Sex ratio determination and worker reproduction in the slave-making ant Harpagoxenus sublaevis

Summary In a population of the monogynous slave-making ant Harpagoxenus sublaevis in S.E. Sweden, the mean proportion of dry weight investment in queens was 0.54. This result differed significantly from 0.75 but not from 0.5, matching the prediction from the genetic relatedness hypothesis of sex ratio applied to slave-makers, given (as confirmed by this study) single mating of queens, population-wide mate competition, and relatively low levels of worker male production. Sex investment appeared unaffected by resource availability. In the same 47 colony population sample, fertile slave-maker workers were found in every queenless colony (ca. 30% of all colonies), and in 58% of queen-right colonies. Fertile workers occurred at a significantly higher frequency in the queenless colonies (19.2%) than in the queenright ones (9.8%), confirming that queenless conditions promote worker fertility. Fertile and sterile workers were similar in size. Electrophoretic allozyme analysis of ants from 49 colonies showed that: 1) queens mated singly; 2) female nestmates were full sisters (their regression coefficient of relatedness (±SE) was 0.735±0.044); 3) inbreeding did not occur; 4) queen and worker siblings were not genetically differentiated. Worker male production in queenright colonies was neither confirmed nor ruled out by the genetic data. However, production data indicated that queenless workers produced between 4.4 and 21.6% of all males. Overall colony productivity was largely determined by slave number, itself positively correlated with the number of slave-maker workers. There was an abrupt switch from all worker to all sexual production as colony size rose, as predicted by life history models. In queenright colonies, fertile slave-makers did not discernibly reduce colony productivity. Such workers occurred in queenright colonies with most slaves, suggesting they exploited energetic surpluses. Worker reproduction in H. sublaevis therefore appears to have greater influence at the level of individual behaviour than at colony or population level.     

Intracolonial behavioral variation in worker oviposition,oophagy, and larval care in queenless honey bee colonies

Summary Two experiments were performed to determine whether worker reproduction in queenless honey bee colonies is influenced by colony genetic structure. In Experiment 1, allozyme analyses of workers and worker-derived drone larvae revealed that in half the colonies, there were genotypic differences in worker egg-laying behavior (presumed to involve actual oviposition), but biases in drone production were not always consistent with biases in egg-laying behavior. In Experiment 2, allozyme analyses again revealed intracolonial differences in egg-laying behavior and in behavior patterns thought to involve oophagy and larval care. Data support the hypothesis of a genetic influence on this intracolonial behavioral variation. Differences in the genotypic distributions of worker-derived drones relative to workers engaged in oviposition behavior in queenless colonies may be a consequence of genetic variability for egg production or for treatment of eggs and larvae (possibly coupled with kin recognition), or both. Offprint requests to: G.E. Robinson     

Fight or flight? A geographic mosaic in host reaction and potency of a chemical weapon in the social parasite <Emphasis Type="Italic">Harpagoxenus sublaevis</Emphasis>

Ant social parasites use chemical warfare to facilitate host colony takeover, which is a critical but recurring step in their life cycle. Many slave-making ants use the secretion of the Dufour gland to manipulate host behaviour during parasitic nest foundation and slave raids. Harpagoxenus sublaevis applies this chemical weapon onto defending Leptothorax host workers, which elicits deadly fights amongst them. Host species are expected to evolve counter-adaptations against this behavioural manipulation and in this study we investigated the geographic structure of this co-evolving trait. We compared the effectiveness of the parasitic gland secretion from different H. sublaevis populations in host colonies from various sites and analysed the occurrence of local adaptation. The two host species L. muscorum and L. acervorum generally showed different responses to the parasites’ chemical weapon: L. acervorum attacked nestmates treated with Dufour gland secretion, while L. muscorum workers fled. Flight, instead of intraspecific fights, is an adaptive host reaction as it results in fewer host fatalities during raids. Beside interspecific host differences, we found a geographic mosaic of host resistance: parasites from a German population strongly manipulated the behaviour of both sympatric Leptothorax populations. Russian or Italian hosts instead did not react with intracolonial aggression, but fled when confronted with the gland secretion of their sympatric parasite. Not only variation in host resistance explains differences in the effectiveness of the parasitic gland secretion but also interpopulational differences in its chemical composition, which were revealed by gas chromatography and mass spectrometry.     

Regulation of reproduction by dominant workers in bumblebee (Bombus terrestris) queenright colonies

The mechanisms of regulating worker reproduction in bumblebees were studied by direct behavioral observations and by measuring ovarian development and juvenile hormone (JH) biosynthesis rates in workers under different social conditions. Workers in the last stage of Bombus terrestris colony development (the competition phase) had the lowest ovarian development and JH biosynthesis rates. Callows introduced into colonies immediately after queen removal (dequeened colonies) demonstrated a significant increase in ovarian development before, but not during, the competition phase. These findings differ from the higher ovarian development in colonies during the competition phase predicted by the prevailing hypothesis that worker reproduction starts in response to a decrease in queen inhibition. Reproduction of callows housed with dominant workers in small queenless groups was inhibited as in queenright colonies. This suggests that the reduced ovarian development and JH biosynthesis rates observed in dequeened and normally developing colonies during the competition phase also reflect inhibition by dominant workers. Thus, two distinct stages of inhibition of reproduction seem to exist: (1) before the competition phase, when the queen slows down worker ovarian development and prevents oviposition; (2) during the competition phase, when dominant workers inhibit ovarian development of other workers. Between these stages there seems to be a temporal “window” of enhanced worker reproductive development. The queen's typical switch to haploid egg production was not associated with changes in worker ovarian development or JH biosynthesis rates. These findings suggest that regulation of worker reproduction in B. terrestris is not determined by simple changes in the queen's inhibition capacity or by the sex of offspring and that the worker's role is more important than previously believed. Received: 18 March 1998 / Accepted after revision: 18 July 1998     

Worker rank, reproductive status and cuticular hydrocarbon signature in the ant, Pachycondyla cf. inversa

Workers in de-queened colonies of the neotropical ant, Pachycondyla cf. inversa, form linear or near-linear dominance hierarchies by violent antennation and biting. In these rank orders, social status and ovarian activity are on average highly correlated. Whereas the presence of a fertile queen appears to be sufficient to prevent workers from laying eggs, fertile workers do not completely control reproduction by their nestmates, suggesting that workers are able to differentiate between an egg-laying queen and an egg-laying worker. Here we show that the composition of cuticular hydrocarbons of egg-laying workers is quantitatively and qualitatively different from that of non-laying workers and resembles the hydrocarbon blend of the queen but does not completely match it. Furthermore, using discriminant analysis, it was possible to distinguish workers with four different classes of ovarian development based only on their cuticular hydrocarbon profiles. Fertility-associated changes in cuticular hydrocarbons may play an important role in the behavioural regulation of reproduction in this ant.     

Reproductive plasticity in bumblebee workers (<Emphasis Type="Italic">Bombus terrestris</Emphasis>)—reversion from fertility to sterility under queen influence

Worker sterility in the bumblebee Bombus terrestris is conditional and is linked to the social development of the colony. Workers refrain from reproducing or overtly challenging the queen until gyne production has initiated, at the so-called competition point (CP). It is not known whether this behavior is hard-wired or workers show reproductive plasticity. It also remains unclear whether worker reproductive decision is under queen and/or worker control. In this study, we tested worker reproductive plasticity in an attempt to assess whether and under which conditions worker sterility/fertility are reversible. We introduced egg-laying workers into colonies with different social structures for 1 week then monitored their reproductive status. We revealed a remarkable reproductive plasticity in the introduced workers that was social-condition-dependent. In the presence of a pre-CP queen, the introduced workers reverted to sterility, whereas in the presence of a post-CP queen, such workers remained egg-layer. Reversion to sterility does not occur when direct contact with the queen is prevented, as the introduced workers remained egg-layer in the queenright colonies with a confined queen. Egg-laying workers that were introduced into queenless colonies mostly maintained their fertility regardless of colony social phase. This shows that worker transition from cooperative to selfish behavior is reversible depending on the social context.     

Colony level and within colony level selection in honeybees

Summary Although honeybee workers are usually infertile, in queenless colonies some workers can develop ovaries and produce offspring. Therefore the classical Darwinian fitness of workers is not zero. Experimental studies in the Cape honey bee (Apis mellifera capensis) reveal a huge genetic variation for individual fitness of workers. The present study with a one locus, two allele model for reproductive dominance of workers shows that a balanced system between colony level and individual within colony selection plausibly explains the phenomenon of a high genetic variance of worker fitness. In particular, a frequent occurrence of queenless colonies in the population leads to stable polymorphic equilibria. Also the multiple mating system of the honey bee queen supports the propagation of alleles causing reproductive dominance of workers.     

Division of labor and slave raid initiation in slave-making ants

In social insect societies, division of labor, i.e., workers of a colony specializing in different tasks, is thought to improve colony performance. Workers of social parasitic slave-making ants focus on a single task, searching for and raiding host colonies to replenish their slave workforce. However, in the North American slavemaker Protomognathus americanus, some workers do not partake in raids but remain inside the colony. We analyzed raid participation, fertility, and cuticular hydrocarbon profiles of slavemaker workers and slaves to understand these behavioral differences and the regulation of division of labor in slavemaker colonies. Raid observations showed that some workers were repeatedly involved in raiding activities (exterior workers), whereas others stayed inside the nest (interior workers). Exterior workers were always infertile, while half of the interior workers were fertile. Analysis of cuticular hydrocarbons demonstrated differences between the groups. We also detected chemical differences between interior and exterior slaves, indicating an influence of the individuals’ tasks on their cuticular profiles. Task- and fertility-related profiles may allow selective nestmate recruiting. Division of labor should also adapt to varying conditions. Since slave raids are dangerous, they should only be initiated when the colony needs additional slaves. Exclusively fed by their slaves, slavemaker workers could determine this need via their nutritional status. In an experiment with various feeding regimes, colonies subjected to a lower food provisioning rate showed increased proportions of slavemaker workers searching for host colonies. Division of labor in slave-making ants, therefore, might be flexible and can change depending on the colonies’ needs.     

Conflicts,social economics and life history strategies in ants

Summary This paper presents a life history model for a perennial social insect colony. The model's purpose is to explore the evolutionary consequences, in terms of fitness of different parties within the colony, of alternative life history strategies. The model has been specifically developed for colonies of the slave-making ant, Harpagoxenus sublaevis, which has reproductive workers organized in dominance orders. It incorporates empirically obtained parameters, and uses computer algorithms based on numerical optimization to determine the optimum policy for a colony queen in allocating resources between workers, queens, and males. Variants of the model also consider alternative situations in which either (1) orphaned workers do not slave-raid, or (2) workers are sterile. The results correspond closely to data on colony growth and reproductive allocation obtained from the field. They suggest that a colony queen would suffer reduced fitness in the two theoretical cases as compared to the real situation. Reproduction by orphaned workers posthumously enhances the colony queen's fitness because a queen with sterile workers cannot produce enough extra sexuals in her lifetime to balance her loss in grandson production. The results also suggest that the division of labour between slave-raiding and nonraiding workers observed in H. sublaevis colonies can be explained as an outcome of worker-worker reproductive conflict: reproductively-inhibited subordinate workers can increase their inclusive fitness by slaveraiding for dominant, nonraiding egg-layers. These findings emphasize the evolutionary importance of the orphanage period and of intracolony conflict in monogynous social insect colonies.     

Queen acceptance and the complexity of nestmate discrimination in the Argentine ant

In most social insect species, individuals recognize and behave aggressively towards non-nestmate conspecifics to maintain colony integrity. However, introduced populations of the invasive Argentine ant, Linepithema humile, exhibit pronounced variation in intraspecific aggression denoting diversity in nestmate recognition behavior, which possibly shapes their social structure and the varying levels of unicoloniality observed among these populations. One approach to better understand differential aggression behaviors towards conspecifics and recognition cue perception and response in L. humile is to examine variation in nestmate discrimination capability among genetically distinct colonies under different social contexts. Consequently, we investigated the dynamics of queen and worker recognition in southeastern US L. humile queenless and queenright colonies by measuring rates of non-nestmate worker and queen adoption and intercolony genetic similarity. Aggression levels between colony pairs differed and were associated with non-nestmate worker, but not queen adoption. Adoption of queens and workers was a function of host colony origin, while colony queen number affected adoption of queens, but not workers, with queens more readily accepted by queenless hosts. Fecundity of adopted non-nestmate queens was comparable to that of rejected non-nestmate and host colony queens, suggesting that queen fecundity did not affect adoption decisions. Genetic similarity between colonies ranged from 30 to 77% alleles shared, with more genetically similar colonies showing lower levels of intraspecific aggression. Non-nestmate queens and workers that were more genetically similar to host colony workers were more likely to be adopted. We provide the first evidence for the role of L. humile colony queen number on queen discrimination and suggest an effect of resident queens on worker conspecific acceptance thresholds. Our findings indicate a role for genetically based cues in L. humile nestmate recognition. However, subtle discrimination capability seems to be influenced by the social context, as demonstrated by more frequent recognition errors in queenless colonies.     

Worker drift and egg dumping by queens in wild Bombus terrestris colonies

Wild bumblebee colonies are hard to find and often inaccessible, so there have been few studies of the genetic structure of bumblebees within natural colonies, and hence, it is not clear how frequently events such as worker reproduction, worker drift and queen usurpation take place. This study aimed to quantify the occurrence of natal-worker reproduction, worker drift and drifter reproduction within 14 wild colonies of Bombus terrestris in Central Scotland. Four unlinked microsatellites were used to identify patterns of relatedness of the colonies’ adults and broods. In colonies with queens (queenright colonies), worker reproduction accounted for just 0.83 % of males, increasing to 12.11 % in queenless colonies. Four colonies contained a total of six workers which were not daughters of the queen, and were assumed to be drifters, and four male offspring of drifters. Drifting is clearly not common and results in few drifter offspring overall, although drifters produced approximately seven times more offspring per capita than workers that remained in their natal colony. Unexpectedly, two colonies contained clusters of sister workers and juvenile offspring that were not sisters to the rest of the adults or brood found in the colonies, demonstrating probable egg dumping by queens. A third colony contained a queen which was not a sister or daughter to the other bees in the colony. Although usurping of bumblebee colonies by queens in early season is well documented, this appears to be the first record of egg dumping, and it remains unclear whether it is being carried out by old queens or newly mated young queens.     

Chemical ecology involved in invasion success of the cuckoo bumblebee <Emphasis Type="Italic">Psithyrus vestalis</Emphasis> and in survival of workers of its host <Emphasis Type="Italic">Bombus terrestris</Emphasis>

In bumblebees all species of the subgenus Psithyrus are social parasites in the nests of their Bombus hosts. In the bumblebee B. terrestris we investigated how colony size influences survival rates of nest entering females of the social parasite Psithyrus vestalis. Furthermore, we studied whether the host worker’s dominance status and age are reflected in its individual scent and whether Psithyrus females use volatiles to selectively kill host workers. The survival rate of Psithyrus vestalis females drops from 100%, when entering colonies with five workers, to 0% for colonies containing 50 host workers. Older host workers, born before the nest invasion, were selectively killed when Psithyrus females entered the nest. In contrast, all workers born after the nest invasion survived. The host workers’ dominance status and age are reflected by their individual odours: newly emerged workers produced a significantly lower total amount of secretions than 4-day-old workers. In chemical analyses of female groups we identified saturated and unsaturated hydrocarbons, aldehydes, and unsaturated wax-type esters of fatty acids. In a discriminant function analysis different worker groups were mainly separated by their bouquets of hydrocarbons. Killed workers release significantly more scent and of a different chemical composition, than survivors. Survivors alter scent production and increase it beyond the level of the killed workers within 1 day of the invasion. The Psithyrus female clearly maintains reproductive dominance utilizing these differences in the odour bouquets as criteria for killing workers that compete for reproduction.     

Social parasitism of queens and workers in the Cape honeybee (<Emphasis Type="Italic">Apis mellifera capensis</Emphasis>)

Workers of a queenless honeybee colony can requeen the colony by raising a new queen from a young worker brood laid by the old queen. If this process fails, the colony becomes hopelessly queenless and workers activate their ovaries to lay eggs themselves. Laying Cape honeybee workers (Apis mellifera capensis) produce female offspring as an additional pathway for requeening. We tested the frequency of successful requeening in ten hopelessly queenless colonies. DNA genotyping revealed that only 8% of all queens reared in hopelessly queenless colonies were the offspring of native laying worker offspring. The vast majority of queens resulted from parasitic takeovers by foreign queens (27%) and invading parasitic workers (19%). This shows that hopelessly queenless colonies typically die due to parasitic takeovers and that the parasitic laying workers are an important life history strategy more frequently used than in providing a native queen to rescue the colony. Parasitism by foreign queens, which might enter colonies alone or accompanied by only a small worker force is much more frequent than previously considered and constitutes an additional life history strategy in Cape honeybees.     

Colony founding,queen dominance and oligogyny in the Australian meat ant Iridomyrmex purpureus

Summary Field observations and laboratory experiments demonstrate that in the Australian meat ant, Iridomyrmex purpureus, the modes of colony founding are remarkably diverse. New colonies can originate from single foundresses (haplometrosis), or foundress associations (pleometrosis), or by colony budding, or the adoption of newly-mated queens that dig founding chambers next to mature nests (probably their natal nests, as workers protect them and may help them dig). Readoption of foundresses and pleometrosis lead to the coexistence of several queens in one nest. We discovered a striking antagonistic behavior among coexisting queens in young colonies, in the form of ritualized antennation bouts. These interactions result in a reproductive rank order in which dominant queens inhibit egg-laying by subordinates, but escalation into physical fighting is rare. Workers ignore queen dominance interactions and treat all queens equally. The first quantitative ethogram of dominance display behavior between multiple ant queens, and its reproductive consequences, is presented. As a colony grows, queens become intolerant of each other's presence and permanently separate within the nest. Once separated, queens appear to be equal in status, laying approximately equal numbers of eggs. All queens continue to be tolerated by workers, even when the colony has reached a size of several thousand workers and begun to produce reproductives. Such mature nests of I. purpureus fulfill the criteria of oligogyny, defined by worker tolerance toward more than one queen and antagonism among queens, such that a limited number of fully functional queens are spaced far apart within a single colony. Oligogynous colonies can arise in this species by pleometrotic founding (primary oligogyny) or by adoption of queens into existing nests (secondary oligogyny). The adaptive significance of the complex system of colony founding, queen dominance and oligogyny in I. purpureus is discussed.     

Worker reproductive parasitism and drift in the western honeybee Apis mellifera

When a honeybee (Apis spp.) colony loses its queen and is unable to rear a new one, some of the workers activate their ovaries and produce eggs. When a colony has a queen (i.e., it is queenright) almost all worker-laid eggs are eaten, but when hopelessly queenless, the workers become more tolerant of worker-laid eggs and rear some of them to adult drones. This increased tolerance renders a queenless colony vulnerable to worker reproductive parasitism, wherein unrelated workers enter the colony and lay eggs. Here, we show that the proportion of unrelated (non-natal) workers significantly decreases after an Apis mellifera colony becomes queenless. The remaining non-natal workers are as likely to have activated ovaries as natal workers, yet they produce more eggs than natal workers, resulting in significantly higher reproductive success for non-natal workers. In a second experiment, we provided queenless and queenright workers with a choice to remain in their own colony or to join a queenless or queenright colony nearby. The experiment was set up such that worker movement was unlikely to be due to simple orientation errors. Very few workers joined another colony, and there was no preference for workers to drift into or out of queenless or queenright colonies, in accordance with the proportion of non-natal workers declining significantly after becoming queenless in the first experiment.     

The social behavior of Leptothorax allardycei (Hymenoptera,Formicidae): time budgets and the evolution of worker reproduction

Summary The use of time by workers of the ant Leptothorax allardycei (Mann) is examined. Theoretical predictions are developed concerning the maximal lowering in colony reproductive output that is consistent with the evolution of worker production. Measurements are made of the effect of aggression on colony efficiency. Analysis of time budgets indicate that a typical ant spends a large fraction (0.55) of its time quiescent and another large fraction of time (0.32) involved in undifferentiated activity. Dominance activity and brood care together make up about 11% of the total time. The amount of time spent on dominance activity is negatively related to the amount of time spent on brood care, but positively related to the amount of time that an ant is active. The amount of time that an ant has available for brood care which is actually spent on brood care declines with the amount of time spent on dominance activity. The amount of time that a worker spends feeding liquid food to larvae is a function of hierarchy rank; alpha spends the most time, beta less and gamma still less. The spread of the trait of worker reproduction is examined theoretically, with particular regard to the associated costs to colony reproduction. Worker reproduction can spread through a population, under a variety of formulations, provided the cost to colony reproduction is less than some critical value in the neighborhood of 0.17–0.22 of the total colony output. The cost of worker reproduction in L. allardycei is estimated in two ways: as a time cost and as a reduction in the total number of brood tended per unit time. The two estimates of cost are 0.15 and 0.13 respectively. The reproductive options of the worker caste and the division of reproductive labor vary considerably between species. Reproduction by workers yields fitness differences between workers and results in competition among workers with the result that colony efficiency is affected.     

How the social parasitic bumblebee <Emphasis Type="Italic">Bombus bohemicus</Emphasis> sneaks into power of reproduction

Social parasitism is widespread in many groups of social living hymenopteran species and has also evolved in the genus Bombus. Cuckoo bumblebees (subgenus Psithyrus) are obligate brood parasites in nests of other bumblebee species. After nest usurpation and the killing of the host queen, the parasite female has to control worker reproduction in order to accomplish and maintain reproductive dominance and to ensure her reproductive success. The aim of our study was to examine whether the generalist parasitic bumblebee Bombus bohemicus monopolizes and prevents worker reproduction by physical or chemical means and to identify possible odor compounds involved therein. We performed bioassays with callow workers of the host Bombus terrestris and have shown that B. bohemicus females are able to suppress host worker ovarian development, when these host workers are under the direct influence of the parasite female. Furthermore, by chemical analyses, we have demonstrated that the parasite females adjust to the odor profiles of their host queens in order to maintain the level of fertility signaling inside the host colony although the host queen is absent. We also found that host workers change their odor profile after nest usurpation by the parasite female and consequently, we suggest that the host and parasite are caught up in a chemical arms race.     

Worker self-restraint and policing maintain the queen’s reproductive monopoly in a pseudomyrmecine ant

Division of reproductive labor in insect societies is often based on worker self-restraint and both queen and worker policing. Workers of many hitherto studied wasps, bees and ants do not lay eggs in the presence of a queen. However, it is presently unclear how far these observations in a few select clades can be generalized. We investigated if and how queens maintain a reproductive monopoly in colonies of the elongate twig ant, Pseudomyrmex gracilis, a member of the previously unstudied ant subfamily Pseudomyrmecinae. Colonies are usually headed by a single, singly mated queen (monogyny, monandry). Workers therefore would be more closely related to males produced by other workers (r?=?0.375) than to the sons of queens (r?=?0.25). Nevertheless, workers appear to refrain from laying male-destined eggs in the presence of the queen. In queenless conditions, workers form dominance hierarchies by antennal boxing, and only one or a few high-ranking individuals readily begin to lay eggs. When returned into a queenright colony, egg-laying workers are immediately bitten, stung and expelled or killed by other workers. While the composition of cuticular hydrocarbons clearly differed between castes, it less clearly reflected worker ovarian development. An association with worker ovarian development that would allow workers to monitor the reproductive status of nestmates could only be tentatively postulated for certain substances. Our study broadens our knowledge about reproductive conflict in social Hymenoptera and shows that worker sterility in the presence of a queen is more common in monogynous, monandrous ants than expected from relatedness alone.     

Serial polygyny and colony genetic structure in the monogynous queenless ant Diacamma cyaneiventre

Serial polygyny, defined as the temporal succession of several reproductive females in a colony, occurs in some monogynous social insects and has so far attracted little attention. Diacamma cyaneiventre is a queenless ponerine ant found in the south of India. Colonies are headed by one singly mated worker, the gamergate. After the death of the gamergate or her absence following colony fission, the gamergate is replaced by a newly eclosed nestmate worker. After a replacement, colonies go through short-lived periods in which two matrilines of sisters co-occur. This is a situation which can be described as serial polygyny. To measure the consequences of serial polygyny, a genetic analysis was performed on 449 workers from 46 colonies of D. cyaneiventre using five microsatellite loci. The presence of more than one matriline among workers of the same nest was detected in 19% of colonies, indicating a recent change of gamergate. The average genetic relatedness among nestmate workers was 0.751 and did not significantly differ from the theoretical expectation under strict monogyny and monandry (0.75). A simple analytical model of the temporal dynamics of serial polygyny was developed in order to interpret these results. We show that the rate of gamergate turnover relative to the rate of worker turnover is the crucial parameter determining the level of serial polygyny and its effect on the genetic structure of colonies. This parameter, estimated from our data, confirms that serial polygyny occurs in D. cyaneiventre but is not strong enough to influence significantly the average genetic relatedness among workers.     

Polydomy and sexual allocation ratios in the ant Myrmica punctiventris

Summary 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.Offprint requests to: L.E. Snyder at the current address