Differential gene expression in queen-worker caste determination in bumble-bees

Costs of reproduction are present but latent in eusocial bumblebee queens

BACKGROUND

The standard evolutionary theory of ageing proposes that ageing occurs because of a trade-off between reproduction and longevity. Eusocial insect queens exhibit positive fecundity-longevity associations and so have been suggested to be counter-examples through not expressing costs of reproduction and through remodelling conserved genetic and endocrine networks regulating ageing and reproduction. If so, eusocial evolution from solitary ancestors with negative fecundity-longevity associations must have involved a stage at which costs of reproduction were suppressed and fecundity and longevity became positively associated. Using the bumblebee (Bombus terrestris), we experimentally tested whether queens in annual eusocial insects at an intermediate level of eusocial complexity experience costs of reproduction, and, using mRNA-seq, the extent to which they exhibit a remodelling of relevant genetic and endocrine networks. Specifically, we tested whether costs of reproduction are present but latent, or whether a remodelling of relevant genetic and endocrine networks has already occurred allowing queens to reproduce without costs.

RESULTS

We experimentally increased queens' costs of reproduction by removing their eggs, which caused queens to increase their egg-laying rate. Treatment queens had significantly reduced longevity relative to control queens whose egg-laying rate was not increased. Reduced longevity in treatment queens was not caused by increased worker-to-queen aggression or by increased overall activity in queens. In addition, treatment and control queens differed in age-related gene expression based on mRNA-seq in both their overall expression profiles and the expression of ageing-related genes. Remarkably, these differences appeared to occur principally with respect to relative age, not chronological age.

CONCLUSIONS

This study represents the first simultaneously phenotypic and transcriptomic experimental test for a longevity cost of reproduction in eusocial insect queens. The results support the occurrence of costs of reproduction in annual eusocial insects of intermediate social complexity and suggest that reproductive costs are present but latent in queens of such species, i.e. that these queens exhibit condition-dependent positive fecundity-longevity associations. They also raise the possibility that a partial remodelling of genetic and endocrine networks underpinning ageing may have occurred in intermediately eusocial species such that, in unmanipulated conditions, age-related gene expression depends more on chronological than relative age.

Association between parasite microbiomes and caste development and colony structure in a social trematode

Division of labour through the formation of morphologically and functionally distinct castes is a recurring theme in the evolution of animal sociality. The mechanisms driving the differentiation of individuals into distinct castes remain poorly understood, especially for animals forming clonal colonies. We test the association between microbiomes and caste formation within the social trematode Philophthalmus attenuatus, using a metabarcoding approach targeting the bacterial 16S SSU rRNA gene. Clonal colonies of this trematode within snail hosts comprise large reproductive individuals which produce dispersal stages, and small, non-reproducing soldiers which defend the colony against invaders. In colonies extracted directly from field-collected snails, reproductives harboured more diverse bacterial communities than soldiers, and reproductives and soldiers harboured distinct bacterial communities, at all taxonomic levels considered. No single bacterial taxon showed high enough prevalence in either soldiers or reproductives to be singled out as a key driver, indicating that the whole microbial community contributes to these differences. Other colonies were experimentally exposed to antibiotics to alter their bacterial communities, and sampled shortly after treatment and weeks later after allowing for turnover of colony members. At those time points, bacterial communities of the two castes still differed across all antibiotic treatments; however, the caste ratio within colonies changed: after antibiotic disruption and turnover of individuals, new individuals were more likely to become reproductives than in undisturbed control colonies. Our results reveal that each caste has a distinct microbiome; whether the social context affects the microbiota, or whether microbes contribute to modulating the phenotype of individuals, remains to be determined.

Validation of Potential Reference Genes for Real-Time qPCR Analysis in Pharaoh Ant, Monomorium pharaonis (Hymenoptera: Formicidae)

Ants are highly diverse social insects living in colonies consisted of up to millions of individuals with reproductive division of labors. Due to the interests in disclosing the genetic and epigenetic regulation mechanisms underlying the distinct developmental trajectories between castes and division of labor in colonies, many ant species have recently been established as laboratory models for evolutionary development and social behavior studies. These functional studies often request a precise quantification of the relative gene expression level, which relies on a stably expressed reference genes for normalization. A core set of reliable reference genes for this purpose however has not been established yet in ants. In the present study, we tested the expression patterns and amplification efficiencies of 12 abundantly expressed candidate genes in Monomorium pharaonis, one of the few ant species that are suitable for laboratory rearing and experimentation. We quantified the expression levels of these genes by RT-qPCR in seven different conditions: embryo development, sexual development, worker development, adult phenotypes, tissues, and two abiotic manipulative treatments in pharaoh ant. Finally, five genes, elongation factor-1 alpha (EF1A), glyceraldehyde 3-phosphate dehydrogenase (GAPDH), TATA-box-binding protein (TATA), tubulin gamma-2 chain-like (TBLg2), heat shock protein 67B2-like (HSP67) were found to be the most stable reference genes across seven conditions. We also identified the most stable reference genes applicable for each distinct condition and the optimal number of reference genes entailed were evaluated. Our study validates reliable reference genes for RT-qPCR analysis which lays the foundation for future studies in pharaoh ant.

miRNA-34 and miRNA-210 target hexamerin genes enhancing their differential expression during early brain development of honeybee (Apis mellifera) castes

During the honeybee larval stage, queens develop larger brains than workers, with morphological differentiation appearing at the fourth larval phase (L4), just after a boost in nutritional difference both prospective females experience. The molecular promoters of this caste-specific brain development are already ongoing in previous larval phases. Transcriptomic analyses revealed a set of differentially expressed genes in the L3 brains of queens and workers, which represents the early molecular response to differential feeding females receive during larval development. Three genes of this set, hex70b, hex70c and hex110, are more highly transcribed in the brain of workers than in queens. The microRNAs miR-34, miR-210 and miR-317 are in higher levels in the queens' brain at the same phase of larval development. Here, we tested the hypothesis that the brain of workers expresses higher levels of hexamerins than that of queens during key phases of larval development and that this differential hexamerin genes expression is further enhanced by the repressing activity of miR-34, miR-210 and miR-317. Our transcriptional analyses showed that hex70b, hex70c and hex110 genes are differentially expressed in the brain of L3 and L4 larval phases of honeybee queens and workers. In silico reconstructed miRNA-mRNA interaction networks were validated using luciferase assays, which showed miR-34 and miR-210 negatively regulate hex70b and hex110 genes by directly and redundantly binding their 3'UTR (untranslated region) sequences. Taken together, our results suggest that miR-34 and miR-210 act together promoting differential brain development in honeybee castes by downregulating the expression of the putative antineurogenic hexamerin genes hex70b and hex110.

Characterization and Developmental Expression Patterns of Four Hexamerin Genes in the Bumble Bee, Bombus terrestris (Hymenoptera: Apidae)

Hexamerins are members of the hemocyanin superfamily and play essential roles in providing amino acids and energy for the nonfeeding stages of insects. In this study, we cloned and analyzed the expression patterns of four hexamerin genes (hex 70a, hex 70b, hex 70c, and hex 110) at different worker development stages and queen diapause statuses in the bumble bee, Bombus terrestris. The results of this study showed that hex 110 has the longest open reading frame (ORF; 3,297 bp) compared to the ORFs of hex 70a (2,034 bp), hex 70b (2,067 bp), and hex 70c (2,055 bp). The putative translation product of Hex 70a, Hex 70b, Hex70c, and Hex 110 has 677, 688, 684, and 1,098aa with predicted molecular mass of 81.13, 79.69, 81.58, and 119 kDa. In the development stages of workers, the expression levels of hex 70a, hex 70b, and hex 70c increased gradually from the larval stage and exhibited high expression levels at the pink eyed and brown eyed pupae stage, whereas hex 110 exhibited the highest expression level at the larval period. Four hexamerin genes were highly expressed at the prediapause status of queen (P < 0.05), and compared to the eclosion queen, the lowest upregulation was 3.7-fold, and the highest upregulation was 1,742-fold. The expression levels of hex 70b, hex 70c, and hex 110 at diapause were significantly higher than those at postdiapause (P < 0.05). In conclusion, hexamerins may play important roles in queen diapause and metamorphosis of larval and pupal stages.

(Epi)Genetic Mechanisms Underlying the Evolutionary Success of Eusocial Insects

Eusocial insects, such as bees, ants, and wasps of the Hymenoptera and termites of the Blattodea, are able to generate remarkable diversity in morphology and behavior despite being genetically uniform within a colony. Most eusocial insect species display caste structures in which reproductive ability is possessed by a single or a few queens while all other colony members act as workers. However, in some species, caste structure is somewhat plastic, and individuals may switch from one caste or behavioral phenotype to another in response to certain environmental cues. As different castes normally share a common genetic background, it is believed that much of this observed within-colony diversity results from transcriptional differences between individuals. This suggests that epigenetic mechanisms, featured by modified gene expression without changing genes themselves, may play an important role in eusocial insects. Indeed, epigenetic mechanisms such as DNA methylation, histone modifications and non-coding RNAs, have been shown to influence eusocial insects in multiple aspects, along with typical genetic regulation. This review summarizes the most recent findings regarding such mechanisms and their diverse roles in eusocial insects.

Gene expression during larval caste determination and differentiation in intermediately eusocial bumblebees, and a comparative analysis with advanced eusocial honeybees

The queen‐worker caste system of eusocial insects represents a prime example of developmental polyphenism (environmentally‐induced phenotypic polymorphism) and is intrinsic to the evolution of advanced eusociality. However, the comparative molecular basis of larval caste determination and subsequent differentiation in the eusocial Hymenoptera remains poorly known. To address this issue within bees, we profiled caste‐associated gene expression in female larvae of the intermediately eusocial bumblebee Bombus terrestris. In B. terrestris, female larvae experience a queen‐dependent period during which their caste fate as adults is determined followed by a nutrition‐sensitive period also potentially affecting caste fate but for which the evidence is weaker. We used mRNA‐seq and qRT‐PCR validation to isolate genes differentially expressed between each caste pathway in larvae at developmental stages before and after each of these periods. We show that differences in gene expression between caste pathways are small in totipotent larvae, then peak after the queen‐dependent period. Relatively few novel (i.e., taxonomically‐restricted) genes were differentially expressed between castes, though novel genes were significantly enriched in late‐instar larvae in the worker pathway. We compared sets of caste‐associated genes in B. terrestris with those reported from the advanced eusocial honeybee, Apis mellifera, and found significant but relatively low levels of overlap of gene lists between the two species. These results suggest both the existence of low numbers of shared toolkit genes and substantial divergence in caste‐associated genes between Bombus and the advanced eusocial Apis since their last common eusocial ancestor.

Evolution, phylogenetic distribution and functional ecology of division of labour in trematodes

Division of labour has evolved in many social animals where colonies consist of clones or close kin. It involves the performance of different tasks by morphologically distinct castes, leading to increased colony fitness. Recently, a form of division of labour has been discovered in trematodes: clonal rediae inside the snail intermediate host belong either to a large-bodied reproductive caste, or to a much smaller and morphologically distinct ‘soldier’ caste which defends the colony against co-infecting trematodes. We review recent research on this phenomenon, focusing on its phylogenetic distribution, its possible evolutionary origins, and how division of labour functions to allow trematode colonies within their snail host to adjust to threats and changing conditions. To date, division of labour has been documented in 15 species from three families: Himasthlidae, Philophthalmidae and Heterophyidae. Although this list of species is certainly incomplete, the evidence suggests that division of labour has arisen independently more than once in the evolutionary history of trematodes. We propose a simple scenario for the gradual evolution of division of labour in trematodes facing a high risk of competition in a long-lived snail host. Starting with initial conditions prior to the origin of castes (size variation among rediae within a colony, size-dependent production of cercariae by rediae, and a trade-off between cercarial production and other functions, such as defence), maximising colony fitness (R₀) can lead to caste formation or the age-structured division of labour observed in some trematodes. Finally, we summarise recent research showing that caste ratios, i.e. relative numbers of reproductive and soldier rediae per colony, become more soldier-biased in colonies exposed to competition from another trematode species sharing the same snail, and also respond to other stressors threatening the host’s survival or the colony itself. In addition, there is evidence of asymmetrical phenotypic plasticity among individual caste members: reproductives can assume defensive functions against competitors in the absence of soldiers, whereas soldiers are incapable of growing into reproductives if the latter’s numbers are reduced. We conclude by highlighting future research directions, and the advantages of trematodes as model systems to study social evolution.

Morphological characterization and staging of bumble bee pupae

Bumble bees (Hymenoptera: Apidae, Bombus) are important pollinators and models for studying mechanisms underlying developmental plasticity, such as factors influencing size, immunity, and social behaviors. Research on such processes, as well as expanding use of gene-manipulation and gene expression technologies, requires a detailed understanding of how these bees develop. Developmental research often uses time-staging of pupae, however dramatic size differences in these bees can generate variation in developmental timing. To study developmental mechanisms in bumble bees, appropriate staging of developing bees using morphology is necessary. In this study, we describe morphological changes across development in several bumble bee species and use this to establish morphology-based staging criteria, establishing 20 distinct illustrated stages. These criteria, defined largely by eye and cuticle pigmentation patterns, are generalizable across members of the subgenus Pyrobombus, and can be used as a framework for study of other bumble bee subgenera. We examine the effects of temperature, caste, size, and species on pupal development, revealing that pupal duration shifts with each of these factors, confirming the importance of staging pupae based on morphology rather than age and the need for standardizing sampling.

Genetic accommodation and the role of ancestral plasticity in the evolution of insect eusociality

For over a century, biologists have proposed a role for phenotypic plasticity in evolution, providing an avenue for adaptation in addition to 'mutation-first' models of evolutionary change. According to the various versions of this idea, the ability of organisms to respond adaptively to their environment through phenotypic plasticity may lead to novel phenotypes that can be screened by natural selection. If these initially environmentally induced phenotypes increase fitness, then genetic accommodation can lead to allele frequency change, influencing the expression of those phenotypes. Despite the long history of 'plasticity-first' models, the importance of genetic accommodation in shaping evolutionary change has remained controversial - it is neither fully embraced nor completely discarded by most evolutionary biologists. We suggest that the lack of acceptance of genetic accommodation in some cases is related to a lack of information on its molecular mechanisms. However, recent reports of epigenetic transgenerational inheritance now provide a plausible mechanism through which genetic accommodation may act, and we review this research here. We also discuss current evidence supporting a role for genetic accommodation in the evolution of eusociality in social insects, which have long been models for studying the influence of the environment on phenotypic variation, and may be particularly good models for testing hypotheses related to genetic accommodation. Finally, we introduce 'eusocial engineering', a method by which novel social phenotypes are first induced by environmental modification and then studied mechanistically to understand how environmentally induced plasticity may lead to heritable changes in social behavior. We believe the time is right to incorporate genetic accommodation into models of the evolution of complex traits, armed with new molecular tools and a better understanding of non-genetic heritable elements.

Building a new research framework for social evolution: intralocus caste antagonism

The breeding and non‐breeding ‘castes’ of eusocial insects provide a striking example of role‐specific selection, where each caste maximises fitness through different morphological, behavioural and physiological trait values. Typically, queens are long‐lived egg‐layers, while workers are short‐lived, largely sterile foragers. Remarkably, the two castes are nevertheless produced by the same genome. The existence of inter‐caste genetic correlations is a neglected consequence of this shared genome, potentially hindering the evolution of caste dimorphism: alleles that increase the productivity of queens may decrease the productivity of workers and vice versa, such that each caste is prevented from reaching optimal trait values. A likely consequence of this ‘intralocus caste antagonism’ should be the maintenance of genetic variation for fitness and maladaptation within castes (termed ‘caste load’), analogous to the result of intralocus sexual antagonism. The aim of this review is to create a research framework for understanding caste antagonism, drawing in part upon conceptual similarities with sexual antagonism. By reviewing both the social insect and sexual antagonism literature, we highlight the current empirical evidence for caste antagonism, discuss social systems of interest, how antagonism might be resolved, and challenges for future research. We also introduce the idea that sexual and caste antagonism could interact, creating a three‐way antagonism over gene expression. This includes unpacking the implications of haplodiploidy for the outcome of this complex interaction.

Evolutionary constraints shape caste-specific gene expression across 15 ant species

Development of polymorphic phenotypes from similar genomes requires gene expression differences. However, little is known about how morph-specific gene expression patterns vary on a broad phylogenetic scale. We hypothesize that evolution of morph-specific gene expression, and consequently morph-specific phenotypic evolution, may be constrained by gene essentiality and the amount of pleiotropic constraints. Here, we use comparative transcriptomics of queen and worker morphs, that is, castes, from 15 ant species to understand the constraints of morph-biased gene expression. In particular, we investigate how measures of evolutionary constraints at the sequence level (expression level, connectivity, and number of gene ontology [GO] terms) correlate with morph-biased expression. Our results show that genes indeed vary in their potential to become morph-biased. The existence of genes that are constrained in becoming caste-biased potentially limits the evolutionary decoupling of the caste phenotypes, that is, it might result in "caste load" occasioning from antagonistic fitness variation, similarly to sexually antagonistic fitness variation between males and females. On the other hand, we suggest that genes under low constraints are released from antagonistic variation and thus more likely to be co-opted for morph specific use. Overall, our results suggest that the factors that affect sequence evolutionary rates and evolution of plastic expression may largely overlap.

MicroRNAs Associated with Caste Determination and Differentiation in a Primitively Eusocial Insect

In eusocial Hymenoptera (ants, bees and wasps), queen and worker adult castes typically arise via environmental influences. A fundamental challenge is to understand how a single genome can thereby produce alternative phenotypes. A powerful approach is to compare the molecular basis of caste determination and differentiation along the evolutionary trajectory between primitively and advanced eusocial species, which have, respectively, relatively undifferentiated and strongly differentiated adult castes. In the advanced eusocial honeybee, Apis mellifera, studies suggest that microRNAs (miRNAs) play an important role in the molecular basis of caste determination and differentiation. To investigate how miRNAs affect caste in eusocial evolution, we used deep sequencing and Northern blots to isolate caste-associated miRNAs in the primitively eusocial bumblebee Bombus terrestris. We found that the miRNAs Bte-miR-6001-5p and -3p are more highly expressed in queen- than in worker-destined late-instar larvae. These are the first caste-associated miRNAs from outside advanced eusocial Hymenoptera, so providing evidence for caste-associated miRNAs occurring relatively early in eusocial evolution. Moreover, we found little evidence that miRNAs previously shown to be associated with caste in A. mellifera were differentially expressed across caste pathways in B. terrestris, suggesting that, in eusocial evolution, the caste-associated role of individual miRNAs is not conserved.

Molecular mechanisms of phenotypic plasticity in social insects

Polyphenism in insects, whereby a single genome expresses different phenotypes in response to environmental cues, is a fascinating biological phenomenon. Social insects are especially intriguing examples of phenotypic plasticity because division of labor results in the development of extreme morphological phenotypes, such as the queen and worker castes. Although sociality evolved independently in ants, bees, wasps and termites, similar genetic pathways regulate phenotypic plasticity in these different groups of social insects. The insulin/insulin-like growth signaling (IIS) plays a key role in this process. Recent research reveals that IIS interacts with other pathways including target of rapamycin (TOR), epidermal growth factor receptor (Egfr), juvenile hormone (JH) and vitellogenin (Vg) to regulate caste differentiation.

Conservation and modification of genetic and physiological toolkits underpinning diapause in bumble bee queens

Diapause is the key adaptation allowing insects to survive unfavourable conditions and inhabit an array of environments. Physiological changes during diapause are largely conserved across species and are hypothesized to be regulated by a conserved suite of genes (a 'toolkit'). Furthermore, it is hypothesized that in social insects, this toolkit was co-opted to mediate caste differentiation between long-lived, reproductive, diapause-capable queens and short-lived, sterile workers. Using Bombus terrestris queens, we examined the physiological and transcriptomic changes associated with diapause and CO2 treatment, which causes queens to bypass diapause. We performed comparative analyses with genes previously identified to be associated with diapause in the Dipteran Sarcophaga crassipalpis and with caste differentiation in bumble bees. As in Diptera, diapause in bumble bees is associated with physiological and transcriptional changes related to nutrient storage, stress resistance and core metabolic pathways. There is a significant overlap, both at the level of transcript and gene ontology, between the genetic mechanisms mediating diapause in B. terrestris and S. crassipalpis, reaffirming the existence of a conserved insect diapause genetic toolkit. However, a substantial proportion (10%) of the differentially regulated transcripts in diapausing queens have no clear orthologs in other species, and key players regulating diapause in Diptera (juvenile hormone and vitellogenin) appear to have distinct functions in bumble bees. We also found a substantial overlap between genes related to caste determination and diapause in bumble bees. Thus, our studies demonstrate an intriguing interplay between pathways underpinning adaptation to environmental extremes and the evolution of sociality in insects.

Allele specific expression in worker reproduction genes in the bumblebee Bombus terrestris

Methylation has previously been associated with allele specific expression in ants. Recently, we found methylation is important in worker reproduction in the bumblebee Bombus terrestris. Here we searched for allele specific expression in twelve genes associated with worker reproduction in bees. We found allele specific expression in Ecdysone 20 monooxygenase and IMP-L2-like. Although we were unable to confirm a genetic or epigenetic cause for this allele specific expression, the expression patterns of the two genes match those predicted for imprinted genes.

Reproductive workers show queenlike gene expression in an intermediately eusocial insect, the buff-tailed bumble bee Bombus terrestris

Bumble bees represent a taxon with an intermediate level of eusociality within Hymenoptera. The clear division of reproduction between a single founding queen and the largely sterile workers is characteristic for highly eusocial species, whereas the morphological similarity between the bumble bee queen and the workers is typical for more primitively eusocial hymenopterans. Also, unlike other highly eusocial hymenopterans, division of labour among worker subcastes is plastic and not predetermined by morphology or age. We conducted a differential expression analysis based on RNA-seq data from 11 combinations of developmental stage and caste to investigate how a single genome can produce the distinct castes of queens, workers and males in the buff-tailed bumble bee Bombus terrestris. Based on expression patterns, we found males to be the most distinct of all adult castes (2411 transcripts differentially expressed compared to nonreproductive workers). However, only relatively few transcripts were differentially expressed between males and workers during development (larvae: 71 and pupae: 162). This indicates the need for more distinct expression patterns to control behaviour and physiology in adults compared to those required to create different morphologies. Among female castes, reproductive workers and their nonreproductive sisters displayed differential expression in over ten times more transcripts compared to the differential expression found between reproductive workers and their mother queen. This suggests a strong shift towards a more queenlike behaviour and physiology when a worker becomes fertile. This contrasts with eusocial species where reproductive workers are more similar to nonreproductive workers than the queen.

Gene expression patterns associated with caste and reproductive status in ants: worker-specific genes are more derived than queen-specific ones

Variation in gene expression leads to phenotypic diversity and plays a central role in caste differentiation of eusocial insect species. In social Hymenoptera, females with the same genetic background can develop into queens or workers, which are characterized by divergent morphologies, behaviours and lifespan. Moreover, many social insects exhibit behaviourally distinct worker castes, such as brood-tenders and foragers. Researchers have just started to explore which genes are differentially expressed to achieve this remarkable phenotypic plasticity. Although the queen is normally the only reproductive individual in the nest, following her removal, young brood-tending workers often develop ovaries and start to reproduce. Here, we make use of this ability in the ant Temnothorax longispinosus and compare gene expression patterns in the queens and three worker castes along a reproductive gradient. We found the largest expression differences between the queen and the worker castes (~2500 genes) and the smallest differences between infertile brood-tenders and foragers (~300 genes). The expression profile of fertile workers is more worker-like, but to a certain extent intermediate between the queen and the infertile worker castes. In contrast to the queen, a high number of differentially expressed genes in the worker castes are of unknown function, pointing to the derived status of hymenopteran workers within insects.

Transcriptome analysis of the Asian honey bee Apis cerana cerana

Background

The Eastern hive honey bee, Apis cerana cerana is a native and widely bred honey bee species in China. Molecular biology research about this honey bee species is scarce, and genomic information for A. c. cerana is not currently available. Transcriptome and expression profiling data for this species are therefore important resources needed to better understand the biological mechanisms of A. c. cerana. In this study, we obtained the transcriptome information of A. c. cerana by RNA-sequencing and compared gene expression differences between queens and workers of A. c. cerana by digital gene expression (DGE) analysis.

Results

Using high-throughput Illumina RNA sequencing we obtained 51,581,510 clean reads corresponding to 4.64 Gb total nucleotides from a single run. These reads were assembled into 46,999 unigenes with a mean length of 676 bp. Based on a sequence similarity search against the five public databases (NR, Swissport, GO, COG, KEGG) with a cut-off E-value of 10⁻⁵ using BLASTX, a total of 24,630 unigenes were annotated with gene descriptions, gene ontology terms, or metabolic pathways. Using these transcriptome data as references we analyzed the gene expression differences between the queens and workers of A. c. cerana using a tag-based digital gene expression method. We obtained 5.96 and 5.66 million clean tags from the queen and worker samples, respectively. A total of 414 genes were differentially expressed between them, with 189 up-regulated and 225 down-regulated in queens.

Conclusions

Our transcriptome data provide a comprehensive sequence resource for future A. c. cerana study, establishing an important public information platform for functional genomic studies in A. c. cerana. Furthermore, the DGE data provide comprehensive gene expression information for the queens and workers, which will facilitate our understanding of the molecular mechanisms of the different physiological aspects of the two castes.

Physiological variation as a mechanism for developmental caste-biasing in a facultatively eusocial sweat bee

Social castes of eusocial insects may have arisen through an evolutionary modification of an ancestral reproductive ground plan, such that some adults emerge from development physiologically primed to specialize on reproduction (queens) and others on maternal care expressed as allo-maternal behaviour (workers). This hypothesis predicts that variation in reproductive physiology should emerge from ontogeny and underlie division of labour. To test these predictions, we identified physiological links to division of labour in a facultatively eusocial sweat bee, Megalopta genalis. Queens are larger, have larger ovaries and have higher vitellogenin titres than workers. We then compared queens and workers with their solitary counterparts-solitary reproductive females and dispersing nest foundresses-to investigate physiological variation as a factor in caste evolution. Within dyads, body size and ovary development were the best predictors of behavioural class. Queens and dispersers are larger, with larger ovaries than their solitary counterparts. Finally, we raised bees in social isolation to investigate the influence of ontogeny on physiological variation. Body size and ovary development among isolated females were highly variable, and linked to differences in vitellogenin titres. As these are key physiological predictors of social caste, our results provide evidence for developmental caste-biasing in a facultatively eusocial bee.

Polyphenism in social insects: insights from a transcriptome-wide analysis of gene expression in the life stages of the key pollinator, Bombus terrestris

BACKGROUND

Understanding polyphenism, the ability of a single genome to express multiple morphologically and behaviourally distinct phenotypes, is an important goal for evolutionary and developmental biology. Polyphenism has been key to the evolution of the Hymenoptera, and particularly the social Hymenoptera where the genome of a single species regulates distinct larval stages, sexual dimorphism and physical castes within the female sex. Transcriptomic analyses of social Hymenoptera will therefore provide unique insights into how changes in gene expression underlie such complexity. Here we describe gene expression in individual specimens of the pre-adult stages, sexes and castes of the key pollinator, the buff-tailed bumblebee Bombus terrestris.

RESULTS

cDNA was prepared from mRNA from five life cycle stages (one larva, one pupa, one male, one gyne and two workers) and a total of 1,610,742 expressed sequence tags (ESTs) were generated using Roche 454 technology, substantially increasing the sequence data available for this important species. Overlapping ESTs were assembled into 36,354 B. terrestris putative transcripts, and functionally annotated. A preliminary assessment of differences in gene expression across non-replicated specimens from the pre-adult stages, castes and sexes was performed using R-STAT analysis. Individual samples from the life cycle stages of the bumblebee differed in the expression of a wide array of genes, including genes involved in amino acid storage, metabolism, immunity and olfaction.

CONCLUSIONS

Detailed analyses of immune and olfaction gene expression across phenotypes demonstrated how transcriptomic analyses can inform our understanding of processes central to the biology of B. terrestris and the social Hymenoptera in general. For example, examination of immunity-related genes identified high conservation of important immunity pathway components across individual specimens from the life cycle stages while olfactory-related genes exhibited differential expression with a wider repertoire of gene expression within adults, especially sexuals, in comparison to immature stages. As there is an absence of replication across the samples, the results of this study are preliminary but provide a number of candidate genes which may be related to distinct phenotypic stage expression. This comprehensive transcriptome catalogue will provide an important gene discovery resource for directed programmes in ecology, evolution and conservation of a key pollinator.

A second generation genetic map of the bumblebee Bombus terrestris (Linnaeus, 1758) reveals slow genome and chromosome evolution in the Apidae

Background

The bumblebee Bombus terrestris is an ecologically and economically important pollinator and has become an important biological model system. To study fundamental evolutionary questions at the genomic level, a high resolution genetic linkage map is an essential tool for analyses ranging from quantitative trait loci (QTL) mapping to genome assembly and comparative genomics. We here present a saturated linkage map and match it with the Apis mellifera genome using homologous markers. This genome-wide comparison allows insights into structural conservations and rearrangements and thus the evolution on a chromosomal level.

Results

The high density linkage map covers ~ 93% of the B. terrestris genome on 18 linkage groups (LGs) and has a length of 2'047 cM with an average marker distance of 4.02 cM. Based on a genome size of ~ 430 Mb, the recombination rate estimate is 4.76 cM/Mb. Sequence homologies of 242 homologous markers allowed to match 15 B. terrestris with A. mellifera LGs, five of them as composites. Comparing marker orders between both genomes we detect over 14% of the genome to be organized in synteny and 21% in rearranged blocks on the same homologous LG.

Conclusions

This study demonstrates that, despite the very high recombination rates of both A. mellifera and B. terrestris and a long divergence time of about 100 million years, the genomes' genetic architecture is highly conserved. This reflects a slow genome evolution in these bees. We show that data on genome organization and conserved molecular markers can be used as a powerful tool for comparative genomics and evolutionary studies, opening up new avenues of research in the Apidae.

Developmental decoupling of alternative phenotypes: insights from the transcriptomes of horn-polyphenic beetles

Developmental mechanisms play an important role in determining the costs, limits, and evolutionary consequences of phenotypic plasticity. One issue central to these claims is the hypothesis of developmental decoupling, where alternate morphs result from evolutionarily independent developmental pathways. We address this assumption through a microarray study that tests whether differences in gene expression between alternate morphs are as divergent as those between sexes, a classic example of developmental decoupling. We then examine whether genes with morph-biased expression are less conserved than genes with shared expression between morphs, as predicted if developmental decoupling relaxes pleiotropic constraints on divergence. We focus on the developing horns and brains of two species of horned beetles with impressive sexual- and morph-dimorphism in the expression of horns and fighting behavior. We find that patterns of gene expression were as divergent between morphs as they were between sexes. However, overall patterns of gene expression were also highly correlated across morphs and sexes. Morph-biased genes were more evolutionarily divergent, suggesting a role of relaxed pleiotropic constraints or relaxed selection. Together these results suggest that alternate morphs are to some extent developmentally decoupled, and that this decoupling has significant evolutionary consequences. However, alternative morphs may not be as developmentally decoupled as sometimes assumed and such hypotheses of development should be revisited and refined.

Identification of a reproductive-specific, putative lipid transport protein gene in a queenless ponerine ant Diacamma sp

Of the various characteristics of social insects, communication for reproductive differentiation is one of the most important and basic social interactions among colony members. To elucidate the molecular basis underlying this process, genes responsible for reproductive differentiation in Diacamma were screened using fluorescent differential display. Differential display, together with real-time quantitative reverse transcriptase-polymerase chain reaction (qRT-PCR), revealed that a gene belonging to the family of cellular retinaldehyde-binding proteins was specifically expressed in the epidermis of the head, legs, and thorax in reproductives. The deduced protein sequence in the coding region, obtained by rapid amplification of cDNA ends (RACE)-PCR, was found to include cellular retinaldehyde-binding domain (CRAL-TRIO domain), suggesting that DiaCRALDCP functions in transportation of lipids, such as cuticular hydrocarbons. DiaCRALDCP transcript levels immediately decreased 1 day after the gemma mutilation, suggesting that DiaCRALDCP is involved in the physiological changes provoked by the behavioral regulation. Considering these results, the social functions of DiaCRALDCP in Diacamma are discussed.

Differential gene expression and protein abundance evince ontogenetic bias toward castes in a primitively eusocial wasp

Polistes paper wasps are models for understanding conditions that may have characterized the origin of worker and queen castes and, therefore, the origin of paper wasp sociality. Polistes is “primitively eusocial” by virtue of having context-dependent caste determination and no morphological differences between castes. Even so, Polistes colonies have a temporal pattern in which most female larvae reared by the foundress become workers, and most reared by workers become future-reproductive gynes. This pattern is hypothesized to reflect development onto two pathways, which may utilize mechanisms that regulate diapause in other insects. Using expressed sequence tags (ESTs) for Polistes metricus we selected candidate genes differentially expressed in other insects in three categories: 1) diapause vs. non-diapause phenotypes and/or worker vs. queen differentiation, 2) behavioral subcastes of worker honey bees, and 3) no a priori expectation of a role in worker/gyne development. We also used a non-targeted proteomics screen to test for peptide/protein abundance differences that could reflect larval developmental divergence. We found that foundress-reared larvae (putative worker-destined) and worker-reared larvae (putative gyne-destined) differed in quantitative expression of sixteen genes, twelve of which were associated with caste and/or diapause in other insects, and they also differed in abundance of nine peptides/proteins. Some differentially-expressed genes are involved in diapause regulation in other insects, and other differentially-expressed genes and proteins are involved in the insulin signaling pathway, nutrient metabolism, and caste determination in highly social bees. Differential expression of a gene and a peptide encoding hexameric storage proteins is especially noteworthy. Although not conclusive, our results support hypotheses of 1) larval developmental pathway divergence that can lead to caste bias in adults and 2) nutritional differences as the foundation of the pathway divergence. Finally, the differential expression in Polistes larvae of genes and proteins also differentially expressed during queen vs. worker caste development in honey bees may indicate that regulatory mechanisms of caste outcomes share similarities between primitively eusocial and advanced eusocial Hymenoptera.

Phenotypic plasticity and diversity in insects

Phenotypic plasticity in general and polyphenic development in particular are thought to play important roles in organismal diversification and evolutionary innovation. Focusing on the evolutionary developmental biology of insects, and specifically that of horned beetles, I explore the avenues by which phenotypic plasticity and polyphenic development have mediated the origins of novelty and diversity. Specifically, I argue that phenotypic plasticity generates novel targets for evolutionary processes to act on, as well as brings about trade-offs during development and evolution, thereby diversifying evolutionary trajectories available to natural populations. Lastly, I examine the notion that in those cases in which phenotypic plasticity is underlain by modularity in gene expression, it results in a fundamental trade-off between degree of plasticity and mutation accumulation. On one hand, this trade-off limits the extent of plasticity that can be accommodated by modularity of gene expression. On the other hand, it causes genes whose expression is specific to rare environments to accumulate greater variation within species, providing the opportunity for faster divergence and diversification between species, compared with genes expressed across environments. Phenotypic plasticity therefore contributes to organismal diversification on a variety of levels of biological organization, thereby facilitating the evolution of novel traits, new species and complex life cycles.

Toward a population genetic framework of developmental evolution: the costs, limits, and consequences of phenotypic plasticity

Adaptive phenotypic plasticity allows organisms to cope with environmental variability, and yet, despite its adaptive significance, phenotypic plasticity is neither ubiquitous nor infinite. In this review, we merge developmental and population genetic perspectives to explore costs and limits on the evolution of plasticity. Specifically, we focus on the role of modularity in developmental genetic networks as a mechanism underlying phenotypic plasticity, and apply to it lessons learned from population genetic theory on the interplay between relaxed selection and mutation accumulation. We argue that the environmental specificity of gene expression and the associated reduction in pleiotropic constraints drive a fundamental tradeoff between the range of plasticity that can be accommodated and mutation accumulation in alternative developmental networks. This tradeoff has broad implications for understanding the origin and maintenance of plasticity and may contribute to a better understanding of the role of plasticity in the origin, diversification, and loss of phenotypic diversity.

Aphid wing dimorphisms: linking environmental and genetic control of trait variation

Both genetic and environmental factors underlie phenotypic variation. While research at the interface of evolutionary and developmental biology has made excellent advances in understanding the contribution of genes to morphology, less well understood is the manner in which environmental cues are incorporated during development to influence the phenotype. Also virtually unexplored is how evolutionary transitions between environmental and genetic control of trait variation are achieved. Here, I review investigations into molecular mechanisms underlying phenotypic plasticity in the aphid wing dimorphism system. Among aphids, some species alternate between environmentally sensitive (polyphenic) and genetic (polymorphic) control of wing morph determination in their life cycle. Therefore, a traditional molecular genetic approach into understanding the genetically controlled polymorphism may provide a unique avenue into not only understanding the molecular basis of polyphenic variation in this group, but also the opportunity to compare and contrast the mechanistic basis of environmental and genetic control of similar dimorphisms.

Changes in reproductive roles are associated with changes in gene expression in fire ant queens

In species with social hierarchies, the death of dominant individuals typically upheaves the social hierarchy and provides an opportunity for subordinate individuals to become reproductives. Such a phenomenon occurs in the monogyne form of the fire ant, Solenopsis invicta, where colonies typically contain a single wingless reproductive queen, thousands of workers and hundreds of winged nonreproductive virgin queens. Upon the death of the mother queen, many virgin queens shed their wings and initiate reproductive development instead of departing on a mating flight. Workers progressively execute almost all of them over the following weeks. To identify the molecular changes that occur in virgin queens as they perceive the loss of their mother queen and begin to compete for reproductive dominance, we collected virgin queens before the loss of their mother queen, 6 h after orphaning and 24 h after orphaning. Their RNA was extracted and hybridized against microarrays to examine the expression levels of approximately 10,000 genes. We identified 297 genes that were consistently differentially expressed after orphaning. These include genes that are putatively involved in the signalling and onset of reproductive development, as well as genes underlying major physiological changes in the young queens.

Novel microsatellite DNA loci for Bombus terrestris (Linnaeus, 1758)

We present details and characteristics of 123 novel polymorphic microsatellite DNA loci for Bombus terrestris. Thirty-four of these loci have been tested in nine other Bombus species and 25 of them showed polymorphisms in at least one species. These microsatellite DNA loci together with the already established 60 loci will be useful for characterizing wild and managed populations of B. terrestris and other Bombus species as well as for detailed genetic studies in including mapping studies and genome annotations.

Genetic and genomic analyses of the division of labour in insect societies

Division of labour--individuals specializing in different activities--features prominently in the spectacular success of the social insects. Until recently, genetic and genomic analyses of division of labour were limited to just a few species. However, research on an ever-increasing number of species has provided new insight, from which we highlight two results. First, heritable influences on division of labour are more pervasive than previously imagined. Second, different forms of division of labour, in lineages in which eusociality has arisen independently, have evolved through changes in the regulation of highly conserved molecular pathways associated with several basic life-history traits, including nutrition, metabolism and reproduction.

Genetic caste determination in termites: out of the shade but not from Mars

Several ant species are known with genetic effects on caste determination but, for termites, the role of environment has been assumed to be omnipotent. Now Hayashi et al. report that commitment to the nymph and worker pathways in Reticulitermes speratus follows a simple model involving two alleles at a sex-linked locus. The spread of this system of genetic caste determination seems best explained by selection at the colony level. This remarkable system may be widely applicable throughout termites, although it cannot be universal, and may provide a window into causal aspects of the molecular biology of caste determination.

Temperature-induced gene expression associated with different thermal reaction norms for growth rate

Although nearly all organisms are subject to fluctuating temperature regimes in their natural habitat, little is known about the genetics underlying the response to thermal conditions, and even less about the genetic differences that cause individual variation in thermal response. Here, we aim to elucidate possible pathways involved in temperature-induced phenotypic plasticity of growth rate. Our model organism is the collembolan Orchesella cincta that occurs in a wide variety of habitats and is known to be adapted to local thermal conditions. Because sequence information is lacking in O. cincta, we constructed cDNA libraries enriched for temperature-responsive genes using suppression subtractive hybridization. We compared gene expression of O. cincta with steep thermal reaction norms (high plasticity) to those with flat thermal reaction norms (low plasticity) for juvenile growth after exposure to a temperature switch composed of a cooling or a warming treatment. Using suppression subtractive hybridization, we found differential expression of ten nuclear genes, including several genes involved in energy metabolism, such as pantothenate kinase and carbonic anhydrase. In addition, seven mitochondrial genes were found in the cloned subtracted library, but further analysis showed this was caused by allelic variation in mitochondrial genes in our founder population, and that a specific haplotype was associated with high thermal responsiveness. Future work will focus on candidate genes from pathways such as the oxidative phosphorylation and biosynthesis of coenzyme A which are possibly involved in thermal responsiveness of juvenile growth rate.

Genome-wide analysis reveals differences in brain gene expression patterns associated with caste and reproductive status in honey bees (Apis mellifera)

A key characteristic of eusocial species is reproductive division of labour. Honey bee colonies typically have a single reproductive queen and thousands of sterile workers. Adult queens differ dramatically from workers in anatomy, physiology, behaviour and lifespan. Young female workers can activate their ovaries and initiate egg laying; these 'reproductive' workers differ from sterile workers in anatomy, physiology, and behaviour. These differences, however, are on a much smaller scale than those observed between the queen and worker castes. Here, we use microarrays to monitor expression patterns of several thousand genes in the brains of same-aged virgin queens, sterile workers, and reproductive workers. We found large differences in expression between queens and both worker groups (~2000 genes), and much smaller differences between sterile and reproductive workers (221 genes). The expression patterns of these 221 genes in reproductive workers are more queen-like, and may represent a core group of genes associated with reproductive physiology. Furthermore, queens and reproductive workers preferentially up-regulate genes associated with the nurse bee behavioural state, which supports the hypothesis of an evolutionary link between worker division of labour and molecular pathways related to reproduction. Finally, several functional groups of genes associated with longevity in other species are significantly up-regulated in queens. Identifying the genes that underlie the differences between queens, sterile workers, and reproductive workers will allow us to begin to characterize the molecular mechanisms underlying the evolution of social behaviour and large-scale remodelling of gene networks associated with polyphenisms.

Molecular determinants of caste differentiation in the highly eusocial honeybee Apis mellifera

Background

In honeybees, differential feeding of female larvae promotes the occurrence of two different phenotypes, a queen and a worker, from identical genotypes, through incremental alterations, which affect general growth, and character state alterations that result in the presence or absence of specific structures. Although previous studies revealed a link between incremental alterations and differential expression of physiometabolic genes, the molecular changes accompanying character state alterations remain unknown.

Results

By using cDNA microarray analyses of >6,000 Apis mellifera ESTs, we found 240 differentially expressed genes (DEGs) between developing queens and workers. Many genes recorded as up-regulated in prospective workers appear to be unique to A. mellifera, suggesting that the workers' developmental pathway involves the participation of novel genes. Workers up-regulate more developmental genes than queens, whereas queens up-regulate a greater proportion of physiometabolic genes, including genes coding for metabolic enzymes and genes whose products are known to regulate the rate of mass-transforming processes and the general growth of the organism (e.g., tor). Many DEGs are likely to be involved in processes favoring the development of caste-biased structures, like brain, legs and ovaries, as well as genes that code for cytoskeleton constituents. Treatment of developing worker larvae with juvenile hormone (JH) revealed 52 JH responsive genes, specifically during the critical period of caste development. Using Gibbs sampling and Expectation Maximization algorithms, we discovered eight overrepresented cis-elements from four gene groups. Graph theory and complex networks concepts were adopted to attain powerful graphical representations of the interrelation between cis-elements and genes and objectively quantify the degree of relationship between these entities.

Conclusion

We suggest that clusters of functionally related DEGs are co-regulated during caste development in honeybees. This network of interactions is activated by nutrition-driven stimuli in early larval stages. Our data are consistent with the hypothesis that JH is a key component of the developmental determination of queen-like characters. Finally, we propose a conceptual model of caste differentiation in A. mellifera based on gene-regulatory networks.

Differential gene expression between adult queens and workers in the ant Lasius niger

Ants and other social insects forming large societies are generally characterized by marked reproductive division of labour. Queens largely monopolize reproduction whereas workers have little reproductive potential. In addition, some social insect species show tremendous lifespan differences between the queen and worker caste. Remarkably, queens and workers are usually genotypically identical, meaning that any phenotypic differences between the two castes arise from caste-specific gene expression. Using a combination of differential display, microarrays and reverse Northern blots, we found 16 genes that were differentially expressed between adult queens and workers in the ant Lasius niger, a species with highly pronounced reproductive division of labour and a several-fold lifespan difference between queens and workers. RNA ligase mediated rapid amplification of cDNA ends (RLM-RACE) and gene walking were used to further characterize these genes. On the basis of the molecular function of their nearest homologues, three genes appear to be involved in reproductive division of labour. Another three genes, which were exclusively overexpressed in queens, are possibly involved in the maintenance and repair of the soma, a candidate mechanism for lifespan determination. In-depth functional analyses of these genes are now needed to reveal their exact role.

Gene expression and the evolution of phenotypic diversity in social wasps

Background

Organisms are capable of developing different phenotypes by altering the genes they express. This phenotypic plasticity provides a means for species to respond effectively to environmental conditions. One of the most dramatic examples of phenotypic plasticity occurs in the highly social hymenopteran insects (ants, social bees, and social wasps), where distinct castes and sexes all arise from the same genes. To elucidate how variation in patterns of gene expression affects phenotypic variation, we conducted a study to simultaneously address the influence of developmental stage, sex, and caste on patterns of gene expression in Vespula wasps. Furthermore, we compared the patterns found in this species to those found in other taxa in order to investigate how variation in gene expression leads to phenotypic evolution.

Results

We constructed 11 different cDNA libraries derived from various developmental stages and castes of Vespula squamosa. Comparisons of overall expression patterns indicated that gene-expression differences distinguishing developmental stages were greater than expression differences differentiating sex or caste. Furthermore, we determined that certain sets of genes showed similar patterns of expression in the same phenotypic forms of different species. Specifically, larvae upregulated genes related to metabolism and genes possessing structural activity. Surprisingly, our data indicated that at least a few specific gene functions and at least one specific gene family are important components of caste differentiation across social insect taxa.

Conclusion

Despite research on various aspects of development originating from model systems, growth in understanding how development is related to phenotypic diversity relies on a growing literature of contrasting studies in non-model systems. In this study, we found that comparisons of patterns of gene expression with model systems highlighted areas of conserved and convergent developmental evolution across diverse taxa. Indeed, conserved biological functions across species implicated key functions related to how phenotypes are built. Finally, overall differences between social insect taxa suggest that the independent evolution of caste arose via distinct developmental trajectories.

A CENTENNIAL CELEBRATION FOR QUANTITATIVE GENETICS

Quantitative genetics is at or is fast approaching its centennial. In this perspective I consider five current issues pertinent to the application of quantitative genetics to evolutionary theory. First, I discuss the utility of a quantitative genetic perspective in describing genetic variation at two very different levels of resolution, (1) in natural, free-ranging populations and (2) to describe variation at the level of DNA transcription. Whereas quantitative genetics can serve as a very useful descriptor of genetic variation, its greater usefulness is in predicting evolutionary change, particularly when used in the first instance (wild populations). Second, I review the contributions of Quantitative trait loci (QLT) analysis in determining the number of loci and distribution of their genetic effects, the possible importance of identifying specific genes, and the ability of the multivariate breeder's equation to predict the results of bivariate selection experiments. QLT analyses appear to indicate that genetic effects are skewed, that at least 20 loci are generally involved, with an unknown number of alleles, and that a few loci have major effects. However, epistatic effects are common, which means that such loci might not have population-wide major effects: this question waits upon (QTL) analyses conducted on more than a few inbred lines. Third, I examine the importance of research into the action of specific genes on traits. Although great progress has been made in identifying specific genes contributing to trait variation, the high level of gene interactions underlying quantitative traits makes it unlikely that in the near future we will have mechanistic models for such traits, or that these would have greater predictive power than quantitative genetic models. In the fourth section I present evidence that the results of bivariate selection experiments when selection is antagonistic to the genetic covariance are frequently not well predicted by the multivariate breeder's equation. Bivariate experiments that combine both selection and functional analyses are urgently needed. Finally, I discuss the importance of gaining more insight, both theoretical and empirical, on the evolution of the G and P matrices.

Caste development and reproduction: a genome-wide analysis of hallmarks of insect eusociality

The honey bee queen and worker castes are a model system for developmental plasticity. We used established expressed sequence tag information for a Gene Ontology based annotation of genes that are differentially expressed during caste development. Metabolic regulation emerged as a major theme, with a caste-specific difference in the expression of oxidoreductases vs. hydrolases. Motif searches in upstream regions revealed group-specific motifs, providing an entry point to cis-regulatory network studies on caste genes. For genes putatively involved in reproduction, meiosis-associated factors came out as highly conserved, whereas some determinants of embryonic axes either do not have clear orthologs (bag of marbles, gurken, torso), or appear to be lacking (trunk) in the bee genome. Our results are the outcome of a first genome-based initiative to provide an annotated framework for trends in gene regulation during female caste differentiation (representing developmental plasticity) and reproduction.

A core linkage map of the bumblebeeBombus terrestris

The bumblebee Bombus terrestris is an economically important pollinator and an emerging model species in quantitative and population genetics. We generated genetic linkage maps for 3 independent mapping populations of B. terrestris. The linkage map with the highest resolution had 21 linkage groups, which adequately represents the haploid chromosome number of B. terrestris (n = 18). This map can be considered saturated, with an average marker distance of 10.3 cM and an estimated genome coverage of 81%. Using flow cytometry, we have estimated the genome size of this species to be 625 Mb. With an estimated total recombination genome length of 2760 cM, this results in a ratio of 226 kb/cM between the physical and genetic genome sizes. A recurring set of microsatellites and amplified fragment length polymorphism (AFLP) markers allowed the alignment of 14 linkage groups between the 3 maps. We propose to adopt this core map as a reference tool for future genetic and molecular work in B. terrestris.

Differential gene expression and phenotypic plasticity in behavioural castes of the primitively eusocial wasp, Polistes canadensis

Understanding how a single genome can produce a variety of different phenotypes is of fundamental importance in evolutionary and developmental biology. One of the most striking examples of phenotypic plasticity is the female caste system found in eusocial insects, where variation in reproductive (queens) and non-reproductive (workers) phenotypes results in a broad spectrum of caste types, ranging from behavioural through to morphological castes. Recent advances in genomic techniques allow novel comparisons on the nature of caste phenotypes to be made at the level of the genes in organisms for which there is little genome information, facilitating new approaches in studying social evolution and behaviour. Using the paper wasp Polistes canadensis as a model system, we investigated for the first time how behavioural castes in primitively eusocial insect societies are associated with differential expression of shared genes. We found that queens and newly emerged females express gene expression patterns that are distinct from each other whilst workers generally expressed intermediate patterns, as predicted by Polistes biology. We compared caste-associated genes in P. canadensis with those expressed in adult queens and workers of more advanced eusocial societies, which represent four independent origins of eusociality. Nine genes were conserved across the four taxa, although their patterns of expression and putative functions varied. Thus, we identify several genes that are putatively of evolutionary importance in the molecular biology that underlies a number of caste systems of independent evolutionary origin.

Caste-based differences in gene expression in the polyembryonic wasp Copidosoma floridanum

The polyembryonic parasitoid Copidosoma floridanum produces two larval castes, soldiers and reproductives, during development within its host. Soldier larvae defend the brood against competitors while reproductive larvae develop into adult wasps. As with other caste-forming insects, the distinct morphological and behavioral features of soldier and reproductive larvae likely involve differential gene expression. In this study we used a bi-directional suppression subtractive hybridization (SSH) approach to isolate differentially expressed genes from C. floridanum soldier and reproductive larvae. We isolated 230 novel expressed sequence tags (ESTs) from the two subtractions (114 soldier/116 reproductive ESTs). Among these ESTs were sequences with significant similarity to genes coding for serine proteinases, proteinase inhibitors, odorant-binding and chemosensory proteins, and cuticular proteins. Also, three novel genes were isolated that resemble one another in conceptual translation and share the cysteine spacing pattern of short scorpion toxins and insect defensins. Reverse transcription-polymerase chain reaction (RT-PCR) analysis of 20 ESTs from the two libraries indicated that 85% were differentially expressed in one caste or the other. We conclude that our SSH strategy was effective in identifying a number of genes differentially expressed in soldier and reproductive larvae and that several of these genes will be useful in characterizing caste-specific gene networks in C. floridanum.