Differences in body sizes and physiological conditions among castes in the ponerine ant Cryptopone sauteri (Hymenoptera: Formicidae)

In several poneroid ant species, mated workers alone undertake reproduction. The reproductive systems of such species have been examined extensively. However, few studies have investigated species with alate queens, which reproduce after shedding their wings. We compared body sizes and the numbers of ovarioles between queens and workers in the ant Cryptopone sauteri with alate queens. We also compared ovariole development between the castes to evaluate their reproductive systems. Approximately 60% of the nests collected were queenless. We often detected unmated queens in the nests throughout the year, but did not obtain strong evidence for their reproduction. Although significant differences were observed in the number of ovarioles and body characteristics between the queens and workers, the differences were not as prominent as those observed in Formicinae and Myrmicinae. We propose two alternative hypotheses, failure of nuptial flight or postponement of reproduction, to explain the presence of unmated queens in the nests.     

Male production by workers in the polygynous ant Prolasius advenus

The ability of workers to produce male individuals is reported here for the first time in a species of the formicine ant genus Prolasius. We show that Prolasius advenus workers possess ovaries and demonstrate that they are able to produce adult males in queenless colonies. We also experimentally tested the influence of queen volatiles on the level of worker reproduction. Workers produced fewer eggs in treatments where they could perceive odors from queens. Some volatile compounds emitted by queens may thus have a signaling or inhibitory effect on worker reproduction. This effect of queen presence did not entirely stop worker reproduction, however, as adult males still emerged under these conditions. Worker-produced males were absent only in treatments with the physical presence of queens. Dissections of workers collected from queenright nests in the field revealed signs of egg-laying activity in more than half of individuals. Together, these results suggest that in nature P. advenus workers produce males at least in orphaned colonies or in situations where the physical presence of queens is limited.     

Intrinsic survival advantage of social insect queens depends on reproductive activation

The central trade‐off between reproduction and longevity dominates most species' life history. However, no mortality cost of reproduction is apparent in eusocial species, particularly social insects in the order Hymenoptera: one or a few individuals (typically referred to as queens) in a group specialize on reproduction and are generally longer lived than all other group members (typically referred to as workers), despite having the same genome. However, it is unclear whether this survival advantage is due to social facilitation by the group or an intrinsic, individual property. Furthermore, it is unknown whether the correlation between reproduction and longevity is due to a direct mechanistic link or an indirect consequence of the social role of the reproductives. To begin addressing these questions, we performed a comparison of queen and worker longevity in the ant Cardiocondyla obscurior under social isolation conditions. Survival of single queens and workers was compared under laboratory conditions, monitoring and controlling for brood production. Our results indicate that there is no intrinsic survival advantage of queens relative to workers unless individuals are becoming reproductively active. This interactive effect of caste and reproduction on life expectancy outside of the normal social context suggests that the positive correlation between reproduction and longevity in social insect queens is due to a direct link that can activate intrinsic survival mechanisms to ensure queen longevity.     

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.     

Immune priming and pathogen resistance in ant queens

Growing empirical evidence indicates that invertebrates become more resistant to a pathogen following initial exposure to a nonlethal dose; yet the generality, mechanisms, and adaptive value of such immune priming are still under debate. Because life‐history theory predicts that immune priming and large investment in immunity should be more frequent in long‐lived species, we here tested for immune priming and pathogen resistance in ant queens, which have extraordinarily long life span. We exposed virgin and mated queens of Lasius niger and Formica selysi to a low dose of the entomopathogenic fungus Beauveria bassiana, before challenging them with a high dose of the same pathogen. We found evidence for immune priming in naturally mated queens of L. niger. In contrast, we found no sign of priming in virgin queens of L. niger, nor in virgin or experimentally mated queens of F. selysi, which indicates that immune priming in ant queens varies according to mating status and mating conditions or species. In both ant species, mated queens showed higher pathogen resistance than virgin queens, which suggests that mating triggers an up‐regulation of the immune system. Overall, mated ant queens combine high reproductive output, very long life span, and elevated investment in immune defense. Hence, ant queens are able to invest heavily in both reproduction and maintenance, which can be explained by the fact that mature queens will be protected and nourished by their worker offspring.     

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.     

Timekeeping Through Social Contacts: Social Synchronization of Circadian Locomotor Activity Rhythm in the Carpenter Ant Camponotus paria

In ant colonies a large proportion of individuals remain inside nests for most of their lives and come out only when necessary. It is not clear how, in a nest of several thousand individuals, information about local time is communicated among members of the colony. Central to this seem to be circadian clocks, which have an intrinsic ability to keep track of local time by entraining to environmental light-dark, temperature, and social cycles. Here, the authors report the results of their study aimed at understanding the role of cyclic social interactions in circadian timekeeping of a day-active species of carpenter ant Camponotus paria. The authors found that daily social interactions with visitors (worker ants) was able to synchronize the circadian locomotor activity rhythm of host worker ants and queens, in one-on-one (pair-wise) and multi-individual (group-wise) interactions. Interestingly, the outcome of cyclic social interactions was context specific; when visitor workers socially interacted with host workers one-on-one, host workers considered the time of interaction as subjective day, but when visitor workers interacted with a group of workers and queens, the hosts considered the time of interaction as subjective night. These results can be taken to suggest that members of the ant species C. paria keep track of local time by socially interacting with workers (foragers) who shuttle in and out of the colony in search of food. (Author correspondence: vsharma@jncasr.ac.in)     

Polyandry and colony genetic structure in the primitive ant Nothomyrmecia macrops

The Australian endemic ant Nothomyrmecia macrops is considered one of the most ‘primitive’ among living ants. We investigated the genetic structure of colonies to determine queen mating frequencies and nestmate relatedness. An average of 18.8 individuals from each of 32 colonies, and sperm extracted from 34 foraging queens, were genotyped using five highly variable microsatellite markers. Queens were typically singly (65%) or doubly mated (30%), but triple mating (5%) also occurred. The mean effective number of male mates for queens was 1.37. No relationship between colony size and queen mate number was found. Nestmate workers were related by b=0.61 ± 0.03, significantly above the threshold under Hamilton’s rule over which, all else being equal, altruistic behaviour persists, but queens and their mates were unrelated. In 25% of the colonies we detected a few workers that could not have been produced by the resident queen, although there was no evidence for worker reproduction. Polyandry is for the first time recorded in a species with very small mature colonies, which is inconsistent with the sperm‐limitation hypothesis for the mediation of polyandry levels. Facultative polyandry is therefore not confined to the highly advanced ant genera, but may have arisen at an early stage in ant social evolution.     

Discovery of independent‐founding solitary queens in the yellow crazy ant Anoplolepis gracilipes in East Java,Indonesia (Hymenoptera: Formicidae)

We collected four solitary queens of the invasive ant Anoplolepis gracilipes under stones in East Java, Indonesia. They produced nanitic workers by claustral colony foundation. This is the first report of independent colony foundation by queens in this species. The discovery may give an important insight into discussion on the origin of this invasive ant.     

Queen–worker caste ratio depends on colony size in the pharaoh ant (<Emphasis Type="Italic">Monomorium pharaonis</Emphasis>)

The success of an ant colony depends on the simultaneous presence of reproducing queens and non-reproducing workers in a ratio that will maximize colony growth and reproduction. Despite its presumably crucial role, queen–worker caste ratios (the ratio of adult queens to workers) and the factors affecting this variable remain scarcely studied. Maintaining polygynous pharaoh ant (Monomorium pharaonis) colonies in the laboratory has provided us with the opportunity to experimentally manipulate colony size, one of the key factors that can be expected to affect colony level queen–worker caste ratios and body size of eclosing workers, gynes and males. We found that smaller colonies produced more new queens relative to workers, and that these queens and workers both tended to be larger. However, colony size had no effect on the size of males or on the sex ratio of the individuals reared. Furthermore, for the first time in a social insect, we confirmed the general life history prediction by Smith and Fretwell (Am Nat 108:499–506, 1974) that offspring number varies more than offspring size. Our findings document a high level of plasticity in energy allocation toward female castes and suggest that polygynous species with budding colonies may adaptively adjust caste ratios to ensure rapid growth.     

Evolution of wingless reproductives in ants: weakly specialized ergatoid queen instead of gamergates in Platythyrea conradti

Platythyrea conradti is the only species in this genus with ergatoid (= permanently wingless) queens. Colonies lack gamergates (unlike other species in this genus), yet aggressive interactions among queen and workers define a hierarchy. A single fertile queen has the top rank and highranking workers do not lay eggs, except when the queen dies. Colonial reproduction by both alate queens (independent foundation) and gamergates (fission) seems the ancestral state in Platythyrea. Independent foundation can be selected against in some species, causing the loss of alate queens for economic reasons. Thus gamergates become the only reproductives, except in P. conradti in which queens became ergatoid. Gamergates and ergatoid queens are two mutually redundant reproductive phenotypes that allow colonial reproduction by fission. Received 1 July 2005; revised 10 November 2005; accepted 16 November 2005.     

Influence of larvae and workers on egg production of queens of the pharaoh's ant,Monomorium pharaonis (L.)

Summary Social regulation of egg production and weight in queens was studied in relation to presence and absence of larvae and workers in the pharaoh's ant,Monomorium pharaonis (L.).Results were obtained by counting eggs and weighing queens under various conditions.The results confirm the existence of a positive feed-back loop between mated queens and their larvae as evident from a correlation (Y = 4.575 * X + 6.452) between the number of large worker larvae (X) and the egg yield (Y). This correlation seems to relate to the queens preferential feeding on larval secretions. Queens without larvae maintained a low level of egg production of about 6 eggs/day. Queens deprived of larvae as well as workers stopped producing eggs within 24 hours.Repletes, special workers, with greatly distended gasters functioned as a buffer retarding decline of egg production by feeding the queens during short periods without larvae.     

Worker demography and behavior in a supercolonial ant colony: The case of the desert ant Cataglyphis niger

Insect societies headed by multiple queens (polygyny) raise evolutionary questions, such as how does genetic heterogeneity among colony members affect in-nest interactions; or, are all queens equally reproductive or equally treated by workers? Answering such questions requires intensive and continuous observations of in-nest behavior. Here, we addressed these questions in the polygyne supercolonial ant, Cataglyphis niger, using a barcoding system that enables the tracking of individual interactions, together with polymorphic DNA microsatellite markers that indicate the matriline and patriline of all individuals. Our findings that both queens and workers have low interrelatedness corroborate earlier reports regarding the supercolony structure of C. niger. Ovary inspection and worker genotyping revealed that all the queens contribute similarly to nest demography. Tracking positions of individual workers through time revealed that only a small proportion of them are constantly engaged in tending the queens and can be considered as retinue workers. However, genotyping these workers and the attended queens revealed no relationship to genetic relatedness, again typical of a true polygyne and supercolonial species. Unlike invasive supercolonial species, C. niger is native to Israel, enabling us to address questions regarding the driving forces, other than kin selection, that stabilize this society.     

Association between host wing morphology polymorphism and Wolbachia infection in Vollenhovia emeryi (Hymenoptera: Myrmicinae)

Many eusocial insects, including ants, show complex colony structures, distributions, and reproductive strategies. In the ant Vollenhovia emeryi Wheeler (Hymenoptera: Myrmicinae), queens and males are produced clonally, while sterile workers arise sexually, unlike other ant species and Hymenopteran insects in general. Furthermore, there is a wing length polymorphism in the queen caste. Despite its evolutionary remarkable traits, little is known about the population structure of this ant species, which may provide insight into its unique reproductive mode and polymorphic traits. We performed in‐depth analyses of ant populations from Korea, Japan, and North America using three mitochondrial genes (COI, COII, and Cytb). The long‐winged (L) morph is predominant in Korean populations, and the short‐winged (S) morph is very rare. Interestingly, all L morphs were infected with Wolbachia, while all Korean S morphs lacked Wolbachia, demonstrating a association between a symbiont and a phenotypic trait. A phylogenetic analysis revealed that the S morph is derived from the L morph. We propose that the S morph is associated with potential resistance to Wolbachia infection and that Wolbachia infection does not influence clonal reproduction (as is the case in other ant species).     

Do army ant queens re-mate later in life?

Queens of eusocial Hymenoptera are inseminated only during a brief period before they start to lay eggs. This has probably been kin-selected because repeated insemination of old queens would normally be against the inclusive fitness interest of their daughter workers. Army ants have been considered to be the only possible exception to this rule due to their idiosyncratic life-history. We studied two distantly related species of army ants, the African Dorylus (Anomma) molestus and the Neotropical Eciton burchellii and present data from microsatellite genotyping, behavioural observations and sperm counts.We also describe the copulation behaviour of African army ants for the first time. Our results strongly suggest that, contradictory to earlier contentions, army ant queens do not mate repeatedly throughout their life and thus do not constitute an exception among the eusocial Hymenoptera in this respect. Sperm counts for males and queens of both species show that army ant queens have to mate with several males to become fully inseminated. However, sperm limitation by queens is unlikely to have been the prime reason for the evolution of high queen-mating frequencies in this group. Received 5 July 2006; revised 26 September 2006; accepted 11 October 2006.     

Repeated evolution of queen parthenogenesis and social hybridogenesis in Cataglyphis desert ants

Over the last decade, genetic studies on social insects have revealed a remarkable diversity of unusual reproductive strategies, such as male clonality, female clonality, and social hybridogenesis. In this context, Cataglyphis desert ants are useful models because of their unique reproductive systems. In several species, queens conditionally use sexual reproduction and parthenogenesis to produce sterile workers and reproductive queens, respectively. In social hybridogenesis, two distinct genetic lineages coexist within a population, and workers result from mating between partners of different lineages; in contrast, queens and males are both produced asexually by parthenogenesis. Consequently, nonreproductive workers are all interlineage hybrids, whereas reproductives are all pure lineage individuals. Here, we characterized the reproductive systems of 11 species to investigate the distribution of the conditional use of sex and social hybridogenesis in Cataglyphis. We identified one new case in which sexual reproduction was conditionally used in the absence of dependent‐lineage reproduction. We also discovered five new instances of social hybridogenesis. Based on our phylogenetic analyses, we inferred that both the conditional use of sex and social hybridogenesis independently evolved multiple times in the genus Cataglyphis.     

Mating system and population structure in the desert ant Cataglyphis livida

We investigated population genetic structure, mating system, worker reproduction and thelytokous parthenogenesis in the desert ant Cataglyphis livida. Pedigree analyses at polymorphic microsatellite loci show that colonies are headed by a single queen, and that queens are mated with two to eight males. No inbreeding was found in the population sampled. Colonies are genetically differentiated and exhibit no isolation-by-distance pattern, consistent with independent foundation of new colonies. Workers do reproduce and lay haploid (arrhenotokous) eggs in queenless colonies; conversely, we found no evidence of worker reproduction in queenright nests. In contrast with C. cursor, where new queens are produced by thelytokous parthenogenesis, female sexuals and workers of C. livida arise from classical sexual reproduction. We discuss the parallels and contrasts between the mating system and population structure in C. livida and the other Cataglyphis species studied so far.     

Foraging characteristics and intraspecific behaviour of the exotic species Monomorium sydneyense (Hymenoptera: Formicidae) in New Zealand,with implications for its management

Abstract

We attempt to evaluate the biosecurity risk posed by the newly established exotic ant species Monomorium sydneyense Forel, in New Zealand. Aggression was observed between workers from different M. sydneyense nests, indicating that unicoloniality is unlikely. Nests had multiple queens, and nest foundation is apparently via winged queens. The foraging behaviour corresponded to multicoloniality, with workers foraging in close proximity to the nest. In trials during December (2003) and March (2004), workers had a >50% probability of finding food at a distance of 0.8–1 m from their nest, but on cooler study dates (October 2004) our logistic regression model indicated that they would not reach this threshold irrespective of how close the food was to the nest. Although M. sydneyense forage during both night and day, they appeared to be relatively inefficient at locating food. We conclude that under the conditions assessed here, the environmental damage likely to be caused by M. sydneyense is modest compared with other invasive ant species.     

Reproduction,dominance, and caste: endocrine profiles of queens and workers of the ant <Emphasis Type="Italic">Harpegnathos saltator</Emphasis>

The regulation of reproduction within insect societies is a key component of the evolution of eusociality. Differential patterns of hormone levels often underlie the reproductive division of labor observed among colony members, and further task partitioning among workers is also often correlated with differences in juvenile hormone (JH) and ecdysteroid content. We measured JH and ecdysteroid content of workers and queens of the ant Harpegnathos saltator. In this species, new colonies are founded by a single queen, but after she dies workers compete in an elaborate dominance tournament to decide a new group of reproductives termed “gamergates.” Our comparisons revealed that queens, gamergates, and inside workers (non-reproductive) did not differ in levels of JH or ecdysteroids. However, increased JH and decreased ecdysteroid content was observed in outside workers exhibiting foraging behavior. Application of a JH analog to virgin queens of H. saltator, although effective at inducing dealation, failed to promote egg production. Together, these results support the hypothesis that JH has lost its reproductive function in H. saltator to regulate foraging among the worker caste.     

Genetic determination of female castes in a hybridogenetic desert ant

In most social insects, the brood is totipotent and environmental factors determine whether a female egg will develop into a reproductive queen or a functionally sterile worker. However, genetic factors have been shown to affect the female's caste fate in a few ant species. The desert ant Cataglyphis hispanica reproduces by social hybridogenesis. All populations are characterized by the coexistence of two distinct genetic lineages. Queens are almost always found mated with a male of the alternate lineage than their own. Workers develop from hybrid crosses between the genetic lineages, whereas daughter queens are produced asexually via parthenogenesis. Here, we show that the association between genotype and caste in this species is maintained by a ‘hard‐wired’ genetic caste determination system, whereby nonhybrid genomes have lost the ability to develop as workers. Genetic analyses reveal that, in a rare population with multiple‐queen colonies, a significant proportion of nestmate queens are mated with males of their own lineage. These queens fail to produce worker offspring; they produce only purebred daughter queens by sexual reproduction. We discuss how the production of reproductive queens through sexual, intralineage crosses may favour the stability of social hybridogenesis in this species.