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.     

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.     

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).     

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.     

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.     

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.     

Competition over workers: fertility signalling in wingless queens of <Emphasis Type="Italic">Hypoponera opacior</Emphasis>

The ant Hypoponera opacior exhibits alternative reproductive morphs associated with distinct sexual behaviours. Wingless, worker-like gynes mate in fall in their mother nest. Subsequently, these polygynous nests divide and wingless queens disperse on foot accompanied by workers, which help during nest foundation. Local resource competition amongst wingless queens was indicated by male-biased sex ratios. Queens compete over access to workers, and this competition manifests in dominance interactions. We colour-marked queens and workers to study this behaviour. Marked queens were treated aggressively and frequently killed by nestmates, whilst similarly labelled workers were readopted. We hypothesise that mated wingless queens—although they resemble workers in external morphology—differ chemically and that fertile queens with divergent odours are not tolerated. Thereby, sterile H. opacior workers avoid the risk to raise offsprings of unrelated queens. However, when nests split, workers should base their decision which queen to follow not only on relatedness, but also on queen fertility. Cuticular hydrocarbon profiles of queens could serve as an honest signal for workers. Indeed, gas chromatography showed that hydrocarbon composition differs between workers and mated queens and is linked to ovary development in queens. Workers and unmated queens exhibit complex profiles of short-chained hydrocarbons, whilst fertile queens carry few, long-chained alkenes and branched alkanes. Furthermore, ovary length was correlated to the relative amounts of characteristic hydrocarbons of queens. The chemical profile was also associated with eye size—an unalterable trait in adult insects—suggesting that larval nutrition and consequently body size affects adult ovary development.     

Dimorphic Males Display Alternative Reproductive Strategies in the Marine Amphipod Jassa marmorata Holmes (Corophioidea: Ischyroceridae)

Discrete dimorphism of males within a species is often the result of selection for alternative reproductive strategies, and these strategies may be evolutionarily stable (ESS). An ESS may be either mixed (genetically fixed differences) or conditional (flexible differences related to varying environmental conditions) (PARKER 1984). Mature males of the marine amphipod Jassa marmorata are dimorphic. Large ‘major’ males have a greatly enlarged thumb (propodus) on their 2nd gnathopods, while small ‘minor’ males exhibit thumbs that are reduced, and CONIAN (1989) suggested that minors may exhibit a different mating strategy from majors. Ratios of males and females fluctuate seasonally (FRANZ 1989) and female body size is inversely correlated with temperature (FRANZ 1989) so male dimorphism could be a flexible response to varying environmental conditions. I sampled a natural population of J. marmorata over a 1-yr period, quantified major and minor morphology, and measured male behaviour and mating success in experimental arenas that contained varying proportions of male morphs and females. Morphology of the two morphs is discrete; female body size varies with season with significantly smaller individuals in summer and fall; body size predicts morph type; and ratios of majors, minors and females fluctuate seasonally. Finally I showed that majors and minors use different mating tactics to gain access to receptive females, and that these behaviours depend on the male's own morphology and on the environmental setting that it finds itself in. Major males fight, display and attempt to evict other males to mate with receptive females. Minors never fight with, display to or attempt to evict majors, but they infrequently display to and attempt to evict other minor males. Furthermore, mating success of the two morphs was not significantly different and may depend on whether males are with a majority or minority of their own type. These data support the conclusion that major and minor male J. marmorata display evolutionarily stable alternative reproductive strategics, but more work should address the nature of this ESS.     

Potential and realized reproduction by different worker castes in queen-less and queen-right colonies of <Emphasis Type="Italic">Pogonomyrmex badius</Emphasis>

Workers of the Florida harvester ant (Pogonomyrmex badius), the only North American Pogonomyrmex with a polymorphic worker caste, produce males when colonies are orphaned. In this study,we assessed the reproductive potential of workers of each caste group, minors and majors, in the presence and absence of the queen, and tested whether males produced in natural queen-right colonies are derived from workers. Worker size was positively correlated with ovariole number such that major workers had approximately double the number of ovarioles as minor workers. The number of vitellogenic oocytes, a measure of reproductive potential, was greater in major compared to minor workers and increased in both worker castes when queens were removed. Major workers have greater reproductive potential than minors although they represent a minority within the colony (~5% of workers are majors). Worker produced eggs were visible in colonies 28 – 35 days after queen removal. This time lag, from queen removal to egg production, is similar to other ants and bees. Though workers are capable of producing viable eggs, we found no evidence that they do so in queen-right colonies, suggesting that worker reproduction is controlled via some social mechanism (self restraint, policing, or inhibition). This result supports predictions of kin selection theory – that due to multiple mating by the queen workers are more related to queen-produced males than most worker-produced males and should thus favor reproduction by the queen and inhibit reproduction by other workers. Received 25 January 2007; revised 1 May 2007; accepted 21 May 2007.     

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.     

Visual prey discrimination of queen and worker ants by a generalist lizard

Laboratory experiments were conducted to test the hypothesis that widelyforaging generalist lizard,Eumeces okadae, visually discriminates palatable queen ants from unpalatable worker ants. The workers ofCamponotus japonicus andLasius niger were rejected on sight, while the queens of both species were eaten with little prior chemical examination by tongue flicks or licks. Comparison of the lizards' responses towards the workers and wingless queens of similar size indicated that neither body size nor presence or absence of wings accounted for difference in responses toward the 2 ant castes. The lizards probably discriminated different ant castes by body proportions.     

Queen polymorphism in the North American harvester ant,Ephebomyrmex imberbiculus

Summary In the North American harvester antEphebomyrmex imberbiculus, in addition to dealate queens, wingless female reproductives occur that have greatly reduced ocelli and thoracic sutures (intermorphic queens). Both queen types are equivalent in function, and do not differ in ovarian morphology. Colonies may contain several inseminated and egg-laying intermorphic queens. We discuss queen polymorphism in respect to the biology of this desert-dwelling species.     

Cryptically dimorphic caste differences in a Neotropical, swarm-founding paper wasp genus, Parachartergus (Hymenoptera: Vespidae)

Morphological differences in body shape between females of different reproductive conditions (in terms of insemination and ovarian development) were examined in two species of the Neotropical polistine genus Parachartergus: P. smithii and P. fraternus. The present study shows, for the first time, that non‐size‐based morphological divergence between queens and workers occurs in Parachartergus, an epiponine genus once believed to have little or no morphological caste differences. In the P. smithii colony examined, queens were significantly larger than workers in five of the eight body parts measured (head width, eye width, genal width, mesosomal length, wing length, first metasomal tergum width, and width and length of the second tergum), but the mean values of wing length and first and second tergum widths were not significantly different between them. The queen : worker size ratios tended to be greater anteriorly and smaller posteriorly, although the size ratio was greatest in second tergum length. Analysis of covariance (ancova ) with mesosomal length as covariate showed that queens had proportionally wider heads and narrower first terga than did workers. In the P. fraternus colony, size differences between queens and workers were not significant, and there was little or no difference in shape, but queens had significantly proportionally wider first terga than did workers.     

Morphological caste and sex differences in the Taiwanese stingless bee Trigona ventralis hoozana (Hymenoptera: Apidae)

To examine morphological differences among queens, workers and males, 14 external body characters were measured in two colonies of the Taiwanese stingless bee Trigona ventralis hoozana. Queens were largest in all of the body parts measured except eye width and mesoscutum length, and values for most variables in queens did not overlap with those of workers and males. In contrast, the worker : male size ratios for 11 variables were close to 1.0, showing that overall body size and shape of workers resembled that of males rather than of queens. Males had the largest eyes and their mesoscutum length was comparable to that of queens. ancova between 14 morphometric variables and mesoscutum width chosen as standard body size showed that allometric growth in most variables was not linear. Plotting of some variables on mesoscutum width showed that queens had a proportionally wider first metasomal tergum and longer antennal scape, but a proportionally narrower head and eye than workers and males. These tests suggest that the morphological caste differences in this species belong to a category of complete dimorphism.     

Male size and survival: the effects of male combat and bird predation in Dawson's burrowing bees, Amegilladawsoni

1. Males of Dawson's burrowing bee, Amegilla dawsoni , occur in two size classes, large majors and small minors. Major males compete aggressively for emerging females in completely exposed emergence areas, whereas minors often employ an alternative mating tactic, which involves rapid patrolling in the vegetated periphery of emergence areas.
2. Because major males wrestle violently with rivals on the ground, they experience greater wing wear and a higher risk of wing damage than minor males. In addition, males patrolling emergence areas and waiting on the ground for emerging females are more likely to be killed by predatory birds than are peripheral minors.
3. Measurements of longevity based on mark–recapture data suggest that majors are slightly shorter-lived than minor males. If male–male combat and predators do reduce the lifespan of major males relative to minors, the effect would be to decrease the lifetime mating advantage of majors relative to minors.
4. Differential mortality could therefore be a factor in the maintenance of the two forms of males in this species.     

A morphological comparison of alates between monogynous and polygynous colonies of Myrmica kotokui in northernmost Japan

Summary: Social structure and sexual size of two populations of Myrmica kotokui were compared. Most of the colonies were polygynous in one population and monogynous in the other. Mean body size of the queens was larger in the monogynous population than in the polygynous population. Although the polygynous population contained many large queens, their wing length was significantly shorter than that of queens in the monogynous population. Some females in the polygynous population were intermorphic between typical workers and queens, while the females of the monogynous population were clearly divided into worker and queen castes without any intermorphic females. The body size of the males showed a large variation and the mean was not significantly different between the two populations.     

Colony dynamics in a primitively eusocial halictine beeLasioglossum (Dialictus) zephyrum (Hymenoptera: Halictidae)

Summary Proximate control of colony dynamics was studied in the primitively eusocial halictine beeLasioglossum (Dialictus) zephyrum using allozyme markers. The results indicate that workers produce on average 15% of the male brood (range=0–50%) in small laboratory colonies made up of unrelated, single-generation, uninseminated females. This proportion is not influenced by colony size, but is influenced by the relative size of the queen. Large queens are more successful in dominating their workers than are small queens, the queen being defined as the female that is the mother of most of the brood produced in the colony. Older and larger females tend to become queens. Thus, while small differences in age (up to 4 days) influence which female becomes a queen, her ability to control her workers is primarily influenced by her relative size. The proportion of reproduction that is co-opted by the queen is negatively correlated with colony reproductivity (the number of males/day/female). Colony reproductivity is also negatively correlated with the standard deviation in size among females.     

Subcaste‐specific evolution of head size in the ant genus Pheidole

An organism's morphology is constrained by its evolutionary history and the need to meet a variety of potentially competing functions. The ant genus Pheidole is the most species‐rich ant genus and almost every species has a dimorphic worker caste (a few are trimorphic). This separation of workers into two developmentally distinct subcastes (smaller minors and larger majors with distinctively large heads) may partially release individuals from functional constraints on morphology, making Pheidole an ideal genus for addressing questions on the evolution of morphology in relation to ecological specialization. Major workers can perform a variety of tasks, although they are usually specialized for defence, as well as food retrieval and processing. Pheidole species vary in their diet, although many species gather seeds. The major workers mill the seeds using large jaws powered by mandible closer muscles that occupy a large proportion of the head cavity. In the present study, we examined the relationship between seed‐harvesting and morphology in Pheidole, hypothesizing that majors of seed‐harvesting species would have larger heads relative to non‐seed‐harvesters to accommodate the powerful mandibular muscles needed to mill seeds. By taking a phylogenetically controlled comparative approach, we found that majors of seed‐associated Pheidole did not have larger heads (width and length) than majors of non‐seed‐harvesting species. However, the head length of minors (and to a lesser extent head width) was smaller in seed‐harvesters. Additionally, we found the difference in head size between majors and minors was greater in seed‐harvesting species. These morphological differences in diet, however, were not related to changes in the rate of evolution in either seed‐harvesting or non‐seed‐harvesting lineages. These findings suggest that the morphologies of worker subcastes can evolve independently of each other, allowing colonies with polymorphic workers to specialize on new resources or tasks in ways not possible in monomorphic species.     

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.     

Morphometric Identification of Queens,Workers and Intermediates in In Vitro Reared Honey Bees (Apis mellifera)

In vitro rearing is an important and useful tool for honey bee (Apis mellifera L.) studies. However, it often results in intercastes between queens and workers, which are normally are not seen in hive-reared bees, except when larvae older than three days are grafted for queen rearing. Morphological classification (queen versus worker or intercastes) of bees produced by this method can be subjective and generally depends on size differences. Here, we propose an alternative method for caste classification of female honey bees reared in vitro, based on weight at emergence, ovariole number, spermatheca size and size and shape, and features of the head, mandible and basitarsus. Morphological measurements were made with both traditional morphometric and geometric morphometrics techniques. The classifications were performed by principal component analysis, using naturally developed queens and workers as controls. First, the analysis included all the characters. Subsequently, a new analysis was made without the information about ovariole number and spermatheca size. Geometric morphometrics was less dependent on ovariole number and spermatheca information for caste and intercaste identification. This is useful, since acquiring information concerning these reproductive structures requires time-consuming dissection and they are not accessible when abdomens have been removed for molecular assays or in dried specimens. Additionally, geometric morphometrics divided intercastes into more discrete phenotype subsets. We conclude that morphometric geometrics are superior to traditional morphometrics techniques for identification and classification of honey bee castes and intermediates.