The genome of the clonal raider ant Cerapachys biroi

Co-inheritance of recombined chromatids maintains heterozygosity in a parthenogenetic ant

According to Mendel's second law, chromosomes segregate randomly in meiosis. Non-random segregation is primarily known for cases of selfish meiotic drive in females, in which particular alleles bias their own transmission into the oocyte. Here we report a rare example of unselfish meiotic drive for crossover inheritance in the clonal raider ant, Ooceraea biroi, in which both alleles are co-inherited at all loci across the entire genome. This species produces diploid offspring parthenogenetically via fusion of two haploid nuclei from the same meiosis. This process should cause rapid genotypic degeneration due to loss of heterozygosity, which results if crossover recombination is followed by random (Mendelian) segregation of chromosomes. However, by comparing whole genomes of mothers and daughters, we show that loss of heterozygosity is exceedingly rare, raising the possibility that crossovers are infrequent or absent in O. biroi meiosis. Using a combination of cytology and whole-genome sequencing, we show that crossover recombination is, in fact, common but that loss of heterozygosity is avoided because crossover products are faithfully co-inherited. This results from a programmed violation of Mendel's law of segregation, such that crossover products segregate together rather than randomly. This discovery highlights an extreme example of cellular 'memory' of crossovers, which could be a common yet cryptic feature of chromosomal segregation.

An evolutionarily conserved pathway mediated by neuroparsin-A regulates reproductive plasticity in ants

Phenotypic plasticity displayed by an animal in response to different environmental conditions is supposedly crucial for its survival and reproduction. The female adults of some ant lineages display phenotypic plasticity related to reproductive role. In pharaoh ant queens, insemination induces substantial physiological/behavioral changes and implicates remarkable gene regulatory network (GRN) shift in the brain. Here, we report a neuropeptide neuroparsin A (NPA) showing a conserved expression pattern associated with reproductive activity across ant species. Knock-down of NPA in unmated queen enhances ovary activity, whereas injection of NPA peptide in fertilized queen suppresses ovary activity. We found that NPA mainly affected the downstream gene JHBP in the ovary, which is positively regulated by NPA and suppression of which induces elevated ovary activity, and shadow which is negatively regulated by NPA. Furthermore, we show that NPA was also employed into the brain-ovary axis in regulating the worker reproductive changes in other distantly related species, such as Harpegnathos venator ants.

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.

Unselfish meiotic drive maintains heterozygosity in a parthenogenetic ant

According to Mendel's second law, chromosomes segregate randomly in meiosis. Nonrandom segregation is primarily known for cases of selfish meiotic drive in females, in which particular alleles bias their own transmission into the oocyte1,2. Here, we report a rare example of unselfish meiotic drive for crossover inheritance in the clonal raider ant, Ooceraea biroi. This species produces diploid offspring parthenogenetically via fusion of two haploid nuclei from the same meiosis3. This process should cause rapid genotypic degeneration due to loss of heterozygosity, which results if crossover recombination is followed by random (Mendelian) segregation of chromosomes4,5. However, by comparing whole genomes of mothers and daughters, we show that loss of heterozygosity is exceedingly rare, raising the possibility that crossovers are infrequent or absent in O. biroi meiosis. Using a combination of cytology and whole genome sequencing, we show that crossover recombination is, in fact, common, but that loss of heterozygosity is avoided because crossover products are faithfully co-inherited. This results from a programmed violation of Mendel's law of segregation, such that crossover products segregate together rather than randomly. This discovery highlights an extreme example of cellular "memory" of crossovers, which could be a common yet cryptic feature of chromosomal segregation.

A Chemical Counterpart to the Resolution Step of Nature's Intein-Mediated Protein Splicing

In the course of an attempted total chemical synthesis of the ant insulin-like peptide-2 (ILP2) protein molecule, specific cleavage of a backbone peptide bond in a branched ester-linked polypeptide chain with concomitant peptide splicing was observed. The side reaction was investigated in model compounds. Here, we postulate a chemical mechanism for this novel polypeptide backbone cleavage reaction as a chemical counterpart to the resolution step of biochemical intein-mediated protein splicing.

Task-specific odorant receptor expression in worker antennae indicates that sensory filters regulate division of labor in ants

Division of labor (DOL) is a characteristic trait of insect societies, where tasks are generally performed by specialized individuals. Inside workers focus on brood or nest care, while others take risks by foraging outside. Theory proposes that workers have different thresholds to perform certain tasks when confronted with task-related stimuli, leading to specialization and consequently DOL. Workers are presumed to vary in their response to task-related cues rather than in how they perceive such information. Here, we test the hypothesis that DOL instead stems from workers varying in their efficiency to detect stimuli of specific tasks. We use transcriptomics to measure mRNA expression levels in the antennae and brain of nurses and foragers of the ant Temnothorax longispinosus. We find seven times as many genes to be differentially expressed between behavioral phenotypes in the antennae compared to the brain. Moreover, half of all odorant receptors are differentially expressed, with an overrepresentation of the 9-exon gene family upregulated in the antennae of nurses. Nurses and foragers thus apparently differ in the perception of their olfactory environment and task-related signals. Our study supports the hypothesis that antennal sensory filters predispose workers to specialize in specific tasks.

Evolution of five environmentally responsive gene families in a pine-feeding sawfly, Neodiprion lecontei (Hymenoptera: Diprionidae)

A central goal in evolutionary biology is to determine the predictability of adaptive genetic changes. Despite many documented cases of convergent evolution at individual loci, little is known about the repeatability of gene family expansions and contractions. To address this void, we examined gene family evolution in the redheaded pine sawfly Neodiprion lecontei, a noneusocial hymenopteran and exemplar of a pine-specialized lineage evolved from angiosperm-feeding ancestors. After assembling and annotating a draft genome, we manually annotated multiple gene families with chemosensory, detoxification, or immunity functions before characterizing their genomic distributions and molecular evolution. We find evidence of recent expansions of bitter gustatory receptor, clan 3 cytochrome P450, olfactory receptor, and antimicrobial peptide subfamilies, with strong evidence of positive selection among paralogs in a clade of gustatory receptors possibly involved in the detection of bitter compounds. In contrast, these gene families had little evidence of recent contraction via pseudogenization. Overall, our results are consistent with the hypothesis that in response to novel selection pressures, gene families that mediate ecological interactions may expand and contract predictably. Testing this hypothesis will require the comparative analysis of high-quality annotation data from phylogenetically and ecologically diverse insect species and functionally diverse gene families. To this end, increasing sampling in under-sampled hymenopteran lineages and environmentally responsive gene families and standardizing manual annotation methods should be prioritized.

Behavioural individuality determines infection risk in clonal ant colonies

In social groups, infection risk is not distributed evenly across individuals. Individual behaviour is a key source of variation in infection risk, yet its effects are difficult to separate from other factors (e.g., age). Here, we combine epidemiological experiments with chemical, transcriptomic, and automated behavioural analyses in clonal ant colonies, where behavioural individuality emerges among identical workers. We find that: (1) Caenorhabditis-related nematodes parasitise ant heads and affect their survival and physiology, (2) differences in infection emerge from behavioural variation alone, and reflect spatially-organised division of labour, (3) infections affect colony social organisation by causing infected workers to stay in the nest. By disproportionately infecting some workers and shifting their spatial distribution, infections reduce division of labour and increase spatial overlap between hosts, which should facilitate parasite transmission. Thus, division of labour, a defining feature of societies, not only shapes infection risk and distribution but is also modulated by parasites.

Cosegregation of recombinant chromatids maintains genome-wide heterozygosity in an asexual nematode

In asexual animals, female meiosis is modified to produce diploid oocytes. If meiosis still involves recombination, this is expected to lead to a rapid loss of heterozygosity, with adverse effects on fitness. Many asexuals, however, have a heterozygous genome, the underlying mechanisms being most often unknown. Cytological and population genomic analyses in the nematode Mesorhabditis belari revealed another case of recombining asexual being highly heterozygous genome-wide. We demonstrated that heterozygosity is maintained despite recombination because the recombinant chromatids of each chromosome pair cosegregate during the unique meiotic division. A theoretical model confirmed that this segregation bias is necessary to account for the observed pattern and likely to evolve under a wide range of conditions. Our study uncovers an unexpected type of non-Mendelian genetic inheritance involving cosegregation of recombinant chromatids.

Age and Task Modulate Olfactory Sensitivity in the Florida Carpenter Ant Camponotus floridanus

Age-related changes in behavior and sensory perception have been observed in a wide variety of animal species. In ants and other eusocial insects, workers often progress through an ordered sequence of olfactory-driven behavioral tasks. Notably, these behaviors are plastic, and workers adapt and rapidly switch tasks in response to changing environmental conditions. In the Florida carpenter ant, smaller minors typically perform most of the work needed to maintain the colony, while the larger majors are specialized for nest defense and rarely engage in these routine tasks. Here, we investigate the effects of age and task group on olfactory responses to a series of odorant blends in minor and major worker castes. Consistent with their respective roles within the colony, we observed significant age-associated shifts in the olfactory responses of minors as they transitioned between behavioral states, whereas the responses of majors remained consistently low regardless of age. Furthermore, we have identified a unitary compound, 3-methylindole, which elicited significantly higher responses and behavioral aversion in minor nurses than in similarly aged foragers suggesting that this compound may play an important role in brood care. Taken together, our results suggest that age- and task-associated shifts in olfactory physiology may play a critical role in the social organization of ant colonies.

Chromosome-level genome assemblies of two parasitoid biocontrol wasps reveal the parthenogenesis mechanism and an associated novel virus

BACKGROUND

Biocontrol is a key technology for the control of pest species. Microctonus parasitoid wasps (Hymenoptera: Braconidae) have been released in Aotearoa New Zealand as biocontrol agents, targeting three different pest weevil species. Despite their value as biocontrol agents, no genome assemblies are currently available for these Microctonus wasps, limiting investigations into key biological differences between the different species and strains.

METHODS AND FINDINGS

Here we present high-quality genomes for Microctonus hyperodae and Microctonus aethiopoides, assembled with short read sequencing and Hi-C scaffolding. These assemblies have total lengths of 106.7 Mb for M. hyperodae and 129.2 Mb for M. aethiopoides, with scaffold N50 values of 9 Mb and 23 Mb respectively. With these assemblies we investigated differences in reproductive mechanisms, and association with viruses between Microctonus wasps. Meiosis-specific genes are conserved in asexual Microctonus, with in-situ hybridisation validating expression of one of these genes in the ovaries of asexual Microctonus aethiopoides. This implies asexual reproduction in these Microctonus wasps involves meiosis, with the potential for sexual reproduction maintained. Investigation of viral gene content revealed candidate genes that may be involved in virus-like particle production in M. aethiopoides, as well as a novel virus infecting M. hyperodae, for which a complete genome was assembled.

CONCLUSION AND SIGNIFICANCE

These are the first published genomes for Microctonus wasps which have been deployed as biocontrol agents, in Aotearoa New Zealand. These assemblies will be valuable resources for continued investigation and monitoring of these biocontrol systems. Understanding the biology underpinning Microctonus biocontrol is crucial if we are to maintain its efficacy, or in the case of M. hyperodae to understand what may have influenced the significant decline of biocontrol efficacy. The potential for sexual reproduction in asexual Microctonus is significant given that empirical modelling suggests this asexual reproduction is likely to have contributed to biocontrol decline. Furthermore the identification of a novel virus in M. hyperodae highlights a previously unknown aspect of this biocontrol system, which may contribute to premature mortality of the host pest. These findings have potential to be exploited in future in attempt to increase the effectiveness of M. hyperodae biocontrol.

The First Complete Mitochondrial Genome of the Genus Pachycondyla (Formicidae, Ponerinae) and Insights into the Phylogeny of Ants

Ants are the standout group among eusocial insects in terms of their exceptional species richness and ecological dominance. The phylogenetic relationships among the group remain elusive. Mitochondrial genome sequences, as a kind of molecular marker, have been widely utilized in the phylogenetic analysis of insects. However, the number of ant mitogenomes published is still very limited. In this study, we utilized next-generation sequencing to determine the complete mitogenome of Pachycondyla annamita (Formicidae, Ponerinae). This is the first mitogenome from the genus Pachycondyla. Two gene rearrangements were identified in the mitogenome, the transposition of trnQ and trnM and the transposition of trnV and rrnS. The secondary structures of tRNAs were predicted. The tRNA genes trnR and trnS1 lacked the dihydrouridine (DHU) arm, and the trnE lacked the TΨC (T) arm. Phylogenetic analyses of the mitochondrial protein-coding genes under maximum likelihood (ML) and Bayesian inference (BI) criteria resulted in conflicting hypotheses. BI analysis using amino acid data with the site-heterogeneous mixture model produced a tree topology congruent with previous studies. The Formicidae was subdivided into two main clades, namely the "poneroid" clade and the "formicoid" clade. A sister group relationship between Myrmicinae and Formicinae was recovered within the "formicoid" clade.

Age and Task Modulate Olfactory Sensitivity in the Florida Carpenter Ant Camponotus floridanus

Age-related changes in behavior and sensory perception have been observed in a wide variety of animal species. In ants and other eusocial insects, workers often progress through an ordered sequence of olfactory-driven behavioral tasks. Notably, these behaviors are plastic, and workers adapt and rapidly switch tasks in response to changing environmental conditions. In the Florida carpenter ant, smaller minors typically perform most of the work needed to maintain the colony while the larger majors are specialized for nest defense and rarely engage in these routine tasks. Here, we investigate the effects of age and task group on olfactory responses to a series of odorant blends in minor and major worker castes. Consistent with their respective roles within the colony, we observed significant age-associated shifts in the olfactory responses of minors as they transitioned between behavioral states, whereas the responses of majors remained consistently low regardless of age. Furthermore, we identified a unitary compound, 3-methylindole, which elicited significantly higher responses and behavioral aversion in minor nurses than in similarly aged foragers suggesting that this compound may play an important role in brood care. Taken together, our results suggest that age- and task-associated shifts in olfactory physiology may play a critical role in the social organization of ant colonies.

Simple Summary

Florida carpenter ants ( Camponotus floridanus ) live in colonies comprised of thousands of workers. The smallest workers, known as minors, engage in routine tasks such as nursing and foraging while the largest workers, known as majors, are thought to be soldiers specialized for defending the nest. How ant colonies allocate their workforce to address the dynamic and ever-changing needs of the colonies remains an open question in the field, but current evidence suggests that ant social behavior likely results from a combination of genetic/epigenetic, physiological, and systems-level processes. Here, we extend these studies by investigating the role of olfactory sensitivity in regulating ant behavior. Minor workers exhibited significant shifts in olfactory sensitivity and odor coding as they aged and switched tasks. The olfactory sensitivity of majors, however, remained relatively stable as they aged. From these studies, we also identified a single compound, 3-methylindole, which elicited significantly higher olfactory responses and aversive behavior in nurses compared to foragers, suggesting that this chemical may have a role in brood care. Overall, these studies support the hypothesis that changes in olfactory sensitivity play an important role in regulating social behavior in ants.

Sparse and stereotyped encoding implicates a core glomerulus for ant alarm behavior

Ants communicate via large arrays of pheromones and possess expanded, highly complex olfactory systems, with antennal lobes in the brain comprising up to ∼500 glomeruli. This expansion implies that odors could activate hundreds of glomeruli, which would pose challenges for higher-order processing. To study this problem, we generated transgenic ants expressing the genetically encoded calcium indicator GCaMP in olfactory sensory neurons. Using two-photon imaging, we mapped complete glomerular responses to four ant alarm pheromones. Alarm pheromones robustly activated ≤6 glomeruli, and activity maps for the three pheromones inducing panic alarm in our study species converged on a single glomerulus. These results demonstrate that, rather than using broadly tuned combinatorial encoding, ants employ precise, narrowly tuned, and stereotyped representations of alarm pheromones. The identification of a central sensory hub glomerulus for alarm behavior suggests that a simple neural architecture is sufficient to translate pheromone perception into behavioral outputs.

DNMT1 mutant ants develop normally but have disrupted oogenesis

Although DNA methylation is an important gene regulatory mechanism in mammals, its function in arthropods remains poorly understood. Studies in eusocial insects have argued for its role in caste development by regulating gene expression and splicing. However, such findings are not always consistent across studies, and have therefore remained controversial. Here we use CRISPR/Cas9 to mutate the maintenance DNA methyltransferase DNMT1 in the clonal raider ant, Ooceraea biroi. Mutants have greatly reduced DNA methylation, but no obvious developmental phenotypes, demonstrating that, unlike mammals, ants can undergo normal development without DNMT1 or DNA methylation. Additionally, we find no evidence of DNA methylation regulating caste development. However, mutants are sterile, whereas in wild-type ants, DNMT1 is localized to the ovaries and maternally provisioned into nascent oocytes. This supports the idea that DNMT1 plays a crucial but unknown role in the insect germline.

Parthenogenesis in dipterans: a genetic perspective

Parthenogenesis has been documented in almost every phylum of animals, and yet this phenomenon is largely understudied. It has particular importance in dipterans since some parthenogenetic species are also disease vectors and agricultural pests. Here, we present a catalogue of parthenogenetic dipterans, although it is likely that many more remain to be identified, and we discuss how their developmental biology and interactions with diverse environments may be linked to different types of parthenogenetic reproduction. We discuss how the advances in genetics and genomics have identified chromosomal loci associated with parthenogenesis. In particular, a polygenic cause of facultative parthenogenesis has been uncovered in Drosophila mercatorum, allowing the corresponding genetic variants to be tested for their ability to promote parthenogenesis in another species, Drosophila melanogaster. This study probably identifies just one of many routes that could be followed in the evolution of parthenogenesis. We attempt to account for why the phenomenon has evolved so many times in the dipteran order and why facultative parthenogenesis appears particularly prevalent. We also discuss the significance of coarse genomic changes, including non-disjunction, aneuploidy, and polyploidy and how, together with changes to specific genes, these might relate to both facultative and obligate parthenogenesis in dipterans and other parthenogenetic animals.

Evolution: A social parasite was born from a virgin

The sudden appearance of small winged queens within a lineage of asexually reproducing ant workers reveals that such social parasites can appear abruptly. The parasitic queens differ in a large genomic region, suggesting that a supergene instantly equipped the social parasite with a suite of co-adapted traits.

A complete absence of indirect genetic effects on brain gene expression in a highly social context

Genes not only control traits of their carrier organism (known as direct genetic effects or DGEs) but also shape their carrier's physical environment and the phenotypes of their carrier's social partners (known as indirect genetic effects or IGEs). Theoretical research has shown that the effects that genes exert on social partners can have profound consequences, potentially altering heritability and the direction of trait evolution. Complementary empirical research has shown that in various contexts (particularly in animal agriculture) IGEs can explain a large proportion of variation in specific traits. However, little is known about the general prevalence of IGEs. We conducted a reciprocal cross-fostering experiment with two genetic lineages of the clonal raider ant Ooceraea biroi to quantify the relative contribution of DGEs and IGEs to variation in brain gene expression (which underlies behavioural variation). We found that thousands of genes are differentially expressed by DGEs but not a single gene is differentially expressed by IGEs. This is surprising given the highly social context of ant colonies and given that individual behaviour varies according to the genotypic composition of the social environment in O. biroi. Overall, these findings indicate that we have a lot to learn about how the magnitude of IGEs varies across species and contexts.

My road to the ants: A model clade for eco-evo-devo

This chapter is the story of how I pioneered ants as a system for studying eco-evo-devo, a field that integrates developmental biology with ecology and evolutionary biology. One aim of eco-evo-devo is to understand how the interactions between genes and their environments during development facilitates the origin and evolution of novel phenotypes. In a series of six parts, I review some of the key discoveries from my lab on how novel worker caste systems in ants--soldiers and supersoldiers--originated and evolved. I also discuss some of the ideas that emerged from these discoveries, including the role that polyphenisms, hidden developmental potentials, and rudimentary organs play in facilitating developmental and evolutionary change. As superorganisms, I argue that ants are uniquely positioned to reveal types of variation that are often difficult to observe in nature. In doing so, they have the potential to transform our view of biology and provide new perspectives in medicine, agriculture, and biodiversity conservation. With my story I hope to inspire the next generation of biologists to continue exploring the unknown regions of phenotypic space to solve some of our most pressing societal challenges.

Decay of homologous chromosome pairs and discovery of males in the thelytokous fungus-growing ant Mycocepurus smithii

The prevalent mode of reproduction among ants is arrhenotokous parthenogenesis where unfertilized eggs give rise to haploid males and fertilized eggs develop into diploid females. Some ant species are capable of thelytokous parthenogenesis, a type of asexual reproduction where females develop from unfertilized diploid eggs. Thelytoky is well-documented in more than 20 ant species. Cytogenetic data are available for six species demonstrating that some thelytokous ant species are capable of producing males occasionally as well as maintaining their chromosome numbers and proper chromosome pairings. Mycocepurus smithii is a thelytokous fungus-growing ant species that inhabits large parts of Central and South America. Cytogenetic data are unavailable for M. smithii and male individuals were never documented for this species, although the presence of males is expected because genetic recombination was observed in a few sexually reproducing populations in Brazil and haploid sperm was documented from the spermathecae of M. smithii queens. This study aims at comparatively studying asexual and sexual populations of M. smithii using classical and molecular cytogenetic methods to test whether karyotype configuration is modified according to the mode of reproduction in M. smithii. Moreover, we report the discovery of M. smithii males from a sexually reproducing population in the Brazilian state Pará, diagnose the male of M. smithii, and morphologically characterize their spermatozoa. Karyotypic variation was observed within the asexual population (2n = 9, 10, or 11), whereas the chromosome number was fixed in the sexual population (2n = 14, n = 7). Identical karyotypes were maintained within individual M. smithii colonies and karyotype variation was only observed between colonies. In asexual individuals, the karyomorphs showed a decay of homologous chromosome pairs, especially in individuals with the karyomorph 2n = 11, which is potentially caused by relaxed natural selection on proper chromosome pairing. In contrast, females in the sexual population showed proper homologous chromosome pairings. In individuals of both asexual and sexual populations, we find that heterochromatin was localized in centromeric regions and on the short arms of the chromosomes, GC-rich regions were associated with heterochromatic regions, and 18S rDNA genes were located on the largest chromosome pair. This comparative cytogenetic analysis contributes to our understanding about the cytological mechanisms associated with thelytokous parthenogenesis in ants and suggests the decay of chromosome structure in the absence of meiosis and genetic recombination.

The pupal moulting fluid has evolved social functions in ants

Insect societies are tightly integrated, complex biological systems in which group-level properties arise from the interactions between individuals^1-4. However, these interactions have not been studied systematically and therefore remain incompletely known. Here, using a reverse engineering approach, we reveal that unlike solitary insects, ant pupae extrude a secretion derived from the moulting fluid that is rich in nutrients, hormones and neuroactive substances. This secretion elicits parental care behaviour and is rapidly removed and consumed by the adults. This behaviour is crucial for pupal survival; if the secretion is not removed, pupae develop fungal infections and die. Analogous to mammalian milk, the secretion is also an important source of early larval nutrition, and young larvae exhibit stunted growth and decreased survival without access to the fluid. We show that this derived social function of the moulting fluid generalizes across the ants. This secretion thus forms the basis of a central and hitherto overlooked interaction network in ant societies, and constitutes a rare example of how a conserved developmental process can be co-opted to provide the mechanistic basis of social interactions. These results implicate moulting fluids in having a major role in the evolution of ant eusociality.

Evidence from automixis with inverted meiosis for the maintenance of sex by loss of complementation

The adaptive value of sexual reproduction is still debated. A short-term disadvantage of asexual reproduction is loss of heterozygosity, which leads to the unmasking of recessive deleterious mutations. The cost of this loss of complementation is predicted to be higher than the twofold cost of meiosis for most types of asexual reproduction. Automixis with terminal fusion of sister nuclei is especially vulnerable to the effect of loss of complementation. It is found, however, in some taxa including oribatid mites, the most prominent group of ancient asexuals. Here, I show that automixis with terminal fusion is stable if it is associated with inverted meiosis and that this appears to be the case in nature, notably in oribatid mites. The existence of automixis with terminal fusion, and its co-occurrence with inverted meiosis, therefore, is consistent with the hypothesis that loss of complementation is important in the evolution of sexual reproduction.

Hymenoptera Genome Database: new genomes and annotation datasets for improved go enrichment and orthologue analyses

We report an update of the Hymenoptera Genome Database (HGD; http://HymenopteraGenome.org), a genomic database of hymenopteran insect species. The number of species represented in HGD has nearly tripled, with fifty-eight hymenopteran species, including twenty bees, twenty-three ants, eleven wasps and four sawflies. With a reorganized website, HGD continues to provide the HymenopteraMine genomic data mining warehouse and JBrowse/Apollo genome browsers integrated with BLAST. We have computed Gene Ontology (GO) annotations for all species, greatly enhancing the GO annotation data gathered from UniProt with more than a ten-fold increase in the number of GO-annotated genes. We have also generated orthology datasets that encompass all HGD species and provide orthologue clusters for fourteen taxonomic groups. The new GO annotation and orthology data are available for searching in HymenopteraMine, and as bulk file downloads.

The genomic basis of army ant chemosensory adaptations

The evolution of mass raiding has allowed army ants to become dominant arthropod predators in the tropics. Although a century of research has led to many discoveries about behavioural, morphological and physiological adaptations in army ants, almost nothing is known about the molecular basis of army ant biology. Here we report the genome of the iconic New World army ant Eciton burchellii, and show that it is unusually compact, with a reduced gene complement relative to other ants. In contrast to this overall reduction, a particular gene subfamily (9‐exon ORs) expressed predominantly in female antennae is expanded. This subfamily has previously been linked to the recognition of hydrocarbons, key olfactory cues used in insect communication and prey discrimination. Confocal microscopy of the brain showed a corresponding expansion in a putative hydrocarbon response centre within the antennal lobe, while scanning electron microscopy of the antenna revealed a particularly high density of hydrocarbon‐sensitive sensory hairs. E. burchellii shares these features with its predatory and more cryptic relative, the clonal raider ant. By integrating genomic, transcriptomic and anatomical analyses in a comparative context, our work thus provides evidence that army ants and their relatives possess a suite of modifications in the chemosensory system that may be involved in behavioural coordination and prey selection during social predation. It also lays the groundwork for future studies of army ant biology at the molecular level.

A molecular toolkit for superorganisms

Social insects, such as ants, bees, wasps, and termites, draw biologists' attention due to their distinctive lifestyles. As experimental systems, they provide unique opportunities to study organismal differentiation, division of labor, longevity, and the evolution of development. Ants are particularly attractive because several ant species can be propagated in the laboratory. However, the same lifestyle that makes social insects interesting also hampers the use of molecular genetic techniques. Here, we summarize the efforts of the ant research community to surmount these hurdles and obtain novel mechanistic insight into the biology of social insects. We review current approaches and propose novel ones involving genomics, transcriptomics, chromatin and DNA methylation profiling, RNA interference (RNAi), and genome editing in ants and discuss future experimental strategies.

Founder effects on sex determination systems in invasive social insects

Invasive populations are often established from a small number of individuals, and thus have low genetic diversity relative to native-range populations. Social ants, bees and wasps (social Hymenoptera) should be vulnerable to such founder effects on genetic diversity because sex in these species is determined genetically via Complementary Sex Determination (CSD). Under CSD, individuals homozygous at one or more critical sex loci are inviable or develop as infertile diploid males. Low diversity at sex loci leads to increased homozygosity and diploid male production, increasing the chance of colony death. In this review, we identify behavioral, social and reproductive traits that preserve allele richness at sex loci, allow colonies to cope with diploid male production, and eventually restore sex allele diversity in invasive populations of social Hymenoptera that experience founding bottlenecks.

Transposable elements and introgression introduce genetic variation in the invasive ant Cardiocondyla obscurior

Introduced populations of invasive organisms have to cope with novel environmental challenges, while having reduced genetic variation caused by founder effects. The mechanisms associated with this "genetic paradox of invasive species" has received considerable attention, yet few studies have examined the genomic architecture of invasive species. Populations of the heart node ant Cardiocondyla obscurior belong to two distinct lineages, a New World lineage so far only found in Latin America and a more globally distributed Old World lineage. In the present study, we use population genomic approaches to compare populations of the two lineages with apparent divergent invasive potential. We find that the strong genetic differentiation of the two lineages began at least 40,000 generations ago and that activity of transposable elements (TEs) has contributed significantly to the divergence of both lineages, possibly linked to the very unusual genomic distribution of TEs in this species. Furthermore, we show that introgression from the Old World lineage is a dominant source of genetic diversity in the New World lineage, despite the lineages' strong genetic differentiation. Our study uncovers mechanisms underlying novel genetic variation in introduced populations of C. obscurior that could contribute to the species' adaptive potential.

DNA methylation is not a driver of gene expression reprogramming in young honey bee workers

The presence of DNA methylation marks within genic intervals, also called gene body methylation, is an evolutionarily-conserved epigenetic hallmark of animal and plant methylomes. In social insects, gene body methylation is thought to contribute to behavioural plasticity, for example between foragers and nurse workers, by modulating gene expression. However, recent studies have suggested that the majority of DNA methylation is sequence-specific, and therefore cannot act as a flexible mediator between environmental cues and gene expression. To address this paradox, we examined whole-genome methylation patterns in the brains and ovaries of young honey bee workers that had been subjected to divergent social contexts: the presence or absence of the queen. Although these social contexts are known to bring about extreme changes in behavioral and reproductive traits through differential gene expression, we found no significant differences between the methylomes of workers from queenright and queenless colonies. In contrast, thousands of regions were differentially methylated between colonies, and these differences were not associated with differential gene expression in the subset of genes examined. Methylation patterns were highly similar between brain and ovary tissues and only differed in nine regions. These results strongly indicate that DNA methylation is not a driver of differential gene expression between tissues or behavioral morphs. Finally, despite the lack of difference in methylation patterns, queen presence affected the expression of all four DNA methyltransferase genes, suggesting that these enzymes have roles beyond DNA methylation. Therefore, the functional role of DNA methylation in social insect genomes remains an open question.

An oxytocin/vasopressin-related neuropeptide modulates social foraging behavior in the clonal raider ant

Oxytocin/vasopressin-related neuropeptides are highly conserved and play major roles in regulating social behavior across vertebrates. However, whether their insect orthologue, inotocin, regulates the behavior of social groups remains unknown. Here, we show that in the clonal raider ant Ooceraea biroi, individuals that perform tasks outside the nest have higher levels of inotocin in their brains than individuals of the same age that remain inside the nest. We also show that older ants, which spend more time outside the nest, have higher inotocin levels than younger ants. Inotocin thus correlates with the propensity to perform tasks outside the nest. Additionally, increasing inotocin pharmacologically increases the tendency of ants to leave the nest. However, this effect is contingent on age and social context. Pharmacologically treated older ants have a higher propensity to leave the nest only in the presence of larvae, whereas younger ants seem to do so only in the presence of pupae. Our results suggest that inotocin signaling plays an important role in modulating behaviors that correlate with age, such as social foraging, possibly by modulating behavioral response thresholds to specific social cues. Inotocin signaling thereby likely contributes to behavioral individuality and division of labor in ant societies.

The neuropeptides oxytocin and vasopressin modulate social behavior in vertebrates, but their function in invertebrates is not well understood. Using brain staining and pharmacological manipulations, this study shows that a related neuropeptide, inotocin, affects how ants respond to larvae.

Response thresholds alone cannot explain empirical patterns of division of labor in social insects

The effects of heterogeneity in group composition remain a major hurdle to our understanding of collective behavior across disciplines. In social insects, division of labor (DOL) is an emergent, colony-level trait thought to depend on colony composition. Theoretically, behavioral response threshold models have most commonly been employed to investigate the impact of heterogeneity on DOL. However, empirical studies that systematically test their predictions are lacking because they require control over colony composition and the ability to monitor individual behavior in groups, both of which are challenging. Here, we employ automated behavioral tracking in 120 colonies of the clonal raider ant with unparalleled control over genetic, morphological, and demographic composition. We find that each of these sources of variation in colony composition generates a distinct pattern of behavioral organization, ranging from the amplification to the dampening of inherent behavioral differences in heterogeneous colonies. Furthermore, larvae modulate interactions between adults, exacerbating the apparent complexity. Models based on threshold variation alone only partially recapitulate these empirical patterns. However, by incorporating the potential for variability in task efficiency among adults and task demand among larvae, we account for all the observed phenomena. Our findings highlight the significance of previously overlooked parameters pertaining to both larvae and workers, allow the formulation of theoretical predictions for increasing colony complexity, and suggest new avenues of empirical study.

This study uses automated tracking of clonal raider ants and mathematical modeling to reveal how previously overlooked traits of larvae and workers might shape social organization in heterogeneous ant colonies. By incorporating the potential for variability in task efficiency among adults and task demand among larvae, the authors were able to account for all empirically observed phenomena.

A chromosome-level genome assembly of the parasitoid wasp, Cotesia glomerata (Hymenoptera: Braconidae)

Hymenopterans make up about 20% of all animal species, but most are poorly known and lack high-quality genomic resources. One group of important, yet under-studied hymenopterans, are parasitoid wasps in the family Braconidae. Among this under-studied group are braconid wasps in the genus Cotesia; a clade of ~1,000 species routinely used in studies of physiology, ecology, biological control, and genetics. However, our ability to understand these organisms has been hindered by a lack of genomic resources. We helped bridge this gap by generating a high-quality genome assembly for the parasitoid wasp, Cotesia glomerata (Braconidae; Microgastrinae). We generated this assembly using multiple sequencing technologies, including Oxford Nanopore, whole-genome shotgun sequencing, and 3-D chromatin contact information (Hi-C). Our assembly is one of the most contiguous, complete, and publicly available hymenopteran genomes, represented by 3,355 scaffolds with a scaffold N50 of ~28Mb and a BUSCO score of ~99%. Given the genome sizes found in closely related species, our genome assembly was ~50% larger than expected, which was apparently induced by runaway amplification of three types of repetitive elements: simple repeats, Long Terminal Repeats (LTRs), and Long Interspersed Nuclear Elements (LINEs). This assembly is another step forward for genomics across this hyper-diverse, yet understudied, order of insects. The assembled genomic data and metadata files are publicly available via Figshare (https://doi.org/10.6084/m9.figshare.13010549).

Comprehensive analysis of male-free reproduction in Monomorium triviale (Formicidae: Myrmicinae)

We report comprehensive evidence for obligatory thelytokous parthenogenesis in an ant Monomorium triviale. This species is characterized by distinct queen–worker dimorphism with strict reproductive division of labor: queens produce both workers and new queens without mating, whereas workers are completely sterile. We collected 333 nests of this species from 14 localities and three laboratory-reared populations in Japan. All wild queens dissected had no sperm in their spermathecae. Laboratory observation confirmed that virgin queens produced workers without mating. Furthermore, microsatellite genotyping showed identical heterozygous genotypes between mothers and their respective daughters, suggesting an extremely low probability of sexual reproduction. Microbial analysis detected no bacterial genera that are known to induce thelytokous parthenogenesis in Hymenoptera. Finally, the lack of variation in partial sequences of mitochondrial DNA among individuals sampled from across Japan suggests recent rapid spread or selective sweep. M. triviale would be a promising model system of superorganism-like adaptation through comparative analysis with well-studied sexual congeners, including the pharaoh ant M. pharaonis.

Advances in the Study of Olfaction in Eusocial Ants

Over the past decade, spurred in part by the sequencing of the first ant genomes, there have been major advances in the field of olfactory myrmecology. With the discovery of a significant expansion of the odorant receptor gene family, considerable efforts have been directed toward understanding the olfactory basis of complex social behaviors in ant colonies. Here, we review recent pivotal studies that have begun to reveal insights into the development of the olfactory system as well as how olfactory stimuli are peripherally and centrally encoded. Despite significant biological and technical impediments, substantial progress has been achieved in the application of gene editing and other molecular techniques that notably distinguish the complex olfactory system of ants from other well-studied insect model systems, such as the fruit fly. In doing so, we hope to draw attention not only to these studies but also to critical knowledge gaps that will serve as a compass for future research endeavors.

Genomic Features of Parthenogenetic Animals

Evolution without sex is predicted to impact genomes in numerous ways. Case studies of individual parthenogenetic animals have reported peculiar genomic features that were suggested to be caused by their mode of reproduction, including high heterozygosity, a high abundance of horizontally acquired genes, a low transposable element load, or the presence of palindromes. We systematically characterized these genomic features in published genomes of 26 parthenogenetic animals representing at least 18 independent transitions to asexuality. Surprisingly, not a single feature was systematically replicated across a majority of these transitions, suggesting that previously reported patterns were lineage-specific rather than illustrating the general consequences of parthenogenesis. We found that only parthenogens of hybrid origin were characterized by high heterozygosity levels. Parthenogens that were not of hybrid origin appeared to be largely homozygous, independent of the cellular mechanism underlying parthenogenesis. Overall, despite the importance of recombination rate variation for the evolution of sexual animal genomes, the genome-wide absence of recombination does not appear to have had the dramatic effects which are expected from classical theoretical models. The reasons for this are probably a combination of lineage-specific patterns, the impact of the origin of parthenogenesis, and a survivorship bias of parthenogenetic lineages.

Expressions of conventional vitellogenin and vitellogenin-like A in worker brains are associated with a nursing task in a ponerine ant

In eusocial insect colonies, non-reproductive workers often perform different tasks. Tasks of an individual worker are shifted depending on various factors, e.g., age and colony demography. Although a vitellogenin (Vg) gene play regulatory roles in both reproductive and non-reproductive division of labours in a honeybee, it has been shown that the insect Vg underwent multiple gene duplications and sub-functionalisation, especially in apical ant lineages. The regulatory roles of duplicated Vgs were suggested to change evolutionarily among ants, whereas such roles in phylogenetically basal ants remain unclear. Here, we examined the expression patterns of conventional Vg (CVg), Vg-like A, Vg-like B and Vg-like C, as well as Vg receptor, during the task shift in an age-dependent manner and under experimental manipulation of colony demography in a primitive ant Diacamma sp. Expressions of CVg and Vg-like A in a brain were associated with a nursing task. It is suggested that associations of brain expressions of these Vgs with worker tasks were acquired in the basal ant lineage, and that such Vg functions could have sub-functionalised in the derived ant lineage.

Chemoreceptor Diversity in Apoid Wasps and Its Reduction during the Evolution of the Pollen-Collecting Lifestyle of Bees (Hymenoptera: Apoidea)

Chemoreceptors help insects to interact with their environment, to detect and assess food sources and oviposition sites, and to aid in intra- and interspecific communication. In Hymenoptera, species of eusocial lineages possess large chemoreceptor gene repertoires compared with solitary species, possibly because of their additional need to recognize nest-mates and caste. However, a critical piece of information missing so far has been the size of chemoreceptor gene repertoires of solitary apoid wasps. Apoid wasps are a paraphyletic group of almost exclusively solitary Hymenoptera phylogenetically positioned between ant and bee, both of which include eusocial species. We report the chemosensory-related gene repertoire sizes of three apoid wasps: Ampulex compressa, Cerceris arenaria, and Psenulus fuscipennis. We annotated genes encoding odorant (ORs), gustatory, and ionotropic receptors and chemosensory soluble proteins and odorant-binding proteins in transcriptomes of chemosensory tissues of the above three species and in early draft genomes of two species, A. compressa and C. arenaria. Our analyses revealed that apoid wasps possess larger OR repertoires than any bee lineage, that the last common ancestor of Apoidea possessed a considerably larger OR repertoire (∼160) than previously estimated (73), and that the expansion of OR genes in eusocial bees was less extensive than previously assumed. Intriguingly, the evolution of pollen-collecting behavior in the stem lineage of bees was associated with a notable loss of OR gene diversity. Thus, our results support the view that herbivorous Hymenoptera tend to possess smaller OR repertoires than carnivorous, parasitoid, or kleptoparasitic species.

anTraX, a software package for high-throughput video tracking of color-tagged insects

Recent years have seen a surge in methods to track and analyze animal behavior. Nevertheless, tracking individuals in closely interacting, group-living organisms remains a challenge. Here, we present anTraX, an algorithm and software package for high-throughput video tracking of color-tagged insects. anTraX combines neural network classification of animals with a novel approach for representing tracking data as a graph, enabling individual tracking even in cases where it is difficult to segment animals from one another, or where tags are obscured. The use of color tags, a well-established and robust method for marking individual insects in groups, relaxes requirements for image size and quality, and makes the software broadly applicable. anTraX is readily integrated into existing tools and methods for automated image analysis of behavior to further augment its output. anTraX can handle large-scale experiments with minimal human involvement, allowing researchers to simultaneously monitor many social groups over long time periods.

Vitellogenin expression corresponds with reproductive status and caste in a primitively eusocial bee

Vitellogenin (vg) expression is consistently associated with variation in insect phenotypes, particularly egg-laying. Primitively eusocial species, such as eusocial sweat bees, have behaviourally totipotent castes, in which each female is capable of high levels of ovarian development. Few studies have investigated vg expression patterns in primitively eusocial insects, and only one study has focused on a primitively eusocial bee. Here we use a primitively eusocial sweat bee, Lasioglossum laevissimum, and Real Time quantitative PCR (RT-qPCR) to investigate the relationship between vg expression, castes, and variation in phenotypes associated with castes differences. These assays showed that females with high ovarian development had the highest levels of vg expression, and that vg expression levels reflected the reproductive status of females first and caste second. This is in contrast to vg expression patterns observed in advanced eusocial queens and workers, which differ in vg expression based on caste and have caste-specific vg expression patterns. Furthermore, future queens (gynes) do not have ovarian development and had similar vg expression levels to early spring foundresses, which do have ovarian development, supporting Vg's function as a transporter of lipids and amino acids before diapause.

Pheromonal Regulation of the Reproductive Division of Labor in Social Insects

The reproductive altruism in social insects is an evolutionary enigma that has been puzzling scientists starting from Darwin. Unraveling how reproductive skew emerges and maintains is crucial to understand the reproductive altruism involved in the consequent division of labor. The regulation of adult worker reproduction involves conspecific inhibitory signals, which are thought to be chemical signals by numerous studies. Despite the primary identification of few chemical ligands, the action modes of primer pheromones that regulate reproduction and their molecular causes and effects remain challenging. Here, these questions were elucidated by comprehensively reviewing recent advances. The coordination with other modalities of queen pheromones (QPs) and its context-dependent manner to suppress worker reproduction were discussed under the vast variation and plasticity of reproduction during colony development and across taxa. In addition to the effect of QPs, special attention was paid to recent studies revealing the regulatory effect of brood pheromones. Considering the correlation between pheromone and hormone, this study focused on the production and perception of pheromones under the endocrine control and highlighted the pivotal roles of nutrition-related pathways. The novel chemicals and gene pathways discovered by recent works provide new insights into the understanding of social regulation of reproductive division of labor in insects.

Asexual Evolution and Forest Conditions Drive Genetic Parallelism in Phytophthora ramorum

It is commonly assumed that asexual lineages are short-lived evolutionarily, yet many asexual organisms can generate genetic and phenotypic variation, providing an avenue for further evolution. Previous work on the asexual plant pathogen Phytophthora ramorum NA1 revealed considerable genetic variation in the form of Structural Variants (SVs). To better understand how SVs arise and their significance to the California NA1 population, we studied the evolutionary histories of SVs and the forest conditions associated with their emergence. Ancestral state reconstruction suggests that SVs arose by somatic mutations among multiple independent lineages, rather than by recombination. We asked if this unusual phenomenon of parallel evolution between isolated populations is transmitted to extant lineages and found that SVs persist longer in a population if their genetic background had a lower mutation load. Genetic parallelism was also found in geographically distant demes where forest conditions such as host density, solar radiation, and temperature, were similar. Parallel SVs overlap with genes involved in pathogenicity such as RXLRs and have the potential to change the course of an epidemic. By combining genomics and environmental data, we identified an unexpected pattern of repeated evolution in an asexual population and identified environmental factors potentially driving this phenomenon.

Globally invasive populations of the clonal raider ant are derived from Bangladesh

Identifying the native range of invasive species is useful to understand their evolution and natural history, as well as to develop new methods to control potentially harmful introduced organisms. The clonal raider ant, Ooceraea biroi, is an introduced species and an increasingly important social insect model organism, but its native range remains unknown. Here, we report a new series of O. biroi collections from Bangladesh, Singapore, Vietnam and China. We use a molecular phylogeny constructed with five gene fragments from 27 samples to determine that invasive lineages of O. biroi originated in Bangladesh. These lineages may have spread from Bangladesh via the historically significant Bay of Bengal shipping ports. Ooceraea biroi shares multiple features of its biology with other introduced ants, including parthenogenesis, retention of heterozygosity and presence of multiple egg-layers in the colony. Using laboratory rearing and microsatellite markers, we show that colonies collected from disturbed habitat in Bangladesh have these traits in common with colonies from the invasive range. Ancestral populations with sexual reproduction in primary habitats either remain to be discovered or have gone extinct. Our findings advance our understanding of the global spread of the clonal raider ant and highlight a suite of general traits that make certain ants prone to becoming invasive.

Gene expression variation in the brains of harvester ant foragers is associated with collective behavior

Natural selection on collective behavior acts on variation among colonies in behavior that is associated with reproductive success. In the red harvester ant (Pogonomyrmex barbatus), variation among colonies in the collective regulation of foraging in response to humidity is associated with colony reproductive success. We used RNA-seq to examine gene expression in the brains of foragers in a natural setting. We find that colonies differ in the expression of neurophysiologically-relevant genes in forager brains, and a fraction of these gene expression differences are associated with two colony traits: sensitivity of foraging activity to humidity, and forager brain dopamine to serotonin ratio. Loci that were correlated with colony behavioral differences were enriched in neurotransmitter receptor signaling & metabolic functions, tended to be more central to coexpression networks, and are evolving under higher protein-coding sequence constraint. Natural selection may shape colony foraging behavior through variation in gene expression.

Evolution, developmental expression and function of odorant receptors in insects

Animals rely on their chemosensory system to discriminate among a very large number of attractive or repulsive chemical cues in the environment, which is essential to respond with proper action. The olfactory sensory systems in insects share significant similarities with those of vertebrates, although they also exhibit dramatic differences, such as the molecular nature of the odorant receptors (ORs): insect ORs function as heteromeric ion channels with a common Orco subunit, unlike the G-protein-coupled olfactory receptors found in vertebrates. Remarkable progress has recently been made in understanding the evolution, development and function of insect odorant receptor neurons (ORNs). These studies have uncovered the diversity of olfactory sensory systems among insect species, including in eusocial insects that rely extensively on olfactory sensing of pheromones for social communication. However, further studies, notably functional analyses, are needed to improve our understanding of the origins of the Orco-OR system, the mechanisms of ORN fate determination, and the extraordinary diversity of behavioral responses to chemical cues.

Odor coding of nestmate recognition in the eusocial ant Camponotus floridanus

In eusocial ants, aggressive behaviors require the ability to discriminate between chemical signatures such as cuticular hydrocarbons that distinguish nestmate friends from non-nestmate foes. It has been suggested that a mismatch between a chemical signature (label) and the internal, neuronal representation of the colony odor (template) leads to aggression between non-nestmates. Moreover, a definitive demonstration that odorant receptors are responsible for the processing of the chemical signals that regulate nestmate recognition has thus far been lacking. To address these issues, we have developed an aggression-based bioassay incorporating highly selective modulators that target odorant receptor functionality to characterize their role in nestmate recognition in the formicine ant Camponotus floridanus Electrophysiological studies were used to show that exposure to either a volatilized antagonist or an agonist eliminated or dramatically altered signaling, respectively. Administration of these compounds to adult workers significantly reduced aggression between non-nestmates without altering aggression levels between nestmates. These studies provide direct evidence that odorant receptors are indeed necessary and sufficient for mediating aggression towards non-nestmates. Furthermore, our observations support a hypothesis in which rejection of non-nestmates depends on the precise decoding of chemical signatures present on non-nestmates as opposed to the absence of any information or the active acceptance of familiar signatures.

Mapping the chemical and sequence space of the ShKT superfamily

The ShKT superfamily is widely distributed throughout nature and encompasses a wide range of documented functions and processes, from modulation of potassium channels to involvement in morphogenesis pathways. Cysteine-rich secretory proteins (CRISPs) contain a cysteine-rich domain (CRD) at the C-terminus that is similar in structure to the ShK fold. Despite the structural similarity of the CRD and ShK-like domains, we know little of the sequence-function relationships in these families. Here, for the first time, we examine the evolution of the biophysical properties of sequences within the ShKT superfamily in relation to function, with a focus on the ShK-like superfamily. ShKT data were sourced from published sequences in the protein family database, in addition to new ShK-like sequences from the Australian speckled anemone (Oulactis sp.). Our analysis clearly delineates the ShK-like family from the CRDs of CRISP proteins. The four CRISP subclusters separate out into the main phyla of Mammalia, Insecta and Reptilia. The ShK-like family is in turn composed of seven subclusters, the largest of which contains members from across the eukaryotes, with a continuum of intermediate properties. Smaller sub-clusters contain specialised members such as nematode ShK-like sequences. Several of these ShKT sub-clusters contain no functionally characterised sequences. This chemical space analysis should be useful as a guide to select sequences for functional studies and to gain insight into the evolution of these highly divergent sequences with an ancient conserved fold.

Gene expression is more strongly associated with behavioural specialization than with age or fertility in ant workers

The ecological success of social insects is based on division of labour, not only between queens and workers, but also among workers. Whether a worker tends the brood or forages is influenced by age, fertility and nutritional status, with brood carers being younger, more fecund and more corpulent. Here, we experimentally disentangle behavioural specialization from age and fertility in Temnothorax longispinosus ant workers and analyse how these parameters are linked to whole-body gene expression. A total of 3,644 genes were associated with behavioural specialization which is ten times more than associated with age and 50 times more than associated with fertility. Brood carers were characterized by an upregulation of three Vitellogenin (Vg) genes, one of which, Vg-like A, was the most differentially expressed gene that was recently shown experimentally to control the switch from brood to worker care. The expression of Conventional Vg was unlinked to behavioural specialization, age or fertility, which contrasts to studies on bees and some ants. Diversity in Vg/Vg-like copy number and expression bias across ants supports subfunctionalization of Vg genes and indicates that some regulatory mechanisms of division of labour diverged in different ant lineages. Simulations revealed that our experimental dissociation of co-varying factors reduced transcriptomic noise, suggesting that confounding factors could potentially explain inconsistencies across transcriptomic studies of behavioural specialization in ants. Thus, our study reveals that worker gene expression is mainly linked to the worker's function for the colony and provides novel insights into the evolution of sociality in ants.

Sex loss in insects: causes of asexuality and consequences for genomes

Boasting a staggering diversity of reproductive strategies, insects provide attractive models for the comparative study of the causes and consequences of transitions to asexuality. We provide an overview of some contemporary studies of reproductive systems in insects and compile an initial database of asexual insect genome resources. Insect systems have already yielded some important insights into various mechanisms by which sex is lost, including genetic, endosymbiont-mediated, and hybridization. Studies of mutation and substitution after loss of sex provide the strongest empirical support for hypothesized effects of asexuality, whereas there is mixed evidence for ecological hypotheses such as increased parasite load and altered niche breadth in asexuals. Most hypotheses have been explored in a select few taxa (e.g. stick insects, aphids), such that much of the great taxonomic breadth of insects remain understudied. Given the variation in the proximate causes of asexuality in insects, we argue for expanding the taxonomic breadth of study systems. Despite some challenges for investigating sex in insects, the increasing cost-effectiveness of genomic sequencing makes data generation for closely-related asexual and sexual lineages increasingly feasible.