Inbreeding and sex-biased gene flow in the ant Formica exsecta

Abstract.— The objective of this study was to assess breeding and dispersal patterns of both males and females in a monogyne (a single queen per colony) population of ants. Monogyny is commonly associated with extensive nuptial flights, presumably leading to considerable gene flow over large areas. Opposite to these expectations we found evidence of both inbreeding and sex-biased gene flow in a monogyne population of Formica exsecta . We found a significant degree of population subdivision at a local scale (within islands) for queens (females heading established colonies) and workers, but not for colony fathers (the males mated to the colony queens). However, we found little evidence of population subdivision at a larger scale (among islands). More conclusive support for sex-biased gene flow comes from the analysis of isolation by distance on the largest island, and from assignment tests revealing differences in female and male philopatry. The genetic similarity between pairs of queens decreased significantly when geographical distance increased, demonstrating limited dispersal and isolation by distance in queens. By contrast, we found no such pattern for colony fathers. Furthermore, a significantly greater fraction of colony queens were assigned as having originated from the population of residence, as compared to colony fathers. Inbreeding coefficients were significantly positive for workers, but not for mother queens. The queen-male relatedness coefficient of 0.23 (regression relatedness) indicates that mating occurs between fairly close relatives. These results suggest that some monogyne species of ants have complex dispersal and mating systems that can result in genetic isolation by distance over small geographical scales. More generally, this study also highlights the importance of identifying the relevant scale in analyses of population structure and dispersal.     

Chemical basis of nest-mate discrimination in the ant Formica exsecta.

Distinguishing nest-mates from non-nest-mates underlies key animal behaviours, such as territoriality, altruism and the evolution of sociality. Despite its importance, there is very little empirical support for such a mechanism in nature. Here we provide data that the nest-mate recognition mechanism in an ant is based on a colony-specific Z9-alkene signature, proving that surface chemicals are indeed used in ant nest-mate recognition as was suggested 100 years ago. We investigated the cuticular hydrocarbon profiles of 10 Formica exsecta colonies that are composed almost entirely of a Z9-alkene and alkane component. Then we showed that worker aggression is only elicited by the Z9-alkene part. This was confirmed using synthetic Z9-alkene and alkane blends matched to the individual colony profiles of the two most different chemical colonies. In both colonies, only glass beads with 'nest-mate' alkene profiles received reduced aggression. Finally, changing the abundance of a single Z9-alkene on live ants was shown to significantly increase the aggression they received from nest-mates in all five colonies tested. Our data suggest that nest-mate discrimination in the social insects has evolved to rely upon highly sensitive responses to relatively few compounds.     

Polymorphic microsatellite DNA markers in the ant Formica exsecta

Highly polymorphic genetic markers provide a useful tool for estimating important genetic parameters in studies of the evolution of sociality in insects. Here we report 14 polymorphic microsatellite markers developed in the ant Formica exsecta. The number of alleles found ranged between 3 and 18 per locus. These markers were developed for studying genetic population structure and mating structure in F. exsecta populations with varying social organizations (monogyne and polygyne types of societies). Cross‐species amplification indicated that some of the markers might be usable even in species belonging to different subfamilies.     

Coexistence of the social types: genetic population structure in the ant Formica exsecta

The ant Formica exsecta has two types of colonies that exist in sympatry but usually as separate subpopulations: colonies with simple social organization and single queens (M type) or colonial networks with multiple queens (P type). We used both nuclear (DNA microsatellites) and mitochondrial markers to study the transition between the social types, and the contribution of males and females in gene flow within and between the types. Our results showed that the social types had different spatial genetic structures. The M subpopulations formed a fairly uniform population, whereas the P subpopulations were, on average, more differentiated from each other than from the nearby M subpopulations and could have been locally established from the M-type colonies, followed by philopatric behavior and restricted emigration of females. Thus, the relationship between the two social types resembles that of source (M type) and sink (P type) populations. The comparison of mitochondrial (phiST) and nuclear (FST) differentiation indicates that the dispersal rate of males is four to five times larger than that of females both among the P-type subpopulations and between the social types. Our results suggest that evolution toward complex social organization can have an important effect on genetic population structure through changes in dispersal behavior associated with different sociogenetic organizations.     

High levels of multiple Wolbachia infection and recombination in the ant Formica exsecta

Wolbachia bacteria are intracellular symbionts of many arthropod species. Their spread through host populations is promoted by drastic alterations imposed on their hosts' reproductive physiology. In the present study, we analyzed the association between Wolbachia strains and host mitochondrial haplotypes in a Swiss population of the ant Formica exsecta. In this species, female dispersal is extremely limited and the mitochondrial haplotypes are strongly differentiated between and within subpopulations. Our study revealed exceptionally high levels of multiple infection, with all ants harboring four or five distinct Wolbachia strains. Four of these strains were present in all ants analyzed. A fifth strain was associated with only three of the five mitochondrial haplotypes. An analysis of the Wolbachia gene wsp further revealed an unexpected high rate of recombination, with three of the five Wolbachia strains appearing to have arisen by homologous recombination.     

Habitat age, breeding system and kinship in the ant Formica fusca

In polygyne ants (multiple queens per colony) factors that affect the distribution and survival of queens may play a key role in shaping the population-wide mating system and colony kin structure. The aim of this paper was to study the breeding system in two populations of different age in the facultatively polygyne ant Formica fusca. Both the observed numbers of queens, and the relatedness patterns among queens, workers and colony fathers were compared in two adjacent populations (ages 17 years and > 100 years) in Southern Finland. The results showed that both the mating system and colony kin structure differed between the study populations. In the old population the relatedness among workers, queens and colony fathers was high. The queens were also related to their mates, resulting in significant inbreeding in workers, but not in queens. Finally, the number of queens per colony fluctuated between years, suggesting queen turnover, and nest-mate queens shared their reproduction unequally (reproductive skew). In the younger population relatedness among queens and workers was lower than in the old population, and the colony fathers were unrelated. Furthermore, inbreeding was absent, and no conclusive evidence was found for reproductive skew among nest-mate queens. Finally, the number of queens per colony appeared more stable between years, although queen turnover occurred also in this population. The observed differences in dispersal and mating behaviour are discussed in the light of a potential connection between population age and habitat saturation.     

Restricted effective queen dispersal at a microgeographic scale in polygynous populations of the ant Formica exsecta

Ecological constraints on effective dispersal have been suggested to be a key factor influencing social evolution in animal societies as well as the shift from single queen colonies (monogyny) to multiple queen colonies (polygyny) in ants. However, little is known about the effective dispersal patterns of ant queens. Here we investigate the microgeographic genetic structure of mitochondrial haplotypes in polygynous populations of the ant Formica exsecta, both between pastures and among nests within pastures. An analysis of molecular variance revealed a very high genetic differentiation (phiST = 0.72) between pastures, indicating that queens rarely disperse successfully between pastures, despite the fact that pastures were sometimes as close as 1 km. Most of the pastures contained only a single haplotype, and haplotypes were frequently distinct between nearby pastures and even between groups of nests within the same pasture. In the three pastures that contained several haplotypes, haplotypes were not randomly distributed, the genetic differentiation between nests being phiST = 0.17, 0.52, and 0.69. This indicates that most queens are recruited within their parental colonies. However, a large proportion of nests contained more than one haplotype, demonstrating that colonies will sometimes accept foreign queens. The relatedness of mitochondrial genes among nestmates varied between 0.62 and 0.75 when relatedness was measured within each pasture and ranged between 0.72 and 1.0 when relatedness was assessed with all pastures as a reference population. Neighboring nests were more genetically similar than distant ones, and there was significant isolation by distance. This pattern may be due to new nests being formed by budding or by limited effective queen dispersal, probably on foot between neighboring nests. These results show that effective queen dispersal is extremely restricted even at a small geographical scale, a pattern consistent with the idea that ecological constraints are an important selective force leading to the evolution and maintenance of polygyny.     

Tolerance to drug-induced cell death favours the acquisition of multidrug resistance in Leishmania

The control of the protozoan parasite Leishmania relies on few drugs with unknown cellular targets and unclear mode of action. Several antileishmanials, however, were shown to induce apoptosis in Leishmania and this death mechanism was further studied in drug-sensitive and drug-resistant Leishmania infantum. In sensitive parasites, antimonials (SbIII), miltefosine (MF) and amphotericin B (AMB), but not paromomycin (PARO), triggered apoptotic cell death associated with reactive oxygen species (ROS). In contrast, Leishmania mutants resistant to SbIII, MF or AMB not only failed to undergo apoptosis following exposure to their respective drugs, but also were more tolerant towards apoptosis induced by other antileishmanials, provided that these killed Leishmania via ROS production. Such tolerance favored the rapid acquisition of multidrug resistance. PARO killed Leishmania in a non-apoptotic manner and failed to produce ROS. PARO resistance neither protected against drug-induced apoptosis nor provided an increased rate of acquisition of resistance to other antileishmanials. However, the PARO-resistant mutant, but not SbIII-, MF- or AMB-resistant mutants, became rapidly cross-resistant to methotrexate, a model drug also not producing ROS. Our results therefore link the mode of killing of drugs to tolerance to cell death and to a facilitated emergence of multidrug resistance. These findings may have fundamental implications in the field of chemotherapeutic interventions.     

Oxygen stress: a regulator of apoptosis in yeast.

Oxygen radicals are important components of metazoan apoptosis. We have found that apoptosis can be induced in the yeast Saccharomyces cerevisiae by depletion of glutathione or by low external doses of H2O2. Cycloheximide prevents apoptotic death revealing active participation of the cell. Yeast can also be triggered into apoptosis by a mutation in CDC48 or by expression of mammalian bax. In both cases, we show oxygen radicals to accumulate in the cell, whereas radical depletion or hypoxia prevents apoptosis. These results suggest that the generation of oxygen radicals is a key event in the ancestral apoptotic pathway and offer an explanation for the mechanism of bax-induced apoptosis in the absence of any established apoptotic gene in yeast.     

Disease resistance to Pectobacterium carotovorum is negatively modulated by the Arabidopsis Lectin Receptor Kinase LecRK-V.5

Plant stomata function in disease resistance by restricting bacteria entry inside leaves. During plant-bacteria interactions, stomatal closure is initiated by the recognition of Microbe-Associated Molecular Patterns (MAMPs). Recently, we have shown that the Lectin Receptor Kinase V.5 (LecRK-V.5) negatively regulates bacterium- and MAMP-induced stomatal closure upstream of Reactive Oxygen Species (ROS) production mediated by abscisic acid signaling. Closed stomata in lecrk-V.5 mutants are correlated with constitutive high level of ROS in guard cells. Consequently, lecrk-V.5 mutants are more resistant to hemi-biotrophic pathogen Pseudomonas syringae pv tomato DC3000 (Pst DC3000). In this report, we further investigate the role of LecRK-V.5 in resistance against necrotrophic bacteria Pectobacterium carotovorum ssp. carotovorum (Pcc). Upon surface-inoculation lecrk-V.5 mutants exhibited enhanced resistance against Pcc whereas a wild-type level of resistance was observed using infiltration-inoculation, an inoculation method that bypasses the epidermal barrier. Enhanced resistance of dip-inoculated lecrk-V.5 mutants against necrotrophic bacteria, that induce different defense responses than hemi-biotrophic bacteria, further suggests a possible role for LecRK-V.5 in stomatal immunity.     

Rapid anti-pathogen response in ant societies relies on high genetic diversity

Social organisms are constantly exposed to infectious agents via physical contact with conspecifics. While previous work has shown that disease susceptibility at the individual and group level is influenced by genetic diversity within and between group members, it remains poorly understood how group-level resistance to pathogens relates directly to individual physiology, defence behaviour and social interactions. We investigated the effects of high versus low genetic diversity on both the individual and collective disease defences in the ant Cardiocondyla obscurior. We compared the antiseptic behaviours (grooming and hygienic behaviour) of workers from genetically homogeneous and diverse colonies after exposure of their brood to the entomopathogenic fungus Metarhizium anisopliae. While workers from diverse colonies performed intensive allogrooming and quickly removed larvae covered with live fungal spores from the nest, workers from homogeneous colonies only removed sick larvae late after infection. This difference was not caused by a reduced repertoire of antiseptic behaviours or a generally decreased brood care activity in ants from homogeneous colonies. Our data instead suggest that reduced genetic diversity compromises the ability of Cardiocondyla colonies to quickly detect or react to the presence of pathogenic fungal spores before an infection is established, thereby affecting the dynamics of social immunity in the colony.     

Inbreeding and reproductive investment in the ant Formica exsecta

In social animals, inbreeding depression may manifest by compromising care or resources individuals receive from inbred group members. We studied the effects of worker inbreeding on colony productivity and investment in the ant Formica exsecta. The production of biomass decreased with increasing inbreeding, as did biomass produced per worker. Inbred colonies produced fewer gynes (unmated reproductive females), whereas the numbers of males remained unchanged. As a result, sex ratios showed increased male bias, and the fraction of workers increased among the diploid brood. Males raised in inbred colonies were smaller, whereas the weight of gynes remained unchanged. The results probably reflect a trade-off between number and quality of offspring, which is expected if the reproductive success of gynes is more dependent on their weight or condition than it is for males. As males are haploid (with the exception of abnormal diploid males produced in very low frequencies in this population), and therefore cannot be inbred themselves, the effect on their size must be mediated through the workers of the colony. We suggest the effects are caused by the inbred workers being less proficient in feeding the growing larvae. This represents a new kind of social inbreeding depression that may affect sex ratios as well as caste fate in social insects.     

Basal superoxide as a sex-specific immune constraint

There is increasing evidence that reactive oxygen species (ROS), a group of unstable and highly reactive chemical molecules, play a key role in regulating and maintaining life-history trade-offs. Upregulation of ROS in association with immune activation is costly because it may result in an imbalance between pro- and antioxidants and, hence, oxidative damage. Previous research aimed at quantifying this cost has mostly focused on changes in the pro-/antioxidant balance subsequent to an immune response. Here, we test the hypothesis that systemic ROS may constrain immune activation. We show that systemic, pre-challenge superoxide (SO) levels are negatively related to the strength of the subsequent immune response towards the mitogen phytohaemagglutinin in male, but not female painted dragon lizards (Ctenophorus pictus). We therefore suggest that systemic SO constrains immune activation in painted dragon males. We speculate that this may be due to sex-specific selection pressures on immune investment.     

Roles of YUCCAs in auxin biosynthesis and drought stress responses in plants

Auxin, a plant hormone, plays crucial roles in diverse aspects of plant growth and development reacting to and integrating environmental stimuli. Indole-3-acetic acid (IAA) is the major plant auxin that is synthesized by members of the YUCCA (YUC) family of flavin monooxygenases that catalyse a rate-limiting step. Although the paths to IAA biosynthesis are characterized in Arabidopsis, little is known about the corresponding components in potato. Recently, we isolated eight putative StYUC (Solanum tuberosum YUCCA) genes and five putative tryptophan aminotransferase genes in comparison to those found in Arabidopsis.¹ The specific domains of YUC proteins were well conserved in all StYUC amino acid sequences. Transgenic potato (Solanum tuberosum cv. Jowon) overexpressing AtYUC6 showed high-auxin and enhanced drought tolerance phenotypes. The transgenic potatoes also exhibited reduced levels of ROS (reactive oxygen species) compared to control plants. We therefore propose that YUCCA and TAA families in potato would function in the auxin biosynthesis. The overexpression of AtYUC6 in potato establishes enhanced drought tolerance through regulated ROS homeostasis.     

Cortex proliferation in the root is a protective mechanism against abiotic stress

Although as an organ the root plays a pivotal role in nutrient and water uptake as well anchorage, individual cell types function distinctly. Cortex is regarded as the least differentiated cell type in the root, but little is known about its role in plant growth and physiology. In recent studies, we found that cortex proliferation can be induced by oxidative stress. Since all types of abiotic stress lead to oxidative stress, this finding suggests a role for cortex in coping with abiotic stress. This hypothesis was tested in this study using the spy mutant, which has an extra layer of cortex in the root. Interestingly, the spy mutant was shown to be hypersensitive to salt and oxidizing reagent applied to the leaves, but it was as tolerant as the wild type to these compounds in the soil. This result lends support to the notion that cortex has a protective role against abiotic stress arising from the soil.     

Respiratory patterns in nurses of the red wood ant Formica polyctena (Hymenoptera, Formicidae)

Summary Cycles of discontinuous gas exchange (DGCs) and abdominal ventilatory movements were studied in nurses of red wood ant, Formica polyctena, using an electrolytic respirometer and an infra-red (IR) gas analyser or flow-through respirometry. Both respirometry systems were combined with an IR actographic device based on IR-emitting and IR-sensor diodes. After recovering from handling and apparatus stress, lasting 1–3 h, completely motionless intact ants displayed regular DGC. After decapitation the ants displayed DGC whose frequency was somewhat lower than that of the intact individuals (7.17 ± 0.79 mHz and 10.43 ± 01.12 mHz, respectively). In headless ants, there occurred continuous slow movements of legs. Bursts of carbon dioxide in the intact and in the headless ants always coincided with a bout of telescoping movements (contractions) of abdominal segments, which was interpreted as active ventilation. During interburst periods, the headless ants exhibited telescoping movements characterised by rapid protraction, lasting 0.07–0.09 s, followed by a slow retraction of segments, suggesting passive ventilation. The intact ants were very sensitive to the flowing air and tended to be continuously active during flow-through respirometry. The decapitated ants, on the contrary, were insensitive to the air current.Received 26 March 2003; revised 30 July 2003; accepted 6 August 2003.     

Reproductive specialization in multiple-queen colonies of the ant Formica exsecta

In polygynous (multiple queens per nest) colonies of socialinsects, queens can increase their reproductive share by layingmore eggs or by increasing the proportion of eggs that developinto reproductive individuals instead of workers. We used polymorphicmicrosatellite loci to determine the genetically effective contributionof queens to the production of gynes (new queens), males, and2 different cohorts of workers in a polygynous population ofthe ant Formica exsecta. For this purpose, we developed a newmethod that can be used for diploid and haplodiploid organismsto quantify the degree of reproductive specialization amongbreeders in societies where there are too many breeders to ascertainparentage. Using this method, we found a high degree of reproductivespecialization among nest-mate queens in both female- and male-producingcolonies (sex ratio is bimodally distributed in the study population).For example, a high effective proportion of queens (25% and79%, respectively) were specialized in the production of malesin female- and male-producing colonies. Our analyses furtherrevealed that in female-producing colonies, significantly fewerqueens contributed to gyne production than to worker production.Finally, we found significant changes in the identity of queenscontributing to different cohorts of workers. Altogether, thesedata demonstrate that colonies of F. exsecta, and probably thoseof many other highly polygynous social insect species, are composedof reproductive individuals differing in their investment togynes, males, and workers. These findings demonstrate a newaspect of the highly dynamic social organization of complexanimal societies.     

Dimethylsulfoniopropionate,superoxide dismutase and glutathione as stress response indicators in three corals under short-term hyposalinity stress

Corals are among the most active producers of dimethylsulfoniopropionate (DMSP), a key molecule in marine sulfur cycling, yet the specific physiological role of DMSP in corals remains elusive. Here, we examine the oxidative stress response of three coral species (Acropora millepora, Stylophora pistillata and Pocillopora damicornis) and explore the antioxidant role of DMSP and its breakdown products under short-term hyposalinity stress. Symbiont photosynthetic activity declined with hyposalinity exposure in all three reef-building corals. This corresponded with the upregulation of superoxide dismutase and glutathione in the animal host of all three species. For the symbiont component, there were differences in antioxidant regulation, demonstrating differential responses to oxidative stress between the Symbiodinium subclades. Of the three coral species investigated, only A. millepora provided any evidence of the role of DMSP in the oxidative stress response. Our study reveals variability in antioxidant regulation in corals and highlights the influence life-history traits, and the subcladal differences can have on coral physiology. Our data expand on the emerging understanding of the role of DMSP in coral stress regulation and emphasizes the importance of exploring both the host and symbiont responses for defining the threshold of the coral holobiont to hyposalinity stress.     

Functional and evolutionary trade-offs co-occur between two consolidated memory phases in Drosophila melanogaster

Memory is a complex and dynamic process that is composed of different phases. Its evolution under natural selection probably depends on a balance between fitness benefits and costs. In Drosophila, two separate forms of consolidated memory phases can be generated experimentally: anaesthesia-resistant memory (ARM) and long-term memory (LTM). In recent years, several studies have focused on the differences between these long-lasting memory types and have found that, at the functional level, ARM and LTM are antagonistic. How this functional relationship will affect their evolutionary dynamics remains unknown. We selected for flies with either improved ARM or improved LTM over several generations, and found that flies selected specifically for improvement of one consolidated memory phase show reduced performance in the other memory phase. We also found that improved LTM was linked to decreased longevity in male flies but not in females. Conversely, males with improved ARM had increased longevity. We found no correlation between either improved ARM or LTM and other phenotypic traits. This is, to our knowledge, the first evidence of a symmetrical evolutionary trade-off between two memory phases for the same learning task. Such trade-offs may have an important impact on the evolution of cognitive capacities. On a neural level, these results support the hypothesis that mechanisms underlying these forms of consolidated memory are, to some degree, antagonistic.