Queen Castes Research Articles

Taxonomic study on the queens of the Japanese ponerine genera, with a redescription of Ectomomyrmex horni restituted as a valid species (Hymenoptera: Formicidae)

Queen ants of the Japanese ponerine genera are described, with the first key to the Japanese genera based on the queen caste. Important queen characters are enumerated and caste differences are discussed for each genus. Physical and behavioral aspects of functional queens (alate/dealate queen, ergatoid, and gamergate) in the Japanese ponerines are briefly discussed. Ectomomyrmex horni (Forel, 1913), stat. restit., is resurrected from synonymy with Ectomomyrmex javanus Mayr, 1867. A redescription of E. horni is presented based on Japanese and Taiwanese specimens.

Genetic and environmental effects on morphological traits of social phenotypes in wasps.

Many species exhibit distinct phenotypic classes, such as sexes in dioecious species or castes in social species. The evolution of these classes is affected by the genetic architecture governing traits shared between phenotypes. However, estimates of the genetic and environmental factors contributing to phenotypic variation in distinct classes have rarely been examined. We studied the genetic architecture underlying morphological traits in phenotypic classes in the social wasp Vespula maculifrons. Our data revealed patriline effects on a few traits, indicating weak genetic influences on caste phenotypic variation. Interestingly, traits exhibited higher heritability in queens than workers. This result suggests that genetic variation has a stronger influence on trait variation in the queen caste than the worker caste, which is unexpected because queens typically experience direct selection. Moreover, estimates of heritability for traits were correlated between the castes, indicating that variability in trait size was governed by similar genetic architecture in the two castes. However, we failed to find evidence for a significant relationship between caste dimorphism and caste correlation, as would be expected if trait evolution was constrained by intralocus genetic conflict. Our analyses also uncovered variation in the allometric relationships for traits. These analyses suggested that worker traits were proportionally smaller than queen traits for most traits examined. Overall, our data provide evidence for a strong environmental and moderate genetic basis of trait variation among castes. Moreover, our results suggest that selection previously operated on caste phenotype in this species, and phenotypic variation is now governed primarily by environmental differences.

Metagenomic analysis of the honey bee queen microbiome reveals low bacterial diversity and Caudoviricetes phages.

In eusocial insects, the health of the queens-the colony founders and sole reproductive females-is a primary determinant for colony success. Queen failure in the honey bee Apis mellifera, for example, is a major concern of beekeepers who annually suffer colony losses, necessitating a greater knowledge of queen health. Several studies on the microbiome of honey bees have characterized its diversity and shown its importance for the health of worker bees, the female non-reproductive caste. However, the microbiome of workers differs from that of queens, which, in comparison, is still poorly studied. Thus, direct investigations of the queen microbiome are required to understand colony-level microbiome assembly, functional roles, and evolution. Here, we used metagenomics to comprehensively characterize the honey bee queen microbiome. Comparing samples from different geographic locations and breeder sources, we show that the microbiome of queens is mostly shaped by the environment experienced since early life and is predicted to play roles in the breakdown of the diet and protection from pathogens and xenobiotics. We also reveal that the microbiome of queens comprises only four candidate core bacterial species, Apilactobacillus kunkeei, Lactobacillus apis, Bombella apis, and Commensalibacter sp. Interestingly, in addition to bacteria, we show that bacteriophages infect the queen microbiome, for which Lactobacillaceae are predicted to be the main reservoirs. Together, our results provide the basis to understand the honey bee colony microbiome assemblage, can guide improvements in queen-rearing processes, and highlight the importance of considering bacteriophages for queen microbiome health and microbiome homeostasis in eusocial insects.IMPORTANCEThe queen caste plays a central role in colony success in eusocial insects, as queens lay eggs and regulate colony behavior and development. Queen failure can cause colonies to collapse, which is one of the major concerns of beekeepers. Thus, understanding the biology behind the queen's health is a pressing issue. Previous studies have shown that the bee microbiome plays an important role in worker bee health, but little is known about the queen microbiome and its function in vivo. Here, we characterized the queen microbiome, identifying for the first time the present species and their putative functions. We show that the queen microbiome has predicted nutritional and protective roles in queen association and comprises only four consistently present bacterial species. Additionally, we bring to attention the spread of phages in the queen microbiome, which increased in abundance in failing queens and may impact the fate of the colony.

Ooceraeahainingensis sp. nov.: A new Chinese Ooceraea (Hymenoptera, Formicidae, Dorylinae) species with a dealate queen, closely allied to the queenless clonal raider ant O.biroi.

The clonal raider ant, Ooceraeabiroi, is a queenless species that reproduces asexually, and these traits make it an attractive model system for laboratory research. However, it is unclear where on the ant phylogeny these traits evolved, partly because few closely related species have been described and studied. Here, we describe a new raider ant species, Ooceraeahainingensis sp. nov., from Zhejiang, China. This species is closely related to O.biroi but can be distinguished by the following features: 1) workers of O.hainingensis sp. nov. have an obvious promesonotal suture and a metanotal groove, whereas these characters are ambiguous in O.biroi; and 2) the subpetiolar process of O.hainingensis is prominent and anteroventrally directed like a thumb with sublinear posteroventral margin, while in O.biroi, it is anteroventrally directed but slightly backward-bent. Molecular phylogenetic analyses confirm that O.hainingensis is genetically distinct from O.biroi. Importantly, unlike O.biroi, O.hainingensis has a queen caste with wings and well-developed eyes. This suggests that the loss of the queen caste and transition to asexual reproduction by workers is specific to O.biroi and occurred after that species diverged from closely related congeneric species.

A new species and one new wing type of the iron maiden ants from Kachin amber (Hymenoptera: Formicidae: Zigrasimecia)

The Cretaceous is a key period for the evolution of ants. Zigrasimecia is a genus endemic to mid-Cretaceous Kachin amber. In previous studies, four species of the genus based on worker and/or queen castes have been reported and three wing types of gyne within the genus have been described. We report here a new species, Zigrasimecia zui sp. nov., based on a worker specimen, the morphological details of which are investigated and reconstructed by using Micro-CT and other techniques. Moreover, a new wing type is reported based on a gyne specimen. By comparison with the compound eyes and other structures of extant ants, we propose that species of Zigrasimecia had the capability for surface-dwelling or burrowing. This work augments the species diversity of Zigrasimecia, and provides direct evidence for the variety of ecological niches that may have been occupied by members of this genus.

Transcriptomic profiling of castes and of sexually and parthenogenetically produced reproductive females in the termite Cavitermes tuberosus

AbstractIn termites with a true worker caste, the development pattern splits up from early developmental stages: primary reproductives develop through the nymphal line, whereas workers and soldiers follow the apterous developmental line. In some species, such as Cavitermes tuberosus Emerson (Isoptera: Termitidae, Termitinae), secondary reproductives (or neotenics) may also develop through the nymphal line from a transitional stage called aspirant. Aspirants originate mostly from automictic parthenogenetic reproduction. Therefore, C. tuberosus queens originate from sexual (primary queens) or parthenogenetic (neotenic queens) reproduction. A comparison of these two queen castes offers the possibility to better understand core molecular underpinnings of caste development and plasticity in termites. We investigated these molecular mechanisms by using high‐throughput Illumina RNA sequencing of pooled individuals. We first assembled the de novo reference transcriptome of C. tuberosus, and then identified the transcripts consistently co‐expressed across castes, sexes, and two alternative routes to female reproduction. Cavitermes tuberosus final transcriptome had 130 874 transcripts, N50 of 3398, and total length of 213 549 184 bp. We found that female reproductive maturation was characterized by gene expression down‐regulation: primary queens expressed fewer transcripts overall and had the greatest number of down‐regulated transcripts when compared to all other castes. In both secondary and primary queens, biological processes involved in muscle development and contraction, flight, and olfactory learning were enriched in the down‐regulated gene cluster. In contrast, processes related to reproductive development, insulin receptor signaling pathway, isoprenoid biosynthesis, and multiple metabolic processes were enriched among up‐regulated genes in primary queens. Finally, we found that 17% of all transcripts (21852) were differently co‐expressed when females from sexual and parthenogenetic origins were compared, even though the expression profile of core reproductive‐related gene clusters showed a similar trend in all reproductive females despite their origin. Our findings fit the genomic imprinting model predictions of a maternal effect that commonly regulates the expression of core reproductive genes in females from parthenogenetic and non‐parthenogenetic origins, whereas the expression of non‐reproductive genes varies.

Single-cell transcriptomic analysis of honeybee brains identifies vitellogenin as caste differentiation-related factor

SummaryThe honeybee (Apis mellifera) is a well-known eusocial insect. In honeybee colonies, thousands of sterile workers, including nurse and forager bees, perform various tasks within or outside the hive, respectively. The queen is the only fertile female and is responsible for reproduction. The queen and workers share similar genomes but occupy different caste statuses. We established single-cell transcriptomic atlases of brains from queens and worker subcastes and identified five major cell groups: Kenyon, optic lobe, olfactory projection, glial, and hemocyte cells. By dividing Kenyon and glial cells into multiple subtypes based on credible markers, we observed that vitellogenin (vg) was highly expressed in specific glial-cell subtypes in brains of queens. Knockdown of vg at the early larval stage significantly suppressed the development into adult queens. We demonstrate vg expression as a "molecular signature" for the queen caste and suggest involvement of vg in regulating caste differentiation.

A new species of the ant genus Lepisiota Santschi, 1926 (Hymenoptera: Formicidae) from Thailand

Lepisiota Santschi, 1926 is one of the large ant genera, currently comprising 94 species and 44 subspecies. It belongs to the subfamily Formicinae and is distributed in the Old World. A new species, L. chutimae sp. n., is described here from Nakhon Ratchasima Province based on the worker and queen castes. The type series of the new species was collected from a shrub tree at about 3 m above the ground in a primary dry evergreen forest at altitude 250 m.

Extent and complexity of RNA processing in honey bee queen and worker caste development

SummaryThe distinct honeybee (Apis mellifera) worker and queen castes have become a model for the study of genomic mechanisms of phenotypic plasticity. Here we performed a nanopore-based direct RNA sequencing with exceptionally long reads to compare the mRNA transcripts between queen and workers at three points during their larval development. We found thousands of significantly differentially expressed transcript isoforms (DEIs) between queen and worker larvae. These DEIs were formatted by a flexible splicing system. We showed that poly(A) tails participated in this caste differentiation by negatively regulating the expression of DEIs. Hundreds of isoforms uniquely expressed in either queens or workers during their larval development, and isoforms were expressed at different points in queen and worker larval development demonstrating a dynamic relationship between isoform expression and developmental mechanisms. These findings show the full complexity of RNA processing and transcript expression in honey bee phenotypic plasticity.

Morphology and phenology of sexuals and new distribution data on the blind Mediterranean ant Hypoponera abeillei (Hymenoptera, Formicidae)

Hypoponera abeillei is the sole member of the otherwise exclusively Afrotropical abeillei group to occur in the West-Palearctic and since its first description on the basis of two Corsican workers in 1881, it remained little-known. Workers of this species are thought to entirely lack eyes, a rare trait among ants of the region, yet their lifestyle remains undocumented. On the other hand, the male caste has been described from 4 Tunisian specimens in 1921, and queens remained so far undescribed. We present an updated description of the male caste based on 45 specimens and a first description of the queen caste based on 14 specimens. The H. abeillei material we examined comes from Italy, Spain and Malta (mostly originating from coastal localities), and comprises 11 inedit distribution records, including the first findings in the islands of Mallorca, Malta and Sardinia. Moreover, we provide a first phenological overview of the species’ nuptial flights. Our data show that H. abeillei sexuals flight during the summer, mostly in August, and demonstrate that they can easily be distinguished from all the other Hypoponera species inhabiting the Mediterranean region based on their morphology. The remarkable diversity of Mediterranean Hypoponera males and queens suggest that sexuals may have a role in future attempts to understand relationships within this genus, yet the number of species in which sexual castes are documented is still extremely reduced.

Corrieoponenouragues gen. nov., sp. nov., a new Ponerinae from French Guiana (Hymenoptera, Formicidae).

This study describes the worker and queen castes of the Neotropical ponerine Corrieoponenouraguesgen. nov., sp. nov., an ant from the tropical rainforest in French Guiana. Worker morphology of the taxon is compared with those of other Ponerinae and the similarities between them are discussed, refining the definition of character states for some diagnostic characters at the generic level, providing an identification key to the Neotropical genera, and making some adjustments to the taxonomic framework within the subfamily. Descriptions, diagnosis, character discussion, identification key, and glossary are illustrated with more than 300 images and line drawings. Open science is supported by providing access to measurement data for specimens of the new genus, a matrix of character states for all ponerine taxa evaluated in this study, and specimen data for all examined material. The new or revived combinations presented here are Pachycondylaprocidua Emery, comb. rev., Neoponeracuriosa (Mackay and Mackay), comb. nov., Leptogenysbutteli (Forel), comb. nov., and Bothroponeraescherichi (Forel), comb. nov. In addition, Leptogenysbutteli is synonymized with Leptogenysmyops (Emery), syn. nov.

Transcriptome dynamics during metamorphosis of imaginal discs into wings and thoracic dorsum in Apis mellifera castes

BackgroundMuch of the complex anatomy of a holometabolous insect is built from disc-shaped epithelial structures found inside the larva, i.e., the imaginal discs, which undergo a rapid differentiation during metamorphosis. Imaginal discs-derived structures, like wings, are built through the action of genes under precise regulation.ResultsWe analyzed 30 honeybee transcriptomes in the search for the gene expression needed for wings and thoracic dorsum construction from the larval wing discs primordia. Analyses were carried out before, during, and after the metamorphic molt and using worker and queen castes. Our RNA-seq libraries revealed 13,202 genes, representing 86.2% of the honeybee annotated genes. Gene Ontology analysis revealed functional terms that were caste-specific or shared by workers and queens. Genes expressed in wing discs and descendant structures showed differential expression profiles dynamics in premetamorphic, metamorphic and postmetamorphic developmental phases, and also between castes. At the metamorphic molt, when ecdysteroids peak, the wing buds of workers showed maximal gene upregulation comparatively to queens, thus underscoring differences in gene expression between castes at the height of the larval-pupal transition. Analysis of small RNA libraries of wing buds allowed us to build miRNA-mRNA interaction networks to predict the regulation of genes expressed during wing discs development.ConclusionTogether, these data reveal gene expression dynamics leading to wings and thoracic dorsum formation from the wing discs, besides highlighting caste-specific differences during wing discs metamorphosis.

Foraging gene expression patterns in queens, workers, and males in a eusocial insect

Reproductive division of labour is based on biased expression of complementary parental behaviours, brood production (egg-laying) by queens and brood care (in particular, brood-provisioning) by workers. In many social insect species, queens provision brood when establishing colonies at the beginning of a breeding season and reproductive division of labour begins with the emergence of workers. In many social insect species, the expression of foraging (for) mRNA is associated with the intensity of foraging behaviour and therefore brood-provisioning. However, only two studies have compared queen and worker for expression levels and neither accounted for transcript splice variation. In this study, we compare the expression level of the for-α transcript variant across four life stages of the queen caste, two behavioural groups of workers, and males of a eusocial sweat bee Lasioglossum laevissimum (Smith, 1853). Foundresses collected prior to the onset of the foraging season and males had the highest for-α expression levels. All active (post-hibernatory) queens and workers had similar for-α expression levels independent of behaviour. These results suggest that the for gene in L. laevissimum acts as a primer before foraging activity and that caste-specific expression patterns correlate with the timing of foraging activity in queens and workers.

Nano-CT imaging of larvae in the ant Pheidole hyatti reveals coordinated growth of a rudimentary organ necessary for soldier development.

The growth of imaginal discs in holometabolous insects is coordinated with larval growth to ensure the symmetrical and proportional development of the adult appendages. In ants, the differential growth of these discs generates distinct castes-the winged male and queen castes and the wingless worker caste. In the hyperdiverse ant genus Pheidole, the worker caste is composed of two morphologically distinct subcastes: small-headed minor workers and larger, big-headed, soldiers. Although these worker subcastes are completely wingless, soldier larvae develop rudimentary forewing discs that function in generating the disproportionate head-to-body scaling and size of soldiers. It remains unclear, however, how rudimentary forewing discs in soldier larvae are coordinated with other imaginal discs. Here we show, using quantitative nano-CT imaging and three-dimensional analyses, that the increase in the volume of the soldier rudimentary forewing discs is coordinated with larval size as well as with the increase in the volume of the leg and eye-antennal (head) discs. However, relative to larval size, we found that when the rudimentary forewing discs appear during the last larval instar, they are relatively smaller but increase in volume faster than that of the head (eye-antennal) and leg discs. These findings show that the rudimentary wing disc in soldier larvae has evolved novel patterns of inter-organ coordination as compared with other insects to generate the big-headed soldier caste in Pheidole. More generally, our study raises the possibility that novel patterns of inter-organ coordination are a general feature of rudimentary organs that acquire novel regulatory functions during development and evolution.

Size constrains oxygen delivery capacity within but not between bumble bee castes

Bumble bees are eusocial, with distinct worker and queen castes that vary strikingly in size and life-history. The smaller workers rely on energetically-demanding foraging flights to collect resources for rearing brood. Queens can be 3 to 4 times larger than workers, flying only for short periods in fall and again in spring after overwintering underground. These differences between castes in size and life history may be reflected in hypoxia tolerance. When oxygen demand exceeds supply, oxygen delivery to the tissues can be compromised. Previous work revealed hypermetric scaling of tracheal system volume of worker bumble bees (Bombus impatiens); larger workers had much larger tracheal volumes, likely to facilitate oxygen delivery over longer distances. Despite their much larger size, queens had relatively small tracheal volumes, potentially limiting their ability to deliver oxygen and reducing their ability to respond to hypoxia. However, these morphological measurements only indirectly point to differences in respiratory capacity. To directly assess size- and caste-related differences in tolerance to low oxygen, we measured critical PO2 (Pcrit; the ambient oxygen level below which metabolism cannot be maintained) during both rest and flight of worker and queen bumble bees. Queens and workers had similar Pcrit values during both rest and flight. However, during flight in oxygen levels near the Pcrit, mass-specific metabolic rates declined precipitously with mass both across and within castes, suggesting strong size limitations on oxygen delivery, but only during extreme conditions, when demand is high and supply is low. Together, these data suggest that the comparatively small tracheal systems of queen bumble bees do not limit their ability to deliver oxygen except in extreme conditions; they pay little cost for filling body space with eggs rather than tracheal structures.

A third species of the rare ant genus Rotastruma (Hymenoptera, Formicidae) from Cambodia, with illustrated generic diagnoses and key to species

The myrmicine genus Rotastruma is a rarely collected arboreal ant group and only two species, R. recava Bolton and R. stenoceps Bolton, are known from the Oriental Region. A new species, R. epispinasp. nov., is described from Cambodia based on the worker and queen castes. The new species is distinguished from R. recava and R. stenoceps by having the propodeal spine directed upwards and elongated peduncle of the petiole. Herein, we provide an update to the identification key and diagnostic characters for the genus Rotastruma.

Worldwide distribution of Syllophopsis subcoeca (Hymenoptera: Formicidae), an Old-World species long known only from the West Indies

ABSTRACT Syllophopsis (Hymenoptera: Formicidae) is a taxonomically difficult and possibly polyphyletic genus of tiny, inconspicuous ants. Syllophopsis subcoeca (Emery) was described in 1894 from St Thomas, US Virgin Islands. This remained the sole published record of S. subcoeca for more than 100 years. Here, we present new descriptions of the worker and queen castes, and document a greatly expanded known geographic distribution of S. subcoeca in both the New World and the Old World. We compiled published and unpublished S. subcoeca specimen records from 67 sites. These include 59 New World sites on 12 West Indian islands (Barbados, Dominica, Grenada, Guadeloupe, Martinique, Nevis, St Croix, St John, St. Kitts, St Lucia, Tortola and Trinidad) and in southern Florida. In addition, we report records of S. subcoeca from eight Old World sites, in Singapore and on two islands of the Northern Mariana Islands (Tinian and Saipan). Published records of Syllophopsis cf. subcoeca from Australia, Christmas Island and Sri Lanka are probably true S. subcoeca. All other Syllophopsis species appear to have originated in the Old World; therefore, the same is almost certainly true of S. subcoeca. It is likely that S. subcoeca was introduced to the West Indies and Florida through human commerce. Syllophopsis subcoeca appears to be a very recent arrival to Florida, where it is now known from nine sites in Miami-Dade and Broward counties. It remains to be seen whether S. subcoeca will become widespread in Florida. It seems unlikely that this S. subcoeca will become a significant ecological or agricultural pest.

Molecular evolution of bumble bee vitellogenin and vitellogenin-like genes.

Vitellogenin (Vg), a storage protein, has been significantly studied for its egg yolk precursor role in oviparous animals. Recent studies found that vitellogenin and its Vg‐like homologs were fundamentally involved in many other biological processes in social insects such as female caste differences and oxidative stress resilience. In this study, we conducted the first large‐scale molecular evolutionary analyses of vitellogenin coding genes (Vg) and Vg‐like genes of bumble bees, a primitively eusocial insect belonging to the genus Bombus. We obtained sequences for each of the four genes (Vg, Vg‐like‐A, Vg‐like‐B, and Vg‐like‐C) from 27 bumble bee genomes (nine were newly sequenced in this study), and sequences from the two closest clades of Bombus, including five Apis species and five Tetragonula species. Our molecular evolutionary analyses show that in bumble bee, the conventional Vg experienced strong positive selection, while the Vg‐like genes showed overall relaxation of purifying selection. In Apis and Tetragonula; however, all four genes were found under purifying selection. Furthermore, the conventional Vg showed signs of strong positive selection in most subgenera in Bombus, apart from the obligate parasitic subgenus Psithyrus which has no caste differentiation. Together, these results indicate that the conventional Vg, a key pleiotropic gene in social insects, is the most rapidly evolving copy, potentially due to its multiple known social functions for both worker and queen castes. This study shows that concerted evolution and purifying selection shaped the evolution of the Vg gene family following their ancient gene duplication and may be the leading forces behind the evolution of new potential protein function enabling functional social pleiotropy.

Description of the Queen of the Arabian Endemic Dolichoderine Species Tapinoma wilsoni Sharaf & Aldawood, 2012 (Hymenoptera: Formicidae) with the First Illustrated Key to Queens of the Arabian Tapinoma Foerster, 1850

We describe the queen caste of the Arabian endemic species Tapinoma wilsoni Sharaf & Aldawood, 2012 from the type locality (Dhi Ayn Archaeological Village) located in the southwestern mountains of the Kingdom of Saudi Arabia. We present the first illustrated key to the Arabian TapinomaFoerster, 1850 based on the queen castes. Three species are recognised from the Arabian Peninsula,T. melanocephalum (Fabricius, 1793), T. simrothi Krausse, 1911 and T. wilsoni Sharaf & Aldawood, 2012.

RNA m6A Modification Functions in Larval Development and Caste Differentiation in Honeybee (Apis mellifera)

Genetically identical female honeybee larvae with different diets develop into sterile workers or fertile queens. It remains unknown whether the reversible RNA N6-methyladenosine (m6A) mark functionally impact this "caste differentiation." Here, we profile the transcriptome-wide m6A methylome of honeybee queen and worker larvae at three instar stages and discover that m6A methylation dynamics are altered by differential feeding. Multiple methylome comparisons show an obvious increase in m6A marks during larval development and reveal a negative correlation between gene expression and m6A methylation. Notably, we find that worker larvae contain more hypermethylated m6A peaks than do queen larvae, and many caste-differentiation-related transcripts are differentially methylated. Chemical suppression of m6A methylation in worker larvae by 3-deazaadenosine (DAA) reduces overall m6A methylation levels and triggers worker larvae to develop queen caste features. Thus, our study demonstrates that m6A functionally impacts caste differentiation and larval development, yet it does not exclude potential contributions from other factors.