Phylogeography of Pogonomyrmex barbatus and P. rugosus harvester ants with genetic and environmental caste determination

We present a phylogeographic study of at least six reproductively isolated lineages of new world harvester ants within the Pogonomyrmex barbatus and P. rugosus species group. The genetic and geographic relationships within this clade are complex: Four of the identified lineages show genetic caste determination (GCD) and are divided into two pairs. Each pair has evolved under a mutualistic system that necessitates sympatry. These paired lineages are dependent upon one another because their GCD requires interlineage matings for the production of F1 hybrid workers, and intralineage matings are required to produce queens. This GCD system maintains genetic isolation among these interdependent lineages, while simultaneously requiring co-expansion and emigration as their distributions have changed over time. It has also been demonstrated that three of these four GCD lineages have undergone historical hybridization, but the narrower sampling range of previous studies has left questions on the hybrid parentage, breadth, and age of these groups. Thus, reconstructing the phylogenetic and geographic history of this group allows us to evaluate past insights and hypotheses and to plan future inquiries in a more complete historical biogeographic context. Using mitochondrial DNA sequences sampled across most of the morphospecies’ ranges in the U.S.A. and Mexico, we conducted a detailed phylogeographic study. Remarkably, our results indicate that one of the GCD lineage pairs has experienced a dramatic range expansion, despite the genetic load and fitness costs of the GCD system. Our analyses also reveal a complex pattern of vicariance and dispersal in Pogonomyrmex harvester ants that is largely concordant with models of late Miocene, Pliocene, and Pleistocene range shifts among various arid-adapted taxa in North America.     

Genetic caste determination in Pogonomyrmex harvester ants imposes costs during colony founding

Some populations of Pogonomyrmex harvester ants comprise genetically differentiated pairs of interbreeding lineages. Queens mate with males of their own and of the alternate lineage and produce pure-lineage offspring which develop into queens and inter-lineage offspring which develop into workers. Here we tested whether such genetic caste determination is associated with costs in terms of the ability to optimally allocate resources to the production of queens and workers. During the stage of colony founding, when only workers are produced, queens laid a high proportion of pure-lineage eggs but the large majority of these eggs failed to develop. As a consequence, the number of offspring produced by incipient colonies decreased linearly with the proportion of pure-lineage eggs laid by queens. Moreover, queens of the lineage most commonly represented in a given mating flight produced more pure-lineage eggs, in line with the view that they mate randomly with the two types of males and indiscriminately use their sperm. Altogether these results predict frequency-dependent selection on pairs of lineages because queens of the more common lineage will produce more pure-lineage eggs and their colonies be less successful during the stage of colony founding, which may be an important force maintaining the coexistence of pairs of lineages within populations.     

Two alternate mechanisms contribute to the persistence of interdependent lineages in Pogonomyrmex harvester ants

Some populations of Pogonomyrmex harvester ants comprise pairs of highly differentiated lineages with queens mating at random with several males of their own and of the alternate lineage. These queens produce two types of diploid offspring, those fertilized by males of the queens' lineage which develop into new queens and those fertilized by males of the other lineage which mostly develop into functionally sterile workers. This unusual mode of genetic caste determination has been found in 26 populations and a total of four lineage pairs (F(1)-F(2), G(1)-G(2), H(1)-H(2) and J(1)-J(2)) have been described in these populations. Despite the fact that a few interlineage queens are produced, previous studies revealed that there is a complete lack of genetic introgression between lineages. Here we quantify the proportion of interlineage queens produced in each of the four lineage pairs and determine the fate of these queens. In the F(1)-F(2), G(1)-G(2) and H(1)-H(2) lineage pairs, interlineage queens were produced by a minority of colonies. These colonies exclusively produced interlineage queens and workers, suggesting that interlineage eggs can develop into queens in these three pairs of lineages in the absence of competition with pure-lineage brood. An analysis of three key stages of the colony life cycle revealed that colonies headed by interlineage queens failed to grow sufficiently to produce reproductive individuals. In laboratory comparisons, interlineage queens produced fewer viable eggs, with the effect that they raised fewer workers and lost more weight per worker produced than pure-lineage queens. In the J(1)-J(2) lineage pair, we did not find a single interlineage queen, raising the possibility that interlineage eggs have completely lost the ability to develop into queens in this lineage pair. Hence, two distinct mechanisms seem to account for the complete lack of between-lineage gene flow in the F(1)-F(2), G(1)-G(2), H(1)-H(2) and J(1)-J(2) lineage pairs.     

Ergatoid queens and intercastes in ants: Two distinct adult forms which look morphologically intermediate between workers and winged queens

Summary The term ergatogyne is used in ants to describe permanently-wingless female adults which are morphologically intermediate between workers and winged queens. This definition is ambiguous because there are two distinct categories of ergatogynes: ergatoid queens and intercastes. Both have an external appearance (ocelli and alitrunk structure) which combines traditional queen and worker characters, and thus can be confused if they both function as reproductives — however intercastes in most species cannot reproduce.Ergatoid queens have replaced winged queens in a substantial number of species. They are sometimes externally similar to conspecific workers, especially in various ponerine species which exhibit limited size dimorphism between castes. Ergatoid queens retain the specialized attributes of a reproductive caste, including larger ovaries, and they are always the functional egg-layers in a colony. In contrast, conspecific intercastes represent various graded stages in a series connecting workers and winged queens, and they occur together with the queens. These hybrid phenotypes result from deviations from the normal pattern of caste differentiation during larval development. Intercastes generally lack a spermatheca and have no reproductive function; however they can mate in a few leptothoracine ants, and then reproduce instead of winged queens in a proportion of colonies.     

Diet and seed removal rates by the harvester ants Pogonomyrmex rastratus and Pogonomyrmex pronotalis in the central Monte desert, Argentina

Granivorous animals, through seed consumption, may have an important influence on plant abundance, distribution and species composition in desert ecosystems. The aims of this study are twofold: to quantify the diet of Pogonomyrmex rastratus (Mayr) and pogonomyrmex pronotalis (Santschi), and to estimate seed removal per colony of both species throughout their activity season (October-April) in the central Monte desert, Argentina. Both species rely heavily upon seeds, which account for 87–94 % of the items carried to the nests. Their diets are similar, consisting mainly of grass seeds, which represent more than 93 % of the seeds. Among them, three species predominate: Aristida spp., Trichloris crinita and Pappophorum spp. Seasonal variations as well as seed species richness in the diet are also similar between species. However, their food-handling behaviour differs: most caryopses carried by P. pronotalis bear bracts whereas most caryopses carried by P. rastratus lack them. Seed removal per colony by P. rastratus (6 × 10⁴ seeds/colony) and by P. pronotalis (5 × 10⁴ seeds/colony) throughout the season is similar to the one reported for P. occidentalis in North America. However, seed removal per hectare, which could be estimated for P. rastratus (8.3 × 10⁵ seeds/ha), is lower than removal rates reported for the North American species P. barbatus, P. desertorum, P. rugosus and P. californicus, probably because P. rastratus has lower activity levels and smaller colonies than the North American studied species. Received 6 April 2005; revised 1 September 2005; accepted 2 September 2005.     

Colony size explains the lifespan differences between queens and workers in eusocial Hymenoptera

Eusocial Hymenoptera show a unique divergence in lifespan of queens and workers; queens belong to the longest lived insects while workers in most eusocial species have significantly shorter lives. The different phenotypes within a colony emerge through reproductive division of labour, which is a characteristic trait of eusocial animals. Division of labour as a measure of organismal complexity increases with colony size in eusocial species similar to the increase of complexity with size that has been shown for the whole range of living organisms. We show that queen and worker lifespan diverge in closely related species representing the transition from solitary to social life and show that queen and worker lifespan are correlated if colony size is taken into account: with increasing colony size the lifespan differential between queen and worker increases, whereas neither queen nor worker lifespan is associated with colony size. Additionally, the lifespan differential is better explained by colony size than by the weight differences between the castes. The divergence of phenotypes found is in line with the increasing specialization of subunits in larger organisms, which leads to increasing complexity. We argue that division of labour is acting to increase colony efficiency, which in turn shapes the investments made into individuals leading to short‐lived workers and long‐lived queens. Additionally, maintenance investments may be shaped due to the variable extrinsic risk faced by different castes. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2013, 109 , 710–724.     

Characterization and distribution of Pogonomyrmex harvester ant lineages with genetic caste determination

Genetic caste determination has been described in two populations of Pogonomyrmex harvester ants, each comprising a pair of interbreeding lineages. Queens mate with males of their own and of the alternate lineage and produce two types of diploid offspring, those fertilized by males of the queens' lineage which develop into queens and those fertilized by males of the other lineage which develop into workers. Each of the lineages has been shown to be itself of hybrid origin between the species Pogonomyrmex barbatus and Pogonomyrmex rugosus, which both have typical, environmentally determined caste differentiation. In a large scale genetic survey across 35 sites in Arizona, New Mexico and Texas, we found that genetic caste determination associated with pairs of interbreeding lineages occurred frequently (in 26 out of the 35 sites). Overall, we identified eight lineages with genetic caste determination that always co-occurred in the same complementary lineage pairs. Three of the four lineage pairs appear to have a common origin while their relationship with the fourth remains unclear. The level of genetic differentiation among these eight lineages was significantly higher than the differentiation between P. rugosus and P. barbatus, which questions the appropriate taxonomic status of these genetic lineages. In addition to being genetically isolated from one another, all lineages with genetic caste determination were genetically distinct from P. rugosus and P. barbatus, even when colonies of interbreeding lineages co-occurred with colonies of either putative parent at the same site. Such nearly complete reproductive isolation between the lineages and the species with environmental caste determination might prevent the genetic caste determination system to be swept away by gene flow.     

Sex allocation conflict between queens and workers in Formica pratensis wood ants predicts seasonal sex ratio variation

Sex allocation theory predicts parents should adjust their investment in male and female offspring in a way that increases parental fitness. This has been shown in several species and selective contexts. Yet, seasonal sex ratio variation within species and its underlying causes are poorly understood. Here, we study sex allocation variation in the wood ant Formica pratensis. This species displays conflict over colony sex ratio as workers and queens prefer different investment in male and female offspring, owing to haplodiploidy and relatedness asymmetries. It is unique among Formica ants because it produces two separate sexual offspring cohorts per season. We predict sex ratios to be closer to queen optimum in the early cohort but more female‐biased and closer to worker optimum in the later one. This is because the power of workers to manipulate colony sex ratio varies seasonally with the availability of diploid eggs. Consistently, more female‐biased sex ratios in the later offspring cohort over a three‐year sampling period from 93 colonies clearly support our prediction. The resulting seasonal alternation of sex ratios between queen and worker optima is a novel demonstration how understanding constraints of sex ratio adjustment increases our ability to predict sex ratio variation.     

Respiratory physiology and water relations of three species of Pogonomyrmex harvester ants (Hymenoptera: Formicidae)

The respiratory physiology and water relations of three harvester ant species (Pogonomyrmex rugosus Emery, P. occidentalis[Cresson] and P. californicus[Buckley]) were examined at three temperatures (15, 25 and 35°C) using a flow-through respirometry system. As intact ants tended to be active during testing, we performed a parallel set of experiments on individuals rendered motionless by decapitation. Both intact and decapitated ants exhibited discontinuous ventilation. Decapitation caused metabolic rate (V˙CO₂) and burst frequency to decrease in all three species. Burst volume either remained constant or increased after removal of the head, though mass-specific V˙CO₂ was unaffected except in P. rugosus. Mass-specific V˙CO₂s of headless harvesters were comparable with published values derived from motionless specimens of other ant species. The mean Q₁₀ for intact ants of all three species was 2.37; for decapitated insects the mean was 2.32. Respiratory water constituted a small (< 5%) fraction of total loss, and we believe that discontinuous ventilation does not act to conserve water in these organisms, although it may serve other functions.     

Forager abundance and its relationship with colony activity level in three species of South American Pogonomyrmex harvester ants

The proportion of foragers in ant colonies is a fairly constant species-specific characteristic that could be determined by intrinsic or extrinsic factors. If intrinsic factors are relevant, species with similar life history characteristics (e.g., colony size and foraging strategies) would be expected to have a similar proportion of foragers in their colonies. Within the genus Pogonomyrmex, North American species can vary largely in their colony size, whereas only species with small colonies are known in South America. We studied the characteristics of the foraging subcaste in three sympatric South American species of Pogonomyrmex harvester ants, and compared it with the available information on other species of the same genus. We used two mark-recapture methods and colony excavations to estimate the number and proportion of foragers in the colonies of P. mendozanus, P. inermis, and P. rastratus, and to test the relationship between forager external activity levels and abundance per colony. Forager abundance in the three studied species was lower than in most North American species. The percentage of foragers in their colonies ranged 7–15 %, more similar to North American species with large colonies than to those with small colony size. Foraging activity was positively correlated with forager abundance in all three species, implying that colony allocation to number of foragers allows for higher food acquisition. Further comparative studies involving a wider range of traits in South and North American species would allow to unveil the role of environmental factors in shaping each species’ particular traits.     

Recruitment pheromone in the harvester ant genus Pogonomyrmex

Workers of the harvester ant genus Pogonomyrmex employ recruitment trail pheromones discharged from the poison gland. In P. barbatus, P. maricopa, P. occidentalis and P. rugosus we identified three pyrazines [2,5-dimethylpyrazine, trimethylpyrazine and 3-ethyl-2,5-dimethylpyrazine (EDMP)] as major compounds of the volatile part of the poison-gland secretions. Laboratory and field tests revealed EDMP to be the main recruitment pheromone.     

Reproductive potential of colonies of desert harvester ants (Pogonomyrmex desertorum): effects of predation and food

Summary To ascertain what factors affect reproductive output of colonies of the Desert Harvester Ant, Pogonomyrmex desertorum, colonies were subjected to two experiments. The first was a 3-way factorial design, which varied (i) artificial predation intensity, (ii) seed addition, and (iii) insect matter addition. The second experiment used fences to prevent horned lizards (Phrynosoma spp.) from foraging on concentrations of worker ants found at colony entrances. Increased artificial predation caused a decrease in alate production. The effect of predation was strongest in colonies also receiving insect matter. The treatments of adding seeds and adding insect matter by themselves did not have a measurable effect on alate production. Exclusion of horned lizard predators using fencing also had no effect on alate production. Although this study shows that predation has the potential to affect reproductive output (and therefore the potential to have been important in the evolutionary history of P. desertorum) the present study shows no evidence that either present levels of predation or scarcity of food limit reproductive output.     

A comparison of the energy budgets of three species of Pogonomyrmex harvester ants (Hymenoptera: Formicidae)

Summary This investigation compares the energy budgets of three species of Pogonomyrmex harvester ants along an altitudinal transect in southern California. All three demonstrate similar seasonal patterns, with high foraging activity and high respiratory costs in mid-summer and little or no activity during winter. Respiration, predominantely metabolism of the workers, was estimated to account for 84–92% of the energy assimilated by the nests. The remaining 8–16% of the energy was invested in the production of new individuals. However, total production was not correlated with food input. It is suggested that worker care of the brood may be the most important determinant of brood production, and thus food may not be a direct limiting resource in harvester ants.A higher percentage of production is invested in workers than in reproductives in all three species. The species usually partition similar proportions of energy between the production of males and females, but since females are larger, more males are produced. Although the species are in different habitats and have very different numbers of workers per nest, the numbers of sexuals produced per nest are similar. The sex-ratio appears to be ecologically determined. Nests that were provided with additional food invested more energy in the production of males. Control nests, nests which had food removed and older nests invested more in the production of females or invested equally in the production of the two sexes.     

Intra-specific differences in cognition: bumblebee queens learn better than workers

Species’ cognitive traits are shaped by their ecology, and even within a species, cognition can reflect the behavioural requirements of individuals with different roles. Social insects have a number of discrete roles (castes) within a colony and thus offer a useful system to determine how ecological requirements shape cognition. Bumblebee queens are a critical point in the lifecycle of their colony, since its future success is reliant on a single individual''s ability to learn about floral stimuli while finding a suitable nest site; thus, one might expect particularly adept learning capabilities at this stage. I compared wild Bombus vosnesenskii queens and workers on their ability to learn a colour association and found that queens performed better than workers. In addition, queens of another species, B. insularis, a cuckoo species with a different lifecycle but similar requirements at this stage, performed equally well as the non-parasitic queens. To control for differences in foraging experience, I then repeated this comparison with laboratory-based B. impatiens and found that unmated queens performed better than workers. These results add to the body of work on how ecology shapes cognition and opens the door to further research in comparative cognition using wild bees.     

A genetic component in the determination of worker polymorphism in the Florida harvester ant Pogonomyrmex badius

Summary. The interplay of genetic and environmental factors in the determination of social insect castes has long intrigued biologists. Though an overwhelming majority of studies establish that factors such as nutrition, pheromones and temperature determine the developmental fate of worker larvae, genetic components have recently been shown to play a role in the determination of morphological worker castes in leaf-cutting ants. Here we demonstrate that the determination of worker castes in the strongly polyandrous Florida harvester ant, Pogonomyrmex badius, has a genetic component. The overall distribution of caste members among patrilines in our study colonies is significantly different from the intracolonial caste ratio. Though this effect was not apparent in all colonies, our results suggest that workers of different patrilines in P. badius differ significantly in their propensities to develop into a certain worker caste. This genetic basis of worker polymorphism may go unnoticed in many social hymenopterans because of their low intracolonial genetic diversity due to monogamous colony structure. The worker polymorphism of P. badius is a taxonomic isolate and presumably a young trait in the genus. Therefore, a common genetic component of the determination of morphological and behavioral worker castes in social insects might be farranging taxonomically and may even be based on a genetic machinery inherent to all hymenopterans, but dormant in most.Received 3 May 2004; revised 27 October 2004; accepted 8 November 2004.     

Origin and evolution of the dependent lineages in the genetic caste determination system of Pogonomyrmex ants

Hybridizing harvester ants of the Pogonomyrmex barbatus/rugosus complex have an exceptional genetic caste determination (GCD) mechanism. We combined computer simulations, population genomics, and linkage mapping using >1000 nuclear AFLP markers and a partial mtDNA sequence to explore the genetic architecture and origin of the dependent lineages. Our samples included two pairs of hybridizing lineages, and the mitochondrial and nuclear data showed contradicting affinities between them. Clustering of individual genotypes based on nuclear markers indicated some exceptions to the general GCD system, that is, interlineage hybrid genes as well as some pure-line workers. A genetic linkage map of P. rugosus showed one of the highest recombination rates ever measured in insects (14.0 cM/Mb), supporting the view that social insects are characterized by high recombination rates. The population data had 165 markers in which sibling pairs showed a significant genetic difference depending on the caste. The differences were scattered in the genome; 13 linkage groups had loci with F(ST)>0.9 between the hybridizing lineages J1 and J2.The mapping results and the population data indicate that the dependent lineages have been initially formed through hybridization at different points in time but the role of introgression has been insignificant in their later evolution.     

Bonding between workers and queens in the ant genus Myrmica

Bond formation between young workers and queens is described for three species of Myrmica. This caste bond inhibits aggression and develops cooperation. Workers have a bias towards queens of their own species and do not discriminate between a queen of their own colony and a novel conspecific one. They are able to develop bonds with queens of other species of the genus provided they meet them first and well before their state of queenless aggressiveness sets in. A bispecific group, in situation contrived to enable them to meet gradually, will fight a normal group. They will then develop their bond to include the conspecific queen without becoming hostile to the earlier substitute. Evidently a queen of another species is able to provide an outline set of stimuli that is later built up by the conspecific queen, a characteristic of the imprinting development process. Allospecific queens can establish new microsocieties in a way reminiscent of temporary social parasites. The fact that allospecifically bonded workers will fight their one-time litter-mates that have been normally bonded suggests that the two groups have acquired different smells from their specifically distinct queens.     

Identifying the transition between single and multiple mating of queens in fungus-growing ants

Obligate mating of females (queens) with multiple males has evolved only rarely in social Hymenoptera (ants, social bees, social wasps) and for reasons that are fundamentally different from those underlying multiple mating in other animals. The monophyletic tribe of ('attine') fungus-growing ants is known to include evolutionarily derived genera with obligate multiple mating (the Acromyrmex and Atta leafcutter ants) as well as phylogenetically basal genera with exclusively single mating (e.g. Apterostigma, Cyphomyrmex, Myrmicocrypta). All attine genera share the unique characteristic of obligate dependence on symbiotic fungus gardens for food, but the sophistication of this symbiosis differs considerably across genera. The lower attine genera generally have small, short-lived colonies and relatively non-specialized fungal symbionts (capable of living independently of their ant hosts), whereas the four evolutionarily derived higher attine genera have highly specialized, long-term clonal symbionts. In this paper, we investigate whether the transition from single to multiple mating occurred relatively recently in the evolution of the attine ants, in conjunction with the novel herbivorous 'leafcutter' niche acquired by the common ancestor of Acromyrmex and Atta, or earlier, at the transition to rearing specialized long-term clonal fungi in the common ancestor of the larger group of higher attines that also includes the genera Trachymyrmex and Sericomyrmex. We use DNA microsatellite analysis to provide unambiguous evidence for a single, late and abrupt evolutionary transition from exclusively single to obligatory multiple mating. This transition is historically correlated with other evolutionary innovations, including the extensive use of fresh vegetation as substrate for the fungus garden, a massive increase in mature colony size and morphological differentiation of the worker caste.