Proximate mechanisms of male morph determination in the ant Cardiocondyla obscurior

The ant genus Cardiocondyla is characterized by an extraordinary male polyphenism, with winged disperser males and wingless, territorial ergatoid males. Winged males are produced only after the colony has experienced stressful environmental conditions, e.g., a drastic temperature decrease. We investigated the proximate basis of male polyphenism and caste dimorphism in C. obscurior. The critical stage for both morph and caste determination is the end of the second of three instars. Larval development as well as duration of the pupal stage are extended both in winged males and winged females and winged reproductives need on average 8.8 days longer for the development from egg to adult than wingless ergatoid males and workers. Treatment of first and second instar larvae with methoprene, a juvenile hormone analogue, led to the expression of the winged morph, suggesting an important role of juvenile hormone in both sexes. Although queens are produced year-round in contrast to winged males, the proximate basis of variation in morphology is likely to be the same in both sexes. Whereas the larvae themselves appear to be insensitive to the environmental changes, behavioral observations revealed that workers react to stress by changing their behavior towards larvae and in this way trigger them to develop into winged males.     

Shift from independent to dependent colony foundation and evolution of 'multi-purpose' ergatoid queens in Mystrium ants (subfamily Amblyoponinae)

Division of labour improves fitness in animal societies. In ants, queens reproduce, whereas workers perform all other tasks. However, during independent colony founding, queens live as solitary insects and must be totipotent, especially in species where they need to forage. In many ants, solitary founding has been replaced by dependent founding, where queens are continuously helped by nestmate workers. Little is known about the details of this evolutionary transition. Mystrium rogeri from Madagascar and Mystrium camillae from Southeast Asia (subfamily Amblyoponinae) have winged queens, but three congeneric species from Madagascar reproduce with permanently wingless queens instead. We show that this 'ergatoid' caste has distinct body proportions in all three species, expressing a mixture of both queen and worker traits. Ergatoid queens have functional ovaries and spermatheca, and tiny wing rudiments. They can be as numerous as workers within a colony, but only a few mate and reproduce, whereas most behave as sterile helpers. The shape of their mandibles makes them unsuited for hunting and, together with a lack of metabolic reserves (i.e. in the form of wing muscles), this means that ergatoid queens cannot be solitary foundresses. In comparison with winged queens, ergatoid queens are less costly per capita and they experience lower mortality. They remain in their natal colonies where they can either reproduce or function as helpers, making them a 'multi-purpose' caste. Within the Amblyoponinae, ergatoid queens replace winged queens in Onychomyrmex as well. However, in this genus, ergatoid queens are 'sole-purpose', few are produced each year and they reproduce but do not work. Hence, different types of ergatoid queens evolved to replace winged queens in ants.  © 2009 The Linnean Society of London, Biological Journal of the Linnean Society , 2009, 98 , 198–207.     

Morphology and reproductive behaviour of intercastes in the ponerine antPachycondyla obscuricornis

Summary Queens are usually the functional reproductives inPachycondyla obscuricornis, but none were found in one colony collected near Brasilia. In this colony, three adults differed morphologically from workers; although wingless, their thorax was more or less like that of winged queens (scutum and scutellum distinct; metanotum larger than that of workers). Another 11 such individuals were found by opening cocoons. We consider that these are intercastes (sensu Peeters, 1991). Two adult intercastes were mated and laid eggs. In contrast, all the workers were virgin, but some laid distinct trophic eggs which were fed to the two reproductives. Following experimental removal of the intercastes, dominance interactions began among the workers, and reproductive eggs were then laid. Intercastes with a reproductive function have not been reported previously in the Ponerinae. They are distinct from ergatoid queens (permanently wingless reproductive caste) or gamergates (mated egglaying workers).     

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.     

Dimorphic ergatoid males and their reproductive behavior in the ponerine antHypoponera bondroiti

Summary We studied the reproductive behavior of the ponerine antHypoponera bondroiti from Okinawa, Japan. This species has dimorphic wingless ergatoid males (major and minor), dimorphic reproductive females (alate queens and wingless reproductive intercastes), and workers. Workers have neither ovarioles nor spermatheca. Major ergatoid males are the largest colony members. Two major males fought one another in the nest until one disappeared, leaving the other to occupy the nest chambers where queens emerge and mate. Minor ergatoid males also fought one another, although they seemed to be less pugnacious, resulting in occasional cohabitation of multiple minor males in the same nest chamber. Major males never attacked minor ones, allowing them to coexist in the same nest chamber. Minor males seemed to mimic females. Both major and minor males mated with both alate queens and intercastes within the nest. After mating, some alate queens shed their wings and remained in the nest, while the others left the nest for dispersal in the laboratory. Intercastes remained in the nest.     

The caste system of the dolichoderine antTechnomyrmex albipes (Hymenoptera: Formicidae): morphological description of queens,workers and reproductively active intercastes

Females ofTechnomyrmex albipes consist of winged queens, intercastes and workers. In established colonies, reproduction is performed by many intercastes (wingless females which have intermediate phenotypes between queen and worker characters). Dissection and morphological examination revealed that intercastes had a spermatheca, but workers did not. Intercastes can be divided into three classes: major intercastes with three ocelli, medium intercastes with one ocellus, and minor intercases without ocelli. Workers have no ocelli. The thoracic structure of intercastes gradually becomes more complex from minors to majors. The body size of intercastes gradually increases from minors to majors, and so does the number of overioles. The body size distributions of minor intercastes and workers overlap considerably, but the distributions of ovariole numbers overlap less. Winged queens had distinctly larger body sizes, more ovarioles and larger spermathecae than intercastes. Most intercastes were inseminated with developed ovaries and appeared to reproduce. The caste system and reproductive division of labour inT. albipes is compared to those of ant species in which permanently wingless females reproduce.     

Correlations between spatiotemporal changes in gene expression and apoptosis underlie wing polyphenism in the ant Pheidole morrisi

Wing polyphenism, which is the ability of a single genome to produce winged and wingless castes in a colony in response to environmental cues, evolved just once and is a universal feature of ants. The gene network underlying wing polyphenism, however, is conserved in the winged castes of different ant species, but is interrupted at different points in the network in the wingless castes of these species. We previously constructed a mathematical model, which predicts that a key gene brinker (brk) mediates the development and evolution of these different "interruption points" in wingless castes of different ant species. According to this model, brk is upregulated throughout the vestigial wing discs of wingless ant castes to reduce growth and induce apoptosis. Here, we tested these predictions by examining the expression of brk, as well as three other genes up- and downstream of brk-decapentaplegic (dpp), spalt (sal), and engrailed (en)-in the winged reproductive and wingless soldier castes in the ant Pheidole morrisi. We show that expression of these genes is conserved in the wing disc of winged castes. Surprisingly, however, we found that brk expression is absent throughout development of the vestigial soldier forewing disc. This absence is correlated with abnormal growth of the soldier forewing disc as revealed by En expression and morphometric analyses. We also discovered that dpp and sal expression change dynamically during the transition from larval-to-prepupal development, and is spatiotemporally correlated with the induction of apoptosis in soldier forewing disc. Our results suggest that, contrary to our predictions, brk may not be a key gene in the network for suppressing wings in soldiers, and its absence may function to disrupt the normal growth of the soldier forewing disc. Furthermore, the dynamic changes in network interruptions we discovered may be important for the induction of apoptosis, and may be a general feature of gene networks that underlie polyphenism.     

Nutritional versus genetic correlates of caste differentiation in a desert ant

1. In many ant species, caste differentiation stems from trophic differences at the larval stage. Adult workers that feed larvae have great control over the allocation of colony resources to growth (production of workers) versus reproduction (production of queens). However, larval caste fate may also be constrained very early on through direct genetic effects or non‐genetic maternal effects. 2. Here, we combined isotopic and genetic analyses to study the developmental origin of queens and workers in a desert‐dwelling ant, Cataglyphis tartessica (Amor & Ortega, 2014). Queens do not found new colonies alone but rather disperse with workers. As the latter are always wingless, selection pressures on specific queen traits such as flight ability have become relaxed. Though the phylogenetically related species, C. emmae (Forel, 1909) only produces winged queens much larger than workers, C. tartessica produces two types of small queens relative to workers: brachypterous (short‐winged) queens and permanently apterous ergatoid (wingless and worker‐like) queens. 3. Upon emergence, workers and ergatoids have similar δ15N isotopic values, which were lower than those of brachypters, suggesting the latter are fed more protein as larvae. Microsatellite analyses indicated that: (i) colonies are mostly monogynous and monandrous; (ii) both ergatoids and brachypters are equally related to workers; and (iii) in the few polyandrous colonies, patrilines were evenly represented across workers, brachypters and ergatoids. 4. Overall, there was no evidence of genetic caste determination. We suggest that, in contrast to brachypters, ergatoids are selfish individuals that escape the nutritional castration carried out by workers and develop into queens in spite of the colony's collective interests.     

Ergatoid queen development in the ant Myrmecina nipponica: modular and heterochronic regulation of caste differentiation

Caste polyphenism in social insects provides us with excellent opportunities to examine the plasticity and robustness underlying developmental pathways. Several ant species have evolved unusual castes showing intermediate morphologies between alate queens and wingless workers. In some low-temperature habitats, the ant Myrmecina nipponica produces such intermediate reproductives (i.e. ergatoids), which can mate and store sperm but cannot fly. To gain insight into the developmental and evolutionary aspects associated with ergatoid production, we conducted morphological and histological examinations of the post-embryonic development of compound eyes, gonads and wings during the process of caste differentiation. In compound eyes, both the queen-worker and ergatoid-worker differences were already recognized at the third larval instar. In gonads, queen-worker differentiation began at the larval stage, and ergatoid-worker differentiation began between the prepupal and pupal stages. Wing development in ergatoids was generally similar to that in workers throughout post-embryonic development. Our results showed that the developmental rate and timing of differentiation in body parts differed among castes and among body parts. These differences suggest that the rearrangement of modular body parts by heterochronic developmental regulation is responsible for the origination of novel castes, which are considered to be adaptations to specific ecological niches.     

Selection against Aerial Dispersal in Ants: Two Non-Flying Queen Phenotypes in Pogonomyrmex laticeps

The South American seed-harvester ant Pogonomyrmex laticeps has dimorphic queens: ergatoid (permanently wingless) and brachypterous (short, non-functional wings). Surveys in western Argentina indicated that colonies near Chilecito, La Rioja Province, produced only ergatoid queens, while those near Punta Balasto, Catamarca Province (263 km away), produced only brachypterous queens. Brachypterous queens were significantly larger than ergatoid queens for 10 of 11 external characters, but both phenotypes had comparable reproductive potential, i.e., a spermatheca and a similar number of ovarioles. Using normal winged queens of the closely related P. uruguayensis for comparison, we determined that both queen phenotypes in P. laticeps had a full set of dorsal thoracic sclerites, albeit each sclerite was much reduced, whereas workers had a thorax without distinct dorsal sclerites. Sclerites were fused and immobile in ergatoid queens, while they were separable and fully articulated in brachypterous queens. Both phenotypes lacked the big indirect flight muscles, but brachypterous queens retained the tiny direct flight muscles. Overall, this dimorphism across populations indicates that there are alternative solutions to selective pressures against flying queens. We lack field data about colony founding strategy (independent or dependent) for either queen phenotype, but colonies at both sites produced numerous gynes, and we infer that all foundresses initiate colonies independently and are obligate foragers.     

Permanent loss of wings in queens of the ant <Emphasis Type="Italic">Odontomachus coquereli</Emphasis> from Madagascar

Winged queens are the most common reproductives in ants. They are morphologically specialized for independent colony foundation, with wings for long-range dispersal and metabolic reserves to raise the first brood. However independent foundation can sometimes be selected against and replaced by fission, featuring short-range dispersal on the ground and reproductives that are dependent on the wingless workers for all non-reproductive tasks. We investigated the evolutionary consequences of this transition on the morphology of the reproductives by collecting 30 colonies of Odontomachus coquereli from Madagascar, the only species in the genus where winged queens have never been found. Data about colony demography, morphometry, allometry and ovarian dissections showed that the winged queen caste has been replaced by a wingless reproductive caste with distinct body proportions relative to the workers or to congeneric winged queens. The 17 reproductives that we measured exhibited little size variability. A single wingless reproductive was found in each colony, corresponding to ‘ergatoids’ in literature. Several facts suggest that colonies reproduce by fission, notably the relatively constant colony size (19±11 workers). The developmental origins of wingless reproductive phenotypes need investigation; little genetic change may be involved, as seen when Odontomachus larvae are parasitized by nematodes. The sole function of wingless reproductives in O. coquereli is reproduction, and they contrast with multi-purpose wingless reproductives found in other ants, where numerous intermorphs occur in each colony and contribute to sterile tasks. Received 15 December 2006; revised 26 February 2007; accepted 1 March 2007.     

Extreme genetic differences between queens and workers in hybridizing Pogonomyrmex harvester ants

The process of reproductive caste determination in eusocial insect colonies is generally understood to be mediated by environmental, rather than genetic factors. We present data demonstrating unexpected genetic differences between reproductive castes in a variant of the rough harvester ant, Pogonomyrmex rugosus var. fuscatus. Across multiple loci, queens were consistently more homozygous than expected, while workers were more heterozygous. Adult colony queens were divided into two highly divergent genetic groups, indicating the presence of two cryptic species, rather than a single population. The observed genetic differences between castes reflect differential representation of heterospecific and conspecific patrilines in these offspring groups. All workers were hybrids; by contrast, winged queens were nearly all pure-species. The complete lack of pure-species workers indicates a loss of worker potential in pure-species female offspring. Hybrids appear to be bipotential, but do not normally develop into reproductives because they are displaced by pure-species females in the reproductive pool. Genetic differences between reproductive castes are expected to be rare in non-hybridizing populations, but within hybrid zones they may be evolutionarily stable and thus much more likely to occur.     

Wingless and intermorphic males in the ant <Emphasis Type="Italic">Cardiocondyla venustula</Emphasis>

The ant genus Cardiocondyla is characterized by a pronounced male diphenism with wingless fighter males and winged disperser males. Winged males have been lost convergently in at least two species-rich clades. Here, we describe the morphological variability of males of Cardiocondyla venustula from uThukela valley, South Africa. Winged males appear to be absent from this species. However, in addition to wingless (“ergatoid”) males with widely fused thoracic sutures and without ocelli, “intermorphic” males exist that combine the typical morphology of wingless males with characteristics of winged males, e.g., more pronounced thoracic sutures, rudimentary ocelli, and vestigial wings. Similar “intermorphic” males have previously been described from one of several genetically distinct lineages of the Southeast Asian “Cardiocondyla kagutsuchi” complex (Insect. Soc. 52: 274-281, 2005). To determine whether male morphology is associated with distinct clades also in C. venustula, we sequenced a 631 bp fragment of mitochondrial DNA of workers from 13 colonies. We found six haplotypes with a sequence variation of up to 5.7 %. Intermorphic and wingless males did not appear to be associated with a particular of these lineages and within colonies showed the same sequence. Interestingly, two colonies contained workers with different haplotypes, suggesting the occasional migration of queens and/or workers between colonies.     

Live and let die: why fighter males of the ant Cardiocondyla kill each other but tolerate their winged rivals

Unlike most social insects, many Cardiocondyla ant species havetwo male morphs: wingless (ergatoid) males, who remain in thenatal nest, and winged males who disperse but, strangely, beforeleaving may also mate within the nest. Whereas ergatoid malesare highly intolerant of each other and fight among themselves,they tend to tolerate their winged counterparts. This is despitethe fact that these winged males, like ergatoid males, representmating competition. Why should ergatoid males tolerate theirwinged rivals? We developed a mathematical model to addressthis question. Our model focuses on a number of factors likelytoinfluence whether ergatoid males are tolerant of winged males:ergatoid male–winged male relatedness, number of virginqueens, number of winged males, and the number of ejaculatesa winged male has (winged males are sperm limited, whereas ergatoidmales have lifelong spermatogenesis). Surprisingly, we foundthat increasing the number of virgin queens favors a kill strategy,whereas an increase in the other factors favors a let-live strategy;these predictions appear true for C. obscurior and for a numberof other Cardiocondyla species. Two further aspects, unequalinsemination success and multiple mating in queens, were alsoincorporated into the model and predictions made about theireffects on toleration of winged males. The model is applicablemore generally in species that have dimorphic males, such assome other ants, bees, and fig wasps.     

Production and reproductive function of intercastes inMyrmecina gvaminicola nipponica colonies (Hymenoptera: Formicidae)

Summary Many females morphologically intermediate between queens and workers were found in a northernmost population ofMyrmecina graminicola nipponica Wheeler. Dissection and morphological observation revealed that there were three categories of intercastes. Major intercastes were as large as queens in body size, with seven or more ovarioles, but had only one ocellus, unlike queens, which had three ocelli. Medium intercasts had an enlarged mesonotum, one or no ocellus and 2 to 12 ovarioles. Minor intercaste was very simlar to workers in external morphology, but had a spermatheca, unlike workers. Inseminated females constituted 75%, 40% and 28.6% in the major, medium and minor intercastes respectively. Many of the virgin medium and minor intercastes had a small disfunctional spermatheca.In queenright colonies, a single queen was inseminated and had an active ovary. In queenless colonies where the intercastes reproduced, however, some colonies were functionally monogynous, but the others polygynous. The ratio of polygynous colonies to monogynous colonies was lowest in July and highest in September, suggesting that polygyny results from newly inseminated intercastes remaining in their natal nests, although they leave those nests in the season of colonial budding. Queenless colonies containing inseminated intercastes exclusively produced intercastes, while queenright colonies almost exclusively produced queens.     

Morphological variability of intercastes in the ant Temnothorax nylanderi: pattern of trait expression and modularity

Ants have distinct morphological castes (queens and workers), but aberrant queen-worker “intercastes” occasionally occur, both in wild and laboratory conditions. Intercastes are rare, however, such novel phenotypes may have evolutionary significance. Their morphology is highly variable in any given species, providing valuable information about the integration of queen traits (e.g. ocelli, wings, complex segmentation of thorax, large gaster and ovaries, spermatheca). Generally, these traits are all diminished or absent in workers. We used multivariate morphometry to analyze an exceptionally large sample of 101 intercastes of Temnothorax nylanderi. We determined distributions and correlations of traits, and confirmed the mosaic nature of intercastes. Queen-specific traits are not expressed coherently in intercastes, but the possible patterns of trait combination are limited. A large number of small-sized intercastes had disproportionately larger head, ocelli and gaster but smaller thorax. In contrast, queen-like growth of thorax and rudimentary wings only occurred in large-sized intercastes. This is the most comprehensive analysis of intercaste variability, and suggests the existence of constraints on recombination of caste-specific modular traits.     

The dynamics of developmental system drift in the gene network underlying wing polyphenism in ants: a mathematical model

SUMMARY Understanding the complex interaction between genotype and phenotype is a major challenge of Evolutionary Developmental Biology. One important facet of this complex interaction has been called "Developmental System Drift" (DSD). DSD occurs when a similar phenotype, which is homologous across a group of related species, is produced by different genes or gene expression patterns in each of these related species. We constructed a mathematical model to explore the developmental and evolutionary dynamics of DSD in the gene network underlying wing polyphenism in ants. Wing polyphenism in ants is the ability of an embryo to develop into a winged queen or a wingless worker in response to an environmental cue. Although wing polyphenism is homologous across all ants, the gene network that underlies wing polyphenism has evolved. In winged ant castes, our simulations reproduced the conserved gene expression patterns observed in the network that controls wing development in holometabolous insects. In wingless ant castes, we simulated the suppression of wings by interrupting (up- or downregulating) the expression of genes in the network. Our simulations uncovered the existence of four groups of genes that have similar effects on target gene expression and growth. Although each group is comprised of genes occupying different positions in the network, their interruption produces vestigial discs that are similar in size and shape. The implications of our results for understanding the origin, evolution, and dissociation of the gene network underlying wing polyphenism in ants are discussed.     

Association between caste and genotype in the termite Mastotermes darwiniensis Froggatt (Isoptera: Mastotermitidae)

Abstract Termite workers and soldiers differ markedly in their morphology and behaviour. We sought evidence for genetic influences on caste determination in the giant northern termite, Mastotermes darwiniensis , by investigating if workers and soldiers from the same colony differed genetically. The genotypes of 795 termites from 11 distinct colonies were assayed at six polymorphic microsatellite loci. We found that the multilocus genotypes of workers and soldiers from 8 of the 11 colonies did not differ significantly. Thus, the majority of the data provided no evidence for a genetic association with caste in workers and soldiers of M. darwiniensis . However, the genotype frequencies of workers and soldiers from three colonies differed, suggesting that genotype is occasionally associated with caste in this species. The genetic differentiation of castes within these colonies could reflect differences in the propensities of termites with distinct genotypes to develop into particular castes and provide a selective advantage to colonies headed by multiple reproductives.     

Independent colony founding by ergatoid queens in the ant genus Pogonomyrmex: queen foraging provides an alternative to dependent colony founding

Ant queens exhibit two primary strategies to initiate nests, independent colony founding (ICF) by solitary queens and dependent colony founding (DCF) when the queen starts a nest with a group of workers that disperse on foot from the parent nest. Numerous ant species have wingless (ergatoid) queens, and it is generally assumed that these species exhibit obligate DCF because their lack of wing musculature provides them with few resources to divert towards producing their first brood of workers. Thus, ICF by ergatoid queens is viewed as maladaptive because these queens need to take additional dangerous foraging trips to garner sufficient food to rear their first brood of workers. Contrary to this prediction, I document ICF by ergatoid queens for three species of harvester ants in the genus Pogonomyrmex (subfamily Myrmicinae), P. cunicularius cunicularius, P. cunicularius pencosensis, and P. huachucanus. Queens of P. huachucanus were obligate foragers, i.e., no minim workers could be produced without external food, and one queen of P. cunicularius pencosensis was observed foraging in the field. Abundant and/or predictable food resources likely select for the evolution of semi-claustral nest founding and ICF by these ergatoid queens. Under these conditions, foraging time would be minimized and the number and size of minim workers would be maximized. These benefits should increase founding success, which could compensate for loss of long-range dispersal. Overall, this study demonstrates that care should be taken before concluding that ant colonies employ DCF based solely on queen morphology.     

Unique habits of stomodeal trophallaxis in the ponerine antHypoponera sp.

Summary The present paper describes for the first time that adult-to-adult trophallaxis exists in ponerine ants. Furthermore, it shows that the adult castes of the ponerine andHypoponera sp. have different habits of trophallaxis. Workers display a soliciting behavior toward queens, males and workers, but receive regurgitated food only from workers. The workers are forced to stop soliciting for regurgitated food by the whipping behavior of the queen. Callow queens solicit regurgitated food from workers and ergatoid males and receive it, while mature queens do not solicit regurgitation from workers. Ergatoid males receive regurgitated food from workers and regurgitate it to queens. Alate males show no trophallactic food exchange with workers and queens. Trophallactic behavior was never observed between males of either winged or ergatoid types.