0.3 in spring and summer, and increased again to a value close to 0.5 in fall. Laboratory experiments indicate that temperture is a proximate factor influencing the primary sex ratio with a higher proportion of haploid eggs being laid at colder temperatures. Production of queen pupae ceased in mid-June, about three weeks before that of male pupae. After this time only worker pupae were produced. During the period of production of sexuals, the proportion of male pupae ranged from 0.30 to 0.38. Outside this period no males were reared although haploid eggs were produced all the year round by queens. Workers thus exert a control on the secondary sex ratio by eliminating a proportion of the male brood during the period of sexual production and eliminating all the males during the remainder of the cycle. These data are consistent with workers preferring a more female-biased sex ratio than queens. The evolutionary significance of the production of male eggs by queens all the year round is as yet unclear. It may be a mechanism allowing queen replacement in the case of the death of the queens in the colony.     

Reproductive development and ontogeny of queen pheromone production in the fire ant Solenopsis invicta

Queens of the fire ant, Solenopsis invicta Buren, produce several releaser and primer pheromones. Using bioassays, the ontogeny of three of these pheromones related to reproductive development was investigated. Virgin queens, in which the process of wing-shedding (dealation) serves as an indicator of the initiation of reproductive development, were studied. First, the production of two queen pheromones produced in the poison gland was examined. The pheromone responsible for initiation and maintenance of retinue formation, a releaser effect, was found to be produced in detectable quantities 2 days after dealation, at which time queens showed significant ovary development and many (30%) had started laying eggs. A primer pheromone that inhibits alate virgin queens from dealating was detected in queens 3 days following wing-shedding, when 80% of the queens were ovipositing. Second, I examined the onset of a pheromone of unknown glandular origin produced by reproductively active virgin queens which leads to their destruction in queenright colonies by stimulating workers to attack and kill them. This pheromone is secreted in quantities detectable by bioassay 2 days after dealation. Thus, in S. invicta, the ontogeny of three distinct queen pheromones is tightly linked with ovary development and initiation of egg laying. These results demonstrate reproductive and communicative functions are closely associated during the transition from potential to functional queen.     

Ergatoid queen development in the ant Myrmecina nipponica: modular and heterochronic regulation of caste differentiation

Caste polyphenism in social insects provides us with excellent opportunities to examine the plasticity and robustness underlying developmental pathways. Several ant species have evolved unusual castes showing intermediate morphologies between alate queens and wingless workers. In some low-temperature habitats, the ant Myrmecina nipponica produces such intermediate reproductives (i.e. ergatoids), which can mate and store sperm but cannot fly. To gain insight into the developmental and evolutionary aspects associated with ergatoid production, we conducted morphological and histological examinations of the post-embryonic development of compound eyes, gonads and wings during the process of caste differentiation. In compound eyes, both the queen-worker and ergatoid-worker differences were already recognized at the third larval instar. In gonads, queen-worker differentiation began at the larval stage, and ergatoid-worker differentiation began between the prepupal and pupal stages. Wing development in ergatoids was generally similar to that in workers throughout post-embryonic development. Our results showed that the developmental rate and timing of differentiation in body parts differed among castes and among body parts. These differences suggest that the rearrangement of modular body parts by heterochronic developmental regulation is responsible for the origination of novel castes, which are considered to be adaptations to specific ecological niches.     

Evolutionary implications of polygyny in the Argentine ant,Iridomyrmex humilis (Mayr) (Hymenoptera: Formicidae): an experimental study

A comparison of several physiological parameters of queens of Iridomyrmex humilis in experimental monogynous and polygynous colonies showed that queens in monogynous colonies became heavier, had more developed ovaries and laid about twice as many eggs. Workers in monogynous colonies were more attracted to queens, which therefore probably received more food. This may partially explain the higher weight and fecundity of queens in monogynous colonies of Iridomyrmex humilis and possibly other ant species. In polygynous colonies, queens differed greatly in their fecundity. These differences did not appear to be the result of a dominance hierarchy. These results are discussed from an evolutionary point of view. Two hypotheses of mutualism and colony level selection are proposed as an alternative to kin selection which is unlikely to be the exclusive selective influence in the evolution of polygyny either in I. humilis or in most other ant species.     

Influence of queen weight and colony origin on worker response in Solenopsis geminata

Abstract. The influence of weight and colony origin of the queen of Solenopsis geminata (F.) (Hymenoptera: Formicidae) on worker attraction is studied under laboratory conditions. In the first experiment, worker response to individual queens of different weight from the same colony is evaluated. Heavier queens are more attractive than smaller queens to their own workers. In subsequent experiments, the colony origin effect is investigated and worker response to a pair of queens of the same weight from the same or different colonies is compared. When queens are from the same colony, workers do not show a significant preference between queens. However, when queens are from a different colony, workers are significantly more attracted to their own queen than to the foreign queen. Finally, the response of workers to queens of different weight from the same or different colonies is investigated. In both cases, workers are significantly more attracted to a heavier queen than a lighter queen, even if the lighter queen is their own queen. A putative pheromonal component (E)‐6‐(1‐pentenyl)‐2H‐2‐pyranone, is not positively correlated with queen weight.     

Brood development into sexual females depends on the presence of a queen but not on temperature in an ant dispersing by colony fission,Aphaenogaster senilis

Abstract. 1. In eusocial insects, colony fission is a mode of dispersal by which a young queen leaves her nest with some workers to found a new colony. In these species, adult females (workers and the queen) should allocate most resources to increasing their colony size, which constrains the possibility of fission. In contrast, developing diploid larvae should have a preference for becoming a queen and having their own offspring, rather than becoming workers and rearing the offspring of other females. 2. In the ant Aphaenogaster senilis, queens are produced in very small numbers, suggesting that adult females control larval development. We used a 6‐year series of data on more than 300 nests to determine the annual cycle of worker and queen production. Although both overlapped, the latter mostly occurred in the second half of the summer, after a major peak of worker emergence. Young queens were also often produced in nests whose reproductive queen had died, thus allowing her replacement. Overall, we estimate that only 0.07% of diploid larvae actually develop into gynes. 3. Laboratory experiments indicated that brood is bipotent until the second larval instar. Diploid larval development into queen was favoured by the removal of the mother queen, but was not affected by rearing temperature. 4. Our data suggest that most diploid broods are forced by the adults to develop into workers rather than into gynes. However, when the queen is not present due to death or after a fission event, a few larvae are allowed to develop into gynes. One way for workers to limit the development of larvae might be by controlling the amount of food they receive.     

Selfish strategies and honest signalling: reproductive conflicts in ant queen associations

Social insects offer unique opportunities to test predictions regarding the evolution of cooperation, life histories and communication. Colony founding by groups of unrelated queens, some of which are later killed, may select for selfish reproductive strategies, honest signalling and punishment. Here, we use a brood transfer experiment to test whether cofounding queens of the ant Lasius niger ‘selfishly’ adjust their productivity when sharing the nest with future competitors. We simultaneously analysed queen cuticular hydrocarbon (CHC) profiles to investigate whether queens honestly signal their reproductive output or produce dishonest, manipulative signals, providing a novel test of the evolutionary significance of queen pheromones. Queens produced fewer workers when their colony contained ample brood, but only in the presence of competitors, suggesting selfish conservation of resources. Several CHCs correlated with reproductive maturation, and to a lesser extent with productivity; the same hydrocarbons were more abundant on queens that were not killed, suggesting that workers select productive queens using these chemical cues. Our results highlight the role of honest signalling in the evolution of cooperation: whenever cheaters can be reliably identified, they may incur sanctions that reduce the incentive to be selfish.     

Kin structure and queen execution in the Argentine ant Linepithema humile

Every spring, workers of the Argentine Ant Linepithema humile kill a large proportion of queens within their nests. Although this behaviour inflicts a high energetic cost on the colonies, its biological significance has remained elusive so far. An earlier study showed that the probability of a queen being executed is not related to her weight, fecundity, or age. Here we test the hypothesis that workers collectively eliminate queens to which they are less related, thereby increasing their inclusive fitness. We found no evidence for this hypothesis. Workers of a nest were on average not significantly less related to executed queens than to surviving ones. Moreover, a population genetic analysis revealed that workers were not genetically differentiated between nests. This means that workers of a given nest are equally related to any queen in the population and that there can be no increase in average worker–queen relatedness by selective elimination of queens. Finally, our genetic analyses also showed that, in contrast to workers, queens were significantly genetically differentiated between nests and that there was significant isolation by distance for queens.     

Evolutionary relationship of fat body endoreduplication and queen fecundity in termites

Endoreduplication or nuclear genome replication without cell division is widely observed in the metabolically active tissues of plants and animals. The fat body cells of adult female insects produce abundant yolk proteins and become polyploid, which is assumed to accelerate egg production. Recently, it was reported that in termites, endopolyploidy in the fat body occurs only in queens but not in the other females; however, the relationship between the fecundity and ploidy level in the fat body remains unclear. Termite queens exhibit a huge variation in their egg‐producing capacity among different species; queens in the species with a foraging lifestyle, in which workers leave the nest to forage outside, are much more fecund than those in the species living in a single piece of wood. In this study, we conducted ploidy analyses on three foragings and three wood‐dwelling termites via flow cytometry. In all the species, the fat body of queens contained significantly more polyploid cells than that of other nonreproductive females, considering their body size effect. However, the male fat body, which is not involved in yolk production, did not show consistency in polyploid cell numbers among the species studied. Moreover, highly fecund queens in foraging termites exhibit higher levels of endopolyploidy in their fat body than those with less fecundity in wood‐dwelling termites. These results suggest that endopolyploidy in the fat body of termite queens can boost their egg production, and the level of endopolyploidy in their fat body is linked to their fecundity. Our study provides a novel insight into the evolutionary relationship between endoreduplication and caste specialization in social insects.     

Queen adoption in the polygynous and polydomous ant, Leptothorax curvispinosus

Alate female reproductives of the facultatively polygynous andpolydomous ant, Lepiothorax curuispinosus, were reared fromfield-collected nests, mated, and introduced into either theirparental nests or alien conspecific nests. The 41 queens introducedinto alien nests were usually attacked and rejected (97. 6%),but one queen was accepted after initial aggression. The 27queens introduced into their parental nests received a variableresponse. Some were accepted without any apparent aggression(59. 3%), but others were strongly attacked and rejected (40.7%). Sequential introductions of up to four queens into particularparental nests indicated that nests consistently either acceptor reject their mated offspring. The presence or absence ofresident queens in parental nests had no apparent influenceon the acceptance of offspring queens. Nests that accepted queenshad significantly fewer workers than those that did not, butthis slight difference is unlikely to explain these dichotomousresults and could be spurious. Dissections of the introducedqueens revealed that 79. 0% were inseminated and 98. 3% haddeveloping, yolked eggs in their ovarioles, but these variableshad no apparent effect on acceptability. Similarly, the sizeof the introduced queens and the time that elapsed between matingand introduction had no apparent effect. The consistent responseof parental nests in either accepting or rejecting their matedoffspring indicates a mechanism of queen number regulation inthis species that involves characteristics of the colony ornest rather than variability among offspring queens. This mechanismcould be responsible for maintaining relatively low numbersof queens and high genetic relatedness in colonies (or individualnests) while promoting flexibility in colony reproduction bycolony fission ("budding") and the dispersal of young queens.This mechanism could also involve an important conflict of interestbetween parental colonies and their mated offspring and mightcontribute to the evolution of socially parasitic colony foundationstrategies. Acceptance of mated offspring by their parentalcolonies might only occur during certain periods in colony development,depend on the level of genetic diversity within the colony (ornest), reflect the condition of the colony, nest, queen(s),brood, or local habitat, or result from a genetic polymorphism.     

Lack of inbreeding avoidance in the Argentine ant Linepithema humile

Although workers might increase their inclusive fitness by favoringcloser over more distant kin, evidence suggest that nepotismgenerally does not occur within colonies of social insects.It has been suggested that this may be due to the cost of recognitionerrors. We tested whether recognition occurs in a system wherea better than random ability to recognize kin should be selected for. Using DNA microsatellites, we show that sexuals of theArgentine ant Linepithema humile fail to use genetic cues toavoid sib-mating. When offspring of two queens were allowedto mate, the percentage of matings among siblings was not significantlylower than expected under the hypothesis of random mating.The finding that sexuals fail to use genetic cues to avoid sib-matings cannot be attributed to the cost of recognitionerrors because any recognition system that would lead to abetter than random ability to avoid sib-mating should be selectedfor when there are costs to inbreeding. These data are thusconsistent with the view that kin recognition mediated solelyby genetic cues might be intrinsically error prone within coloniesof social insects.     

Population and colony structure and morphometrics in the queen dimorphic harvester ant, <Emphasis Type="Italic">Pogonomyrmex pima</Emphasis>

The North American seed-harvester ant Pogonomyrmex (Ephebomyrmex) pima displays a dimorphism that consists of winged (alate) and wingless (intermorph) queens; both types of queens are fully reproductive. Microsatellite allele frequencies and a mitochondrial phylogeny demonstrate (1) alate and intermorph queens represent an intraspecific wing polymorphism, and (2) an absence of assortative mating and inbreeding by males. Surveys at our field site in southcentral Arizona, USA, demonstrated that only one type of queen (intermorph or dealate) occurred in each colony, including those excavated during the season in which reproductive sexuals were present. Colony structure appeared to vary by queen type as most intermorph colonies contained multiple mated queens. Alternatively, dealate queen colonies rarely contained a mated queen. Our inability to find mated dealate queens in these colonies probably resulted from difficulty in excavating the entire colony and reproductive queen, especially given that these colonies were only excavated over one day. A morphometric analysis demonstrated that intermorph queens are intermediate in size to that of workers and alate queens, but that intermorph queens retain all of the specialized anatomical features of alate queens (except for wings). Some colonies had queens that foraged and performed nest maintenance activities, and these queens sometimes accounted for a significant portion of colony foraging trips. Dissections revealed that these queens were uninseminated; some of these queens produced males in the laboratory. Received 24 October 2006; revised 1 December 2006; accepted 8 December 2006.     

The role of exogenous JH I,JH III and Anti-JH (precocene II) on queen induction of 4.5-day-old worker honey bee larvae

Worker larvae at an age of 4½ days were fed one of several mixtures of reconstituted royal jelly adjusted to a refractive index of 1.3825 and supplemented with JH I, JH III or Anti-JH (precocene II). In addition, juvenile hormone was topically applied to larvae of the same age. It was readily apparent that caste induction is concentration-dependent and that 4?-day-old worker larvae can still develop into queens under laboratory conditions, providing that they have not stopped feeding or can be induced to commence feeding again. These findings are contrary to the general belief that queen induction is not possible after a socalled sensitive period of 3–3½ days. Queens resulted only from honey bee larvae exposed to royal jelly containing 1 μg of JH I. In addition, oral application at this concentration resulted in the only case in which the normal mean weights of worker honey bees were exceeded. All other concentrations of juvenile hormone were not sufficient to initiate queen induction, although its lower concentration may have influenced the production of intercastes.Precocene II did not play a role in queen induction and it also did not interfere with the growth of developing larvae or adults. In addition, the lack of malformations in honey bees treated with precocene II indicates that the use of such a compound as a control agent in insect populations will probably not be detrimental to honey bee larvae that are at least 4½ days old. However, large doses of precocene will quickly kill most 3½-day-old honey bee larvae.The evidence presented here clearly indicates that caste determination is regulated by the endocrine system in honey bee larvae. Food intake in honey bee larvae may well be regulated by the endocrine system. Thus, an apparently inhibited corpus allatum (C.A.) could be reactivated by food intake coupled with juvenile hormone. The food intake restriction that worker larvae normally encounter in the hive probably results in a cessation of C.A. activity. The increase in food intake by queen larvae, on the other hand, carries an increase in growth and accompanying morphological changes necessary for queen development. This concept may also explain the development of intercastes encountered in in vitro studies. Only those larvae that follow a normal food intake sequence, i.e. moderate during the first 3–4 days or so, will develop into queens. Conversely, those larvae that take in too much food during the early portion of development may achieve incomplete development of the neurosecretory system and, thus, develop into intercastes.     

When invasive ants meet: effects of outbreeding on queen performance in the tramp ant Cardiocondyla itsukii

Most disturbed habitats in the tropics and subtropics harbor numerous species of invasive ants, and occasionally the same species has been introduced repeatedly from multiple geographical sources. We examined how experimental crossbreeding between sexuals from different populations affects the fitness of queens of the tramp ant Cardiocondyla itsukii, which is widely distributed in Asia and the Pacific Islands. Eggs laid by queens that mated with nestmate males had a higher hatchi ng rate than eggs laid by queens mated to males from neighboring (Hawaii x Kauai) or distant introduced populations (Hawaii/Kauai x Okinawa). Furthermore, inbreeding queens had a Ion ger lifespan and produced a less female-biased offspring sex ratio than queens from allopatric mating. This suggests that the genetic divergence between different source populations may already be so large that in case of multiple invasions eventual crossbreeding might negatively affect the fitness of tramp ants.