Queen control of sex ratio in fire ants (original) (raw)
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Sex Allocation Conflict in Ants: When the Queen Rules
Current Biology, 2006
Insect societies are paramount examples of cooperation, yet they also harbor internal conflicts whose resolution depends on the power of the opponents . The male-haploid, female-diploid sex-determining system of ants causes workers to be more related to sisters than to brothers, whereas queens are equally related to daughters and sons . Workers should thus allocate more resources to females than to males, while queens should favor an equal investment in each sex. Female-biased sex allocation and manipulation of the sex ratio during brood development suggest that workers prevail in many ant species . Here, we show that queens of Formica selysi strongly influenced colony sex allocation by biasing the sex ratio of their eggs. Most colonies specialized in the production of a single sex. Queens in female-specialist colonies laid a high proportion of diploid eggs, whereas queens in male-specialist colonies laid almost exclusively haploid eggs, which constrains worker manipulation. However, the change in sex ratio between the egg and pupae stages suggests that workers eliminated some male brood, and the population sex-investment ratio was between the queens' and workers' equilibria. Altogether, these data provide evidence for an ongoing conflict between queens and workers, with a prominent influence of queens as a result of their control of egg sex ratio.
Primary sex ratio regulation by queens in ants (Formicidae) and other social Hymenoptera
Myrmecological News, 2012
Sex allocation theory describes how parents should bias investment in either sons or daughters when each gives a different fitness return. Over the past decades, social Hymenoptera have increasingly been used as models for testing the predictions of sex ratio theory. Their haplodiploid sex determining system gives mothers considerable control over the proportion of each offspring sex by selective fertilization of eggs. Moreover, the great diversity in life-history strategies and breeding systems has allowed detailed tests of quantitative predictions linking sex ratios to environmental or genetic factors at the colony and population levels. Nevertheless, the vast majority of theoretical and experimental treatments devoted to sex allocation in ants, bees and wasps have focused on sex ratio at emergence or adult stage, rather than primary sex ratio adjustment (the proportion of each sex at oviposition). Sex ratio at emergence may be adaptive, but it may also result from sex-specific di...
Sex ratio determination by queens and workers in the ant Pheidole desertorum
Animal Behaviour, 2000
Because workers in colonies of eusocial Hymenoptera are more closely related to sisters than to brothers, theory predicts workers should bias investment in reproductive broods to favour reproductive females over males. However, conflict between queens and workers is predicted. Queens are equally related to daughters and sons, and should act to prevent workers from biasing investment. Previous study of the ant Pheidole desertorum showed that workers are nearly three times more closely related to reproductive females than males; however, the investment sex ratio is very near equal, consistent with substantial queen control of workers. Near-equal investment is produced by an equal frequency of colonies whose reproductive broods consist of only females (female specialists) and colonies whose reproductive broods consist of only males or whose sex ratios are extremely male biased (male specialists). Because natural selection should act on P. desertorum workers to bias investment in favour of reproductive females, why do workers in male-specialist colonies rear only (or mostly) males? We tested the hypothesis that queens prevent workers from rearing reproductive females by experimentally providing workers with immature reproductive broods of both sexes. Workers reared available reproductive females, while failing to rear available males. Worker preference for rearing reproductive females is consistent with queens preventing their occurrence in colonies of male specialists. These results provide evidence that queens and workers will act in opposition to determine the sex ratio, a fundamental prediction of queen-worker conflict theory.
Mode of colony foundation influences the primary sex ratio in ants
Animal Behaviour, 1999
In ants, young queens can found new colonies independently (without the help of workers) or dependently (with the help of workers). It has been suggested that differences in the mode of colony founding strongly influence queen survival and colony development. This is because independent queens are constrained to produce a worker force rapidly, before they deplete their body reserves and to resist the intense intercolony competition during the founding stage. By contrast, queens that found colonies dependently remain with the workers, which probably results in a lower mortality rate and earlier production of reproductive offspring. Consequently, in species that found independently, queens of incipient colonies are expected to produce mostly worker brood by laying a lower fraction of haploid (male) eggs than queens in mature colonies; such a difference would not occur in species founding dependently. We compared the primary sex ratio (proportion of male-determined eggs) laid by queens in incipient and mature colonies of two ant species, Lasius niger and Linepithema humile, showing independent and dependent modes of colony founding, respectively. As predicted, L. niger queens of incipient colonies laid a lower proportion of haploid eggs than queens from mature colonies. By contrast, queens of L. humile laid a similar proportion of haploid eggs in both incipient and mature colonies. These results provide the first evidence that (1) the primary sex ratio varies according to the mode of colony foundation, and (2) queens can adjust the primary sex ratio according to the life history stage of the colony in ants.
Primary and secondary sex ratios in monogyne colonies of the fire ant
Animal Behaviour, 1995
Patterns of sex ratios and sex investment ratios play an instrumental role in theoretical and empirical considerations of the evolution and maintenance of insect sociality. An assumption of sex ratio studies in the eusocial Hymenoptera is that workers, who reproduce indirectly by rearing the queen's offspring, have the ability to distinguish the sex of larvae and direct preferential treatment toward developing females. To determine if workers of the monogyne (single queen per colony) form of the fire ant, Solenopsis invicta, have this ability and the extent to which they might use it to influence the sex ratio of the colony's reproductives, we compared the primary sex ratio (ratio of male/female determined eggs produced by the queen) to the secondary sex ratio (ratio of male/female reproductive adults or pupae). We examined this relationship in mature colonies producing sexuals of almost exclusively one sex or the other, a common condition in monogyne colonies of this and other ant species. Queens of male-producing colonies laid about 19% haploid (male) eggs, whereas queens of female-producing colonies laid about 11% haploid eggs. In both cases, the proportion of haploid eggs laid was far higher than the proportion of adult males reared by the workers. These results suggest that workers strongly influence the colony's secondary sex ratio by selective elimination of male larvae, but the magnitude of this influence depends upon the primary sex ratio produced by the queen. Highly male-biased secondary sex ratios produced by workers in relation to more male-biased primary sex ratios may be due to ergonomic constraints that could limit the ability of workers to skew the sex investment ratio.
Local resource competition and sex ratio in the ant Cataglyphis cursor
Behavioral Ecology, 2006
The local resource competition (LRC) hypothesis predicts that wherever philopatric offspring compete for resources with their mothers, offspring sex ratios should be biased in favor of the dispersing sex. In ants, LRC is typically found in polygynous (multiple queen) species where foundation of new nests occurs by budding, which results in a strong population structure and a male-biased population-wide sex ratio. However, under polygyny, the effect of LRC on sex allocation is often blurred by the effect of lowered relatedness asymmetries among colony members. Moreover, environmental factors, such as the availability of resources, have also been shown to deeply influence sex ratio in ants. We investigated sex allocation in the monogynous (single queen) ant Cataglyphis cursor, a species where colonies reproduce by budding and both male and female sexuals are produced through parthenogenesis, so that between-colony variations in relatedness asymmetries should be reduced. Our results show that sex allocation in C. cursor is highly male biased both at the colony and population levels. Genetic analyses indicate a significant isolation-by-distance in the study population, consistent with limited dispersal of females. As expected from asexual reproduction, only weak variations in relatedness asymmetry of workers toward sexual offspring occur across colonies, and they are not associated with colony sex ratio. Inconsistent with the predictions of the resource availability hypothesis, the male bias significantly increases with colony size, and investment in males, but not in females, is positively correlated with total investment in sexuals. Overall, our results are consistent with the predictions of the LRC hypothesis to account for sex ratio variation in this species.
Workers determine queen inheritance of reproduction in a functionally monogynous ant population
Animal Behaviour, 2011
Leptothorax acervorum multiple-queen colony power social behaviour In cooperatively breeding animals there is potential for conflict among group members over reproductive sharing (skew). The outcome of such conflict depends on which individuals have power. Theoretical treatments of skew generally assume that power rests with those directly reproducing, but in eusocial insects, an important group for testing skew theory, nonreproductive workers are known to be influential in other colony conflicts such as sex ratio. We investigated the role of queens and workers in determining queen inheritance of a newly vacant reproductive position in a functionally monogynous (only one queen reproduces) population of the ant Leptothorax acervorum. Multiple-queen colonies with an already established reproductive queen were split in half producing a fragment containing multiple, previously nonreproductive queens. Loss of the established queen simulates a queen turnover event as occurs in nature. We found that worker behaviour influenced which queen filled the vacant reproductive position, with this queen receiving on average less aggression and more grooming from workers than the other queens. This study shows that after loss of the established queen, workers can modify their behaviour towards individual queens, and, contrary to skew models, skew among queens remains high despite changes in relatedness asymmetry. Our results suggest workers collectively hold considerable power over skew among queens. Ó
1994
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 Primary and secondary sex ratios in Iridomyrmex humilis Corresponding Editor: S. A. Frank
Dual mechanism of queen influence over sex ratio in the ant Pheidole pallidula
Behavioral Ecology and Sociobiology, 2005
Social Hymenoptera are general models for the study of parent-offspring conflict over sex ratio, because queens and workers frequently have different reproductive optima. The ant Pheidole pallidula shows a split distribution of sex ratios with most of the colonies producing reproductives of a single sex. Sex ratio specialization is tightly associated with the breeding system, with single-queen (monogynous) colonies producing male biased brood and multiple-queen (polygynous) colonies female-biased brood. Here, we show that this sex specialization is primarily determined by the queen’s influence over colony sex ratio. Queens from monogynous colonies produce a significantly more male-biased primary sex ratio than queens from polygynous colonies. Moreover, queens from monogynous colonies produce a significantly lower proportion of diploid eggs that develop into queens and this is associated with lower rate of juvenile hormone (JH) production compared to queens from polygynous colonies. These results indicate that queens regulate colony sex ratio in two complementary ways: by determining the proportion of female eggs laid and by hormonally biasing the development of female eggs into either a worker or reproductive form. This is the first time that such a dual system of queen influence over colony sex ratio is identified in an ant.