Reproductive isolation between Pogonomyrmex rugosus and two lineages with genetic caste determination

Hybrid speciation occurs when combination of two interspecific genomes results in individuals that are of high fitness but reproductively incompatible with the parental species. Although hybrid speciation is a relatively common source of new species in plants, it appears to be a much rarer occurrence in animal taxa. Here we report on reproductive isolation and range overlap between the rough harvester ant Pogonomyrmex rugosus and two lineages with hybrid genotypes (H 1 and H2). Both lineages obligately interbreed and produce genetically distinct queen and worker offspring, a phenomenon referred to as genetic caste determination (GCD). Diploid offspring produced by gametes of the same lineage develop only into queens, whereas diploid offspring derived from gametes of distinct lineages develop into workers. We investigated small-scale patterns of gene flow between the parent and the two H lineages by sampling along an 80-km transect between a pure P. rugosus population and a two-lineage population. Microsatellite and mitochondrial markers both indicated virtually no gene flow between the parent species and either lineage even at sites where parental and H-lineage colonies co-occurred. The geographic ranges of the parental species and the two-lineage population were essentially parapatric, with a surprisingly narrow band of overlap and evidence of spatial structuring even at microgeographic scales within the transition zone. This suggests that ecological competition with the parent species plays a significant role in determining the evolutionary persistence and current distribution of the hybrid lineages and the genetic caste system.     

Sperm parasitism in ants: selection for interspecific mating and hybridization

Interspecific mating in eusocial Hymenoptera can be favored under certain conditions even if all hybrid offspring are completely infertile. This exploits two key features of the eusocial Hymenoptera: a haplodiploid genetic system and reproductive division of labor in females. Interspecifically mated queens can still produce viable sons that will mate intraspecifically. Apparent reduced fitness resulting from producing infertile daughter gynes can be also offset by advantages conferred by hybrid workers. An important advantage is likely to be superior ability at using marginal habitats. Interspecifically mated queens can nest in sites where intraspecific competition will be low. By mating interspecifically, a queen trades expected reproductive success through female offspring for a higher probability of achieving some reproductive success. Females that mate interspecifically can be considered "sperm parasites" on the males of the other species. I provide evidence that sperm parasitism is responsible for widespread hybridization in North America among two species of the ant subgenus Acanthomyops (genus Lasius), and review evidence for sperm parasitism in other hybridization phenomena in ants. Sperm parasitism in ants represents a novel form of social parasitism in ants and a dispersal polymorphism. It may also act as a precursor to the evolution of some other recently discovered phenomena, such as genetic caste determination.     

All-offspring dispersal in a tropical mammal with resource defense polygyny

In polygynous mammals, males are usually responsible for gene flow while females are predominantly philopatric. However, there is evidence that in a few mammalian species female offspring may disperse to avoid breeding with their father when male tenure exceeds female age at maturity. We investigated offspring dispersal and local population structure in the Neotropical bat Lophostoma silvicolum. The mating system of this species is resource defense polygyny, with the resource being active termite nests, excavated by single males, which are then joined by females. We combined field observations of 14 harems during 3 years and data about the genetic structure within and between these groups, calculated with one mitochondrial locus and nine nuclear microsatellite loci. The results show that both male and female offspring disperse before maturity. In addition, we estimated life span of excavated termite nests and the duration they were occupied by the same male. Our findings suggest that long male tenure of up to 30 months is indeed a likely cause for the observed dispersal by female offspring that can reach maturity at a low age of 6 months. We suggest that dispersal by offspring of both sexes may occur quite frequently in polygynous tropical bats and thus generally may be more common in mammals than previously assumed.     

Patterns of male parentage in the fungus-growing ants

Ant queens from eight species, covering three genera of lower and two genera of higher attine ants, have exclusively or predominantly single mating. The ensuing full-sib colonies thus have a strong potential reproductive conflict between the queen and the workers over male production. This is because, all other things being equal, relatedness incentives should favour traits expressed in both workers and the queen to monopolise the production of the colony's male offspring. Microsatellite genotyping of males from these attine species shows that workers in queenless colonies are able to produce males, but that no worker-produced males were found in queenright colonies. Our results suggest that worker reproduction is rare or even absent in colonies with a fertile queen. This indicates that either the queen directly prevents the workers from raising their own sons, or that worker reproduction is absent in the presence of a fertile queen due to high ergonomic costs of this behaviour.     

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.     

Control of reproduction in social insect colonies: individual and collective relatedness preferences in the paper wasp, Polistes annularis

Social insect colonies often have one or a few queens. How these queens maintain their reproductive monopoly, when other colony members could gain by sharing in the reproduction, is not generally known. DNA microsatellite genotyping is used to determine reproductive interests of various classes of colony members in the paper wasp, Polistes annularis. The relatedness estimates show that the best outcome for most individuals is to be the reproductive egg-layer. For workers, this depends on the sex of offspring: they should prefer to lay their own male eggs, but are indifferent if the queen lays the female eggs. The next-best choice is usually to support the current queen. As a rule, subordinates and workers should prefer the current queen to reproduce over other candidates (though subordinates have no strong preference for the queen over other subordinates, and workers may prefer other workers as a source of male eggs). This result supports the theory that reproductive monopoly stems from the collective preferences of non-reproductives, who suppress each other in favor of the queen. However, we reject the general hypothesis of collective worker control in this species because its predictions about who should succeed after the death of the present queen are not upheld. The first successor is a subordinate foundress even though workers should generally prefer a worker successor. If all foundresses have died, an older worker succeeds as queen, in spite of a collective worker preference for a young worker. The results support the previous suggestion that age serves as a conventional cue serving to reduce conflict over queen succession. Received: 3 May 1996 / Accepted after revision: 22 September 1996     

Worker reproduction in the ponerine ant Ophthalmopone berthoudi: an alternative form of eusocial organization

Summary Colonies of the ponerine Ophthalmopone berthoudi were collected throughout the year. The queen caste is absent. Dissection of large numbers of workers revealed that many of them (up to 100 in one nest) are inseminated and produce eggs. The ovaries are small and contain very few mature oocytes, indicating that there is a slow rate of egg-laying. Workers are produced throughout the year, and all are capable of becoming functional reproductives. However, only those that are sexually-attractive during the limited period of male activity become mated. Thus the percentages of mated laying workers (=gamergates) fluctuate seasonally (Fig. 2). Successive generations of gamergates do not overlap. Observation of nests in the field and in the laboratory indicated that gamergates were never active above ground. There is no aggression between them, and their numbers are not socially regulated. There are few interactions between gamergates and non-reproductive workers, and the former do not acquire more food during termite meals. The notion of parental oppression is undermined by the complete loss of the meen caste, while the nature of the breeding system of this ant leads to the prediction of low relatedness between nestmates.     

Relatedness,recognition errors,and colony fusion in the termite <Emphasis Type="Italic">Nasutitermes corniger</Emphasis>

Loss of aggression between social groups can have far-reaching effects on the structure of societies and populations. We tested whether variation in the genetic structure of colonies of the termite Nasutitermes corniger affects the probability of aggression toward non-nestmates and the ability of unrelated colonies to fuse. We determined the genotypes of workers and soldiers from 120 colonies at seven polymorphic microsatellite loci. Twenty-seven colonies contained offspring of multiple founding queens or kings, yielding an average within-colony relatedness of 0.33. Genotypes in the remaining 93 colonies were consistent with reproduction by a single queen and king or their progeny, with an average within-colony relatedness of 0.51. In standardized assays, the probability of aggression between workers and soldiers from different colonies was an increasing function of within-colony relatedness. The probability of aggression was not affected significantly by the degree of relatedness between colonies, which was near zero in all cases, or by whether the colonies were neighbors. To test whether these assays of aggression predict the potential for colony fusion in the field, we transplanted selected nests to new locations. Workers and soldiers from colonies that were mutually tolerant in laboratory assays joined their nests without fighting, but workers and soldiers that were mutually aggressive in the assays initiated massive battles. These results suggest that the presence of multiple unrelated queens or kings promotes recognition errors, which can lead to the formation of more complex colony structures.     

Incest avoidance,fluctuating asymmetry,and the consequences of inbreeding in Iridomyrmex humilis,an ant with multiple queen colonies

Summary Inbreeding may have important consequences for the genetic structure of social insects and thus for sex ratios and the evolution of sociality and multiple queen (polygynous) colonies. The influence of kinship on mating preferences was investigated in a polygynous ant species, Iridomyrmex humilis, which has within-nest mating. When females were presented simultaneously with a brother that had been reared in the same colony until the pupal stage and an unrelated male produced in another colony, females mated preferentially with the unrelated male. The role of environmental colony-derived cues was tested in a second experiment where females were presented with two unrelated males, one of which had been reared in the same colony until the pupal stage (i.e., as in the previous experiment), while the other had been produced in another colony. In this experiment there was no preferential mating with familiar or unfamiliar males, suggesting that colony-derived cues might not be important in mating preferences. Inbreeding was shown to have no strong effect on the reproductive output of queens as measured by the number of worker and sexual pupae produced. The level of fluctuating asymmetry of workers produced by inbreeding queens was not significantly higher than that of non-inbreeding queens. Finally, colonies headed by inbreeding queens did not produce adult diploid males. Based on the current hypotheses of sex-determination the most plausible explanations for the absence of diploid-male-producing colonies are that (i) workers recognized and eliminated these males early in their development, and/or (ii) there are multiple sex-determining loci in this species. It is suggested that even if inbreeding effects on colony productivity are absent or low, incest avoidance mechanisms may have evolved and been maintained if inbreeding queens produce a higher proportion of unviable offspring. Correspondence to: L. Keller at the present address     

A genetic component to size in queens of the ant,<Emphasis Type="Italic"> Formica truncorum</Emphasis>

The genetic basis of morphological traits in social insects remains largely unexplored. This is even true for individual body size, a key life-history trait. In the social insects, the size of reproductive individuals affects dispersal decisions, so that small size in queens is often associated with reduced dispersal, and large size with long-range dispersal and independent colony founding. Worker size is connected to division of labour when workers specialize in certain tasks according to their size. In many species, variation in worker size has been shown to increase colony performance. In this study, we present the first evidence of an additive genetic component to queen size in ants, using maternal half sib analysis. We also compared intra-colony size variation in colonies with high (queen doubly mated) versus low (queen singly mated) genetic variability. We found a high and significant heritability (h²=0.51) for queen size in one of the two study years, but not in the other. Size variation among queens was greater in colonies headed by a doubly mated queen in one of the study years, but not in the other. This indicates that genetic factors can influence queen size, but that environmental factors may override these under some circumstances. The heritability for worker size was low (h²=0.09) and non-significant. Increased genetic diversity did not increase worker size variation in the colonies. Worker size appeared largely environmentally determined, potentially allowing colonies to adjust worker size ratios to current conditions.Communicated by J. Heinze     

Foundress polyphenism and the origins of eusociality in a facultatively eusocial sweat bee, Megalopta genalis (Halictidae)

The reproductive (queen) and nonreproductive (worker) castes of eusocial insect colonies are a classic example of insect polyphenism. A complementary polyphenism may also exist entirely among females in the reproductive caste. Although less studied, reproductive females may vary in behavior based on size-associated attributes leading to the production of daughter workers. We studied a bee with flexible social behavior, Megalopta genalis, to better understand the potential of this polyphenism to shape the social organization of bee colonies and, by extension, its role in the evolution of eusociality. Our experimental design reduced variation among nest foundresses in life history variables that could influence reproductive decisions, such as nesting quality and early adulthood experience. Within our study population, approximately one third of M. genalis nests were eusocial and the remaining nests never produced workers. Though they do not differ in survival, nest-founding females who do not attempt to produce workers (which we refer to as the solitary phenotype) are significantly smaller and become reproductive later than females who attempt to recruit workers (the social phenotype). Females with the social phenotype are more likely to produce additional broods but at a cost of having some of their first offspring become nonreproductive workers. The likelihood of eusocial organization varies with body size across females of the social phenotype. Thus, fitness consequences associated with size-based plasticity in foundress behavior has colony level effects on eusociality. The potential for size-based polyphenisms among reproductive females may be an important factor to consider in the evolutionary origins of eusociality.     

Heirs and spares: caste conflict and excess queen production in Melipona bees

The caste conflict hypothesis states that there is potential conflict over the caste fate of totipotent immature females in social insects. In most species, an immature female has little control over her fate because workers control her nutrition. However, in Melipona bees, immature females should have considerable control over their own caste fate because they develop on a provision mass in a sealed cell, and because queens are not larger than workers. This may explain why, in Melipona, large numbers of queens are reared only to be executed. (Because Melipona colonies are founded by swarms very few reproductive opportunities for adult queens occur.) This study uses a one-locus genetic model to determine the optimum proportion of females that should develop into queens from the perspective of immature totipotent females who control their own caste fate. For a population in which all colonies are headed by a single, single-mated queen, which is the typical situation in Melipona, the optimum rises from 14-20% as male production by workers declines from 100% to zero. This agrees well with previous studies which, collectively, give an average of 22% of females developing into queens.     

Maternity of replacement queens in the thelytokous Cape honey bee Apis mellifera capensis

Unlike workers of all other honey bee (Apis mellifera) subspecies, workers of the Cape honey bee of South Africa (A. mellifera capensis) reproduce thelytokously and are thus able to produce female offspring that are pseudoclones of themselves. This ability allows workers to compete with their queen over the maternity of daughter queens and, in one extreme case, has led to a clonal lineage of workers becoming a social parasite in commercially managed populations of A. mellifera scutellata. Previous work (Jordan et al., Proc R Soc Lond B Biol Sci 275:345, 2008) showed that, in A. mellifera capensis, 59% of queen cells produced during swarming events contained the offspring of workers and that, of these, 65% were the offspring of non-natal workers. Here, we confirm that a substantial proportion (38.5%) of offspring queens is worker-laid. We additionally show that: (1) Although queens produce most diploid female offspring sexually, we found some homozygous or hemizygous queen offspring, suggesting that queens also reproduce by thelytoky. These parthenogenetic individuals are probably nonviable beyond the larval stage. (2) Worker-laid offspring queens are viable and become the resident queen at the same frequency as do sexually produced queen-laid offspring queens. (3) In this study, all but one of the worker-derived queens were laid by natal workers rather than workers from another nest. This suggests that the very high rates of social parasitism observed in our previous study were enhanced by beekeeping manipulations, which increased movement of parasites between colonies.     

Low queen mating frequency in the seed-harvester ant Pogonomyrmex (Ephebomyrmex) pima: implications for the evolution of polyandry

The evolution of polyandry is a central problem in the study of insect mating systems, and both material and genetic benefits have been proposed to offset the presumed costs of multiple mating. Although most eusocial Hymenoptera queens mate with just one or occasionally two males, high levels of polyandry are exhibited by several taxa, including seed-harvester ants of the genus Pogonomyrmex. Previous studies of queen mating frequency in Pogonomyrmex have focused on monogynous (one queen per colony) species in the subgenus Pogonomyrmex. We performed a genetic mother–offspring analysis of mating frequency in Pogonomyrmex (Ephebomyrmex) pima, a queen-dimorphic species with dealate and intermorph queens that differ in colony structure (intermorph colonies contain multiple queens). Our results demonstrate that both dealate and intermorph queens of P. (E.) pima are typically single maters, unlike their congeners analyzed thus far. Polyandry appears to be a derived trait in Pogonomyrmex, but comparative tests between P. (E.) pima queen morphs and across the genus provide no evidence that it evolved as an adaptation to increase genetic diversity within colonies or to obtain more sperm, respectively.     

Brood production and lineage discrimination in the red harvester ant (Pogonomyrmex barbatus)

In contrast to the system of caste determination in most social insects, reproductive caste determination in some populations of Pogonomyrmex barbatus has a genetic basis. Populations that exhibit genetic caste determination are segregated into two distinct, genetic lineages. Same-lineage matings result in female reproductives, while inter-lineage matings result in workers. To investigate whether founding P. barbatus queens lay eggs of reproductive genotype, and to determine the fate of those eggs, we genotyped eggs, larvae, and pupae produced by naturally inseminated, laboratory-raised queens. We show that founding dependent lineage queens do lay eggs of reproductive genotype, and that the proportion of reproductive genotypes decreases over the course of development from eggs to larvae to pupae. Because queens must mate with a male of each lineage to produce both workers and female reproductives, it would benefit queens to be able to distinguish males of the two lineages. Here we show that P. barbatus males from the two genetic lineages differ in their cuticular hydrocarbon profiles. Queens could use male cuticular hydrocarbons as cues to assess the lineage of males at the mating aggregation, and possibly keep mating until they have mated with males of both lineages.     

Parentage and sex allocation in the facultatively polygynous ant Myrmicatahoensis

Most social groups have the potential for reproductive conflict among group members. Within insect societies, reproduction can be divided among multiple fertile individuals, leading to potential conflicts between these individuals over the parentage of sexual offspring. Colonies of the facultatively polygynous ant Myrmicatahoensis contain from one to several mated queens. In this species, female sexuals were produced almost exclusively by one queen. The parentage of male sexuals was more complex. In accordance with predictions based on worker sex-allocation preferences, male-producing colonies tended to have low levels of genetic relatedness (i.e., high queen numbers). Correspondingly, males were often reared from the eggs of two or more queens in the nest. Further, over half of the males produced appeared to be the progeny of fertile workers, not of queens. Overall investment ratios were substantially more male biased than those predicted by genetic relatedness, suggesting hidden costs associated with the production of female sexuals. These costs are likely to include local resource competition among females, most notably when these individuals are adopted by their maternal nest. Received: 3 March 1998 / Accepted after revision: 20 June 1998     

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

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

Social regulation of ovary activation in ’anarchistic’ honey-bees (Apis mellifera)

Honey-bee (Apis mellifera) colonies exhibit extreme reproductive division of labour. Workers almost always have inactive ovaries and the queen monopolises egg laying. Although extremely rare, ’anarchistic’ colonies exist in which workers produce male offspring despite the presence of the queen. By comparing the rates of ovary activation in anarchistic and wild-type bees fostered to host colonies of different genotype (i.e. anarchist and non-anarchist) and queen status (i.e. queenless and queenright), we investigated the factors involved in inhibiting ovary activation. Fostered anarchist workers always had a higher level of ovary development than fostered wild-type bees in both anarchist and non-anarchist host colonies. Fostered workers of both genotypes had more active ovaries in anarchistic than in wild-type hosts. Fostered workers of both strains also had more active ovaries in queenless than in queenright hosts. The results suggest that selection for worker reproduction in the anarchistic line has both reduced the effects of brood and queen pheromones on worker ovary inhibition and increased the likelihood that workers of the anarchistic line will develop ovaries compared to wild-type workers. Received: 14 June 2000 / Revised: 26 September 2000 / Accepted: 7 October 2000     

Extremely high mating frequency in the Florida harvester ant (<Emphasis Type="Italic">Pogonomyrmex badius</Emphasis>)

High effective multiple queen mating is a rare but taxonomically widespread phenomenon in eusocial Hymenoptera that has arisen convergently in several taxa. In ants, high effective mating frequencies have been confirmed in only two clades: the higher leaf-cutters (Atta, Acromyrmex) and the Pogonomyrmex seed harvesters. We analysed polyandry in Pogonomyrmex badius, which has a life-history unique within the genus, and report the highest numerical mating frequencies thus far recorded in ants. We also show that P. badius is characterized by one of the highest effective mating frequencies hitherto found in ants. It is now clear that all major sub-clades of Pogonomyrmex sensu stricto exhibit high levels of polyandry. Therefore, multiple mating must have arisen early in the evolution of the genus, and may have constituted a mechanism to increase offspring variability for queens that were confronted with increasingly complex levels of organization. Too few congeners have been investigated by the same method to be certain that polyandry in P. badius is really higher than in the rest of the genus. If so, research should concentrate on a possible link between high queen mating frequency and the distinct caste system found in P. badius.Communicated by L. Keller     

Genetic caste determination in harvester ants: possible origin and maintenance by cyto-nuclear epistasis

While reproductive caste in eusocial insects is usually determined by environmental factors, in some populations of the harvester ants, Pogonomyrmex barbatus and P. rugosus, caste has been shown to have a strong genetic component. This system of genetic caste determination (GCD) is characterized by between-caste nuclear variation and high levels of mitochondrial haplotype variation between alternative maternal lineages. Two previous genetic models, involving a single nuclear caste-determining locus or interactions between two nuclear loci, respectively, have been proposed to explain the GCD system. We propose a new model based on interactions between nuclear and mitochondrial genes that can better explain the co-maintenance of distinct nuclear and mitochondrial lineages. In our model, females with coevolved cyto-nuclear gene complexes, derived from intra-lineage mating, develop into gynes, while females with disrupted cyto-nuclear complexes, derived from inter-lineage mating, develop into workers. Both haplodiploidy and inbreeding facilitate the buildup of such coevolved cyto-nuclear complexes within lineages. In addition, the opportunity for both intra-lineage and inter-lineage mating in polyandrous populations facilitates the accumulation of gyne-biasing genes. This model may also help to explain the evolution of workerless social, parasites. We discuss similarities of GCD and cytoplasmic male sterility in plants and how worker production of males would affect the stability of GCD. Finally, we propose experiments and observations that might help resolve the origin and maintenance of this unusual system of caste determination.