Evolution of novel mosaic castes in ants: modularity, phenotypic plasticity, and colonial buffering

Abstract Many ants have independently evolved castes with novel morphology as well as function, such as soldiers and permanently wingless (ergatoid) queens. We present a conceptual model, based on modularity in morphology and development, in which evolutionary innovation is facilitated by the ancestral ant polyphenism of winged queens and wingless workers. We suggest that novel castes evolved from rare intercastes, anomalous mosaics of winged queens and workers, erratically produced by colonies through environmental or genetic perturbations. The colonial environment is highly accommodating and buffers viable intercastes from individual selection. Their cost is limited because they are diluted by the large number of nestmates, yet some can bring disproportionate benefits to their colonies in the context of defense or reproduction (e.g., wingless intercastes able to mate). Useful intercastes will increase in frequency as their morphology is stabilized through genetic accommodation. We show that both soldiers and ergatoid queens are mosaics of winged queens and workers, and they are strikingly similar to some intercastes. Modularity and developmental plasticity together with winged/wingless polyphenism thus allow for the production of highly variable mosaic intercastes, and colonies incubate the advantageous mosaics.     

A sex-linked locus controls wing polymorphism in males of the pea aphid,Acyrthosiphon pisum (Harris)

Discrete variation in wing morphology is a very common phenomenon in insects and has been used extensively in the past 50 years as a model to study the ecology and evolution of dispersal. Wing morph determination can be purely genetic, purely environmental, or some combination of the two. The precise genetic determinants of genetically based wing morph variation are unknown. Here we explore the genetic basis of wing polymorphism in the pea aphid, which can produce either winged or wingless males. We confirm that three types of pea aphid clones coexist in natural populations, those producing winged males only, those producing wingless males only, and those producing a mixture of both. A Mendelian genetic analysis reveals that male wing polymorphism in pea aphids is determined by a single locus, two alleles system. Using microsatellite loci of known location, we show that this locus is on the X chromosome. The existence of a simple genetic determinism for wing polymorphism in a system in which genetic investigation is possible may help investigations on the physiological and molecular mechanisms of genetically-based wing morph variation. This locus could also be used in the search for genes involved in the wing polyphenism described in parthenogenetic females and to investigate the interplay between polymorphisms and polyphenisms.     

Genetic variation for an aphid wing polyphenism is genetically linked to a naturally occurring wing polymorphism

Many polyphenisms are examples of adaptive phenotypic plasticity where a single genotype produces distinct phenotypes in response to environmental cues. Such alternative phenotypes occur as winged and wingless parthenogenetic females in the pea aphid (Acyrthosiphon pisum). However, the proportion of winged females produced in response to a given environmental cue varies between clonal genotypes. Winged and wingless phenotypes also occur in males of the sexual generation. In contrast to parthenogenetic females, wing production in males is environmentally insensitive and controlled by the sex-linked, biallelic locus, aphicarus (api). Hence, environmental or genetic cues induce development of winged and wingless phenotypes at different stages of the pea aphid life cycle. We have tested whether allelic variation at the api locus explains genetic variation in the propensity to produce winged females. We assayed clones from an F2 cross that were heterozygous or homozygous for alternative api alleles for their propensity to produce winged offspring. We found that clones with different api genotypes differed in their propensity to produce winged offspring. The results indicate genetic linkage of factors controlling the female wing polyphenism and male wing polymorphism. This finding is consistent with the hypothesis that genotype by environment interaction at the api locus explains genetic variation in the environmentally cued wing polyphenism.     

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.     

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.     

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.     

Evolution of male morphology in the ant genus Cardiocondyla

The ant genus Cardiocondyla is characterized by a striking male polymorphism, with wingless, local fighter males (ergatoid males) with life-long spermatogenesis, and winged, peaceful disperser males with limited sperm supply. We examined the evolution of male morphology by reconstructing the phylogeny of Cardiocondyla from sequences of the mitochondrial COI/COII and 16S RNA genes from 13 of the 15 species, of which males are known. Data suggest that male polymorphism is ancestral and that winged males were lost convergently in several taxa, such as C. elegans, C. batesii, and C. mauritanica. Saber-shaped mandibles and lethal fighting among adult ergatoid males might probably have been the original condition, from which strong, shear-shaped mandibles and attacks directed predominantly against freshly eclosed, not yet sclerotized males might have evolved once. The evolution of queen number from ancestral polygyny to derived monogyny appears to be associated with a switch in the behavior of ergatoid males from fighting to mutual tolerance.     

A new type of male dimorphism with ergatoid and short-winged males in Cardiocondyla cf. kagutsuchi

Summary. A new type of ant male dimorphism, consisting of wingless (ergatoid) and short-winged (brachypterous) males, was found in a species of the “Cardiocondyla kagutsuchi”- complex from Malaysia. The ergatoid males show the typical morphological and behavioral characteristics of those of many other Cardiocondyla species. The brachypterous males are morphologically intermediate between ergatoid males and typical winged males of other taxa in this genus. On one hand, they share a number of morphological and behavioral features with ergatoid males that might be adaptations to the loss of flight and intranidal mating: aggressive behavior towards rival males, a prolonged spermatogenesis, which is unique in winged males, paler body coloration, smaller compound eyes, shorter antennal funiculi, more rounded heads – perhaps due to the increased development of mandibular muscles, and an angular pronotum, probably for neck protection. Their short wings appear to protect the petiolar joints during fighting. On the other hand, the brachypterous males have not become as specialized as the ergatoids and to some extent keep the nature of the winged males of other species, i.e., they escape from the nest with a higher probability and with less injuries and do not show a reduction of the ocelli. In the sexual production season, the ergatoid males emerged first in small numbers and then both male morphs emerged in large numbers. The sex ratio was extremely female-biased in the earlier stage of sexual production, probably due to local mate competition.Received 13 December 2004; revised 17 February 2005; accepted 22 February 2005.     

Stress grows wings: environmental induction of winged dispersal males in Cardiocondyla ants

Dispersal is advantageous, but, at the same time, it implies high costs and risks. Due to these counteracting selection pressures, many species evolved dispersal polymorphisms, which, in ants, are typically restricted to the female sex (queens). Male polymorphism is presently only known from a few genera, such as Cardiocondyla, in which winged dispersing males coexist with wingless fighter males that mate exclusively inside their maternal nests. We studied the developmental mechanisms underlying these alternative male morphs and found that, first, male dimorphism is not genetically determined, but is induced by environmental conditions (decreasing temperature and density). Second, male morph is not yet fixed at the egg stage, but it differentiates during larval development. This flexible developmental pattern of male morphs allows Cardiocondyla ant colonies to react quickly to changes in their environment. Under good conditions, they invest exclusively in philopatric wingless males. But, when environmental conditions turn bad, colonies start to produce winged dispersal males, even though these males require a many times higher investment by the colony than their much smaller wingless counterparts. Cardiocondyla ants share this potential of optimal resource allocation with other colonial animals and some seed dimorphic plants.     

Alternative mating tactics and extreme male dimorphism in fig wasps

The dimorphisms in morphology and behaviour of male fig wasps are among the most extreme in the animal kingdom, and offer excellent opportunities to test the predictions of certain sexual selection models. Winged males resemble their conspecific females closely, but wingless males are so divergent in form that they have repeatedly been classified into different taxa. Wingless males mate within their natal fig fruits, whereas winged males disperse to mate. Individual species may have winged males, wingless males or both morphs. A key hypothesis proposes that sexual selection on male mating opportunities favours winged males in species with small broods and wingless males in species with large broods. Using data from 114 species in 33 genera, we show that both simple and formal comparative analyses support the correlated evolution of large brood size and male winglessness. Theoretical models further predict that, in male dimorphic species, the proportion of winged males should equal (in cases without local mate competition) or exceed (in cases with local mate competition) the proportion of females developing in fig fruits without wingless males. These predictions are met by eight out of nine male dimorphic species studied. Taken together, the patterns across all species, and between different male dimorphic species, strongly support sexual selection on mating opportunities as the major determinant of male morph ratios in fig wasps.     

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.     

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).     

Decision-making conditions for intra- or inter-nest mating of winged males in the male-dimorphic ant <Emphasis Type="Italic">Cardiocondyla minutior</Emphasis>

Only winged male and female ants generally mate through nuptial flight during the reproductive season. In the ants of Cardiocondyla, the males show wing dimorphism and their reproductive strategies differ depending on the differences in wing morphology. It has been suggested that wingless “ergatoid” males bearing very similar external morphologies to workers mate within natal nests, whereas winged males bearing typical ant male morphology disperse from their nests to mate. However, some behavioral observations suggest that the winged males of some Cardiocondyla ants such as C. obscurior and C. minutior may mate within natal nests before dispersion. We evaluated the factors affecting the mating behaviors of the winged males of C. minutior under laboratory conditions. We found that (1) the winged males remained and mated with virgin females in natal nests when either virgin winged females or the relatively mature pupae of winged females (i.e., at least 10 days) were present in the nest, (2) the winged males dispersed to adjacent nests with virgin winged females when only mated queens and the relatively young pupae of winged females (i.e., <9 days) were present in the nest, and (3) all winged males were accepted by the workers of non-natal nests irrespective of the distance from the natal nests in the field. Although most ergatoid males were accepted by the workers of close non-natal nests, they were all attacked and killed by the workers of distant non-natal nests. These results suggest that intra-nest mating and the dispersion of the winged males of C. minutior are facultatively determined by the condition of winged females (virginity and relative pupal age) in natal nests. Furthermore, our results suggest that winged males are likely to seek mating partners chemically and to mate with virgin winged females.     

Male-specific flight apparatus development in Acyrthosiphon pisum (Aphididae, Hemiptera, Insecta): comparison with female wing polyphenism

Wing polymorphisms observed in many Insecta are important topics in developmental biology and ecology; these polymorphisms are a consequence of trade-offs between flight and other abilities. The pea aphid, Acyrthosiphon pisum, possesses 2 types of wing polymorphisms: One is a genetic wing polymorphism occurring in males, and the other is an environmental wing polyphenism seen in viviparous females. Although genetic and environmental cues for the 2 wing polymorphisms have been studied, differences in their developmental regulation have not been elucidated. In particular, there is little knowledge regarding the developmental processes in male wing polymorphism. Therefore, in this study, the development of flight apparatuses and external morphologies was compared among 3 male wing morphs (winged, wingless, and intermediate). These male developmental processes were subsequently compared with those of female wing morphs. Developmental differences between the male and female polymorphisms were identified in flight muscle development and degeneration but not in wing bud development. Furthermore, the nymphal periods of wingless and intermediate males were significantly shorter than that of winged males, indicating the adaptive significance of male winglessness. Overall, this study indicates that the male and female wing polymorphisms are based on different regulatory systems for flight apparatus development, which are probably the result of different adaptations under different selection pressures.     

Genetic structure and reproductive strategy of the ant Cardiocondyla elegans: strictly monogynous nests invaded by unrelated sexuals

Cardiocondyla elegans is a Mediterranean ant that nests on river banks. It rears only wingless (ergatoid) males that live peacefully in the same nest as opposed to other species of the same genus, which have both peaceful, winged and mutually aggressive 'ergatoid' males. Using microsatellite analysis, we investigated the genetic structure of 21 colonies from three different locations as well as the parentage of sexuals of two colonies of C. elegans. We show that C. elegans is strictly monogynous, and that its nests can contain foreign sexuals. The presence of alien sexuals inside ant nests is described for the first time and probably counteracts inbreeding resulting from matings between siblings. In the laboratory, aggression tests showed that workers only allow alien males to enter their nests, while all winged female sexuals attempting to enter were attacked. Nevertheless, the presence of alien female sexuals in nests in the field seems to result from active carrying behaviour by workers during the reproductive period.     

The critical period for caste determination in Bombus terrestris and its juvenile hormone correlates

The critical period for caste determination and its juvenile hormone (JH III) correlates were studied in Bombus terrestris. Larvae of known age and instar were taken from young colonies, in which they would have been reared as workers, and placed into groups of queenless workers. Under these conditions the critical age for caste determination was 5 days, during the second instar. Endocrine correlates of caste determination were obtained by determining profiles of juvenile hormone titer and juvenile hormone biosynthesis, measured by chiral-specific radioimmunoassay and the in vitro radiochemical assay, respectively. By the middle of the second instar prospective queen larvae had significantly higher rates of juvenile hormone biosynthesis and juvenile hormone titer than prospective worker larvae. Based on the coincidence of timing of both the critical period and the appearance of caste-specific juvenile hormone titer, we suggest that juvenile hormone plays a role in the mechanisms that control caste determination in B. terrestris.     

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.     

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.