Evolutionary genomics of socially polymorphic populations of Pogonomyrmex californicus.

BACKGROUND: Social insects vary considerably in their social organization both between and within species. In the California harvester ant, Pogonomyrmex californicus (Buckley 1867), colonies are commonly founded and headed by a single queen (haplometrosis, primary monogyny). However, in some populations in California (USA), unrelated queens cooperate not only during founding (pleometrosis) but also throughout the life of the colony (primary polygyny). The genetic architecture and evolutionary dynamics of this complex social niche polymorphism (haplometrosis vs pleometrosis) have remained unknown. RESULTS: We provide a first analysis of its genomic basis and evolutionary history using population genomics comparing individuals from a haplometrotic population to those from a pleometrotic population. We discovered a recently evolved (< 200 k years), 8-Mb non-recombining region segregating with the observed social niche polymorphism. This region shares several characteristics with supergenes underlying social polymorphisms in other socially polymorphic ant species. However, we also find remarkable differences from previously described social supergenes. Particularly, four additional genomic regions not in linkage with the supergene show signatures of a selective sweep in the pleometrotic population. Within these regions, we find for example genes crucial for epigenetic regulation via histone modification (chameau) and DNA methylation (Dnmt1). CONCLUSIONS: Altogether, our results suggest that social morph in this species is a polygenic trait involving a potential young supergene. Further studies targeting haplo- and pleometrotic individuals from a single population are however required to conclusively resolve whether these genetic differences underlie the alternative social phenotypes or have emerged through genetic drift.

How Individualized Niches Arise: Defining Mechanisms of Niche Construction, Niche Choice, and Niche Conformance.

Organisms interact with their environments in various ways. We present a conceptual framework that distinguishes three mechanisms of organism-environment interaction. We call these NC3 mechanisms: niche construction, in which individuals make changes to the environment; niche choice, in which individuals select an environment; and niche conformance, in which individuals adjust their phenotypes in response to the environment. Each of these individual-level mechanisms affects an individual's phenotype-environment match, its fitness, and its individualized niche, defined in terms of the environmental conditions under which the individual can survive and reproduce. Our framework identifies how individuals alter the selective regimes that they and other organisms experience. It also places clear emphasis on individual differences and construes niche construction and other processes as evolved mechanisms. The NC3 mechanism framework therefore helps to integrate population-level and individual-level research.

Digest: Better constructing the future than choosing a new habitat.

What strategy should an individual follow in a heterogeneous environment when its phenotype is not optimized for its current environment: make changes to the environment (habitat construction), move to a different place (habitat choice), or both? Scheiner et al. used an individual-based model to investigate the interaction of habitat choice and habitat construction. In most situations, habitat construction was superior to either habitat selection or a mixed strategy.

Digest: Changing environment often easier than changing phenotype.

What strategy should an individual follow when faced with a suboptimal environment: change the environment, adapt to the environment, or both? Scheiner et al. used an individual-based model to address the interaction of plasticity and habitat construction with different life histories in a heterogeneous environment. In most situations, habitat construction was superior to either plasticity or a mixed strategy, but not always, and specific conditions may favor plasticity.

Genome assembly and annotation of the California harvester ant Pogonomyrmex californicus.

The harvester ant genus Pogonomyrmex is endemic to arid and semiarid habitats and deserts of North and South America. The California harvester ant Pogonomyrmex californicus is the most widely distributed Pogonomyrmex species in North America. Pogonomyrmex californicus colonies are usually monogynous, i.e. a colony has one queen. However, in a few populations in California, primary polygyny evolved, i.e. several queens cooperate in colony founding after their mating flights and continue to coexist in mature colonies. Here, we present a genome assembly and annotation of P. californicus. The size of the assembly is 241 Mb, which is in agreement with the previously estimated genome size. We were able to annotate 17,889 genes in total, including 15,688 protein-coding ones with BUSCO (Benchmarking Universal Single-Copy Orthologs) completeness at a 95% level. The presented P. californicus genome assembly will pave the way for investigations of the genomic underpinnings of social polymorphism in the number of queens, regulation of aggression, and the evolution of adaptations to dry habitats.

Digest: Evolution of eusociality favored by split sex ratios under worker-control.

Eusociality has repeatedly independently evolved in ants, bees, and wasps (Hymenoptera), leading to the idea that haplodiploidy may be an important driving factor in this group. Using a modeling approach, Quiñones et al. show that split sex ratios and worker control of sex ratios (achieved by removal of male brood) can promote the initial evolution of helping raise offspring of related individuals. However, over time, these factors can result in social polymorphism, that is, a mix of solitary and social nests, or to eusocial colonies with three different strategies, namely those that produce mostly females, mostly males, or a balanced sex ratio.

Honeybees possess a structurally diverse and functionally redundant set of queen pheromones.

Queen pheromones, which signal the presence of a fertile queen and induce workers to remain sterile, play a key role in regulating reproductive division of labour in insect societies. In the honeybee, volatiles produced by the queen's mandibular glands have been argued to act as the primary sterility-inducing pheromones. This contrasts with evidence from other groups of social insects, where specific queen-characteristic hydrocarbons present on the cuticle act as conserved queen signals. This led us to hypothesize that honeybee queens might also employ cuticular pheromones to stop workers from reproducing. Here, we support this hypothesis with the results of bioassays with synthetic blends of queen-characteristic alkenes, esters and carboxylic acids. We show that all these compound classes suppress worker ovary development, and that one of the blends of esters that we used was as effective as the queen mandibular pheromone (QMP) mix. Furthermore, we demonstrate that the two main QMP compounds 9-ODA and 9-HDA tested individually were as effective as the blend of all four major QMP compounds, suggesting considerable signal redundancy. Possible adaptive reasons for the observed complexity of the honeybee queen signal mix are discussed.

Gene Expression Dynamics in Major Endocrine Regulatory Pathways along the Transition from Solitary to Social Life in a Bumblebee, Bombus terrestris.

Understanding the social evolution leading to insect eusociality requires, among other, a detailed insight into endocrine regulatory mechanisms that have been co-opted from solitary ancestors to play new roles in the complex life histories of eusocial species. Bumblebees represent well-suited models of a relatively primitive social organization standing on the mid-way to highly advanced eusociality and their queens undergo both, a solitary and a social phase, separated by winter diapause. In the present paper, we characterize the gene expression levels of major endocrine regulatory pathways across tissues, sexes, and life-stages of the buff-tailed bumblebee, Bombus terrestris, with special emphasis on critical stages of the queen's transition from solitary to social life. We focused on fundamental genes of three pathways: (1) Forkhead box protein O and insulin/insulin-like signaling, (2) Juvenile hormone (JH) signaling, and (3) Adipokinetic hormone signaling. Virgin queens were distinguished by higher expression of forkhead box protein O and downregulated insulin-like peptides and JH signaling, indicated by low expression of methyl farnesoate epoxidase (MFE) and transcription factor Krüppel homolog 1 (Kr-h1). Diapausing queens showed the expected downregulation of JH signaling in terms of low MFE and vitellogenin (Vg) expressions, but an unexpectedly high expression of Kr-h1. By contrast, reproducing queens revealed an upregulation of MFE and Vg together with insulin signaling. Surprisingly, the insulin growth factor 1 (IGF-1) turned out to be a queen-specific hormone. Workers exhibited an expression pattern of MFE and Vg similar to that of reproducing queens. Males were characterized by high Kr-h1 expression and low Vg level. The tissue comparison unveiled an unexpected resemblance between the fat body and hypopharyngeal glands across all investigated genes, sexes, and life stages.

Epigenetics and locust life phase transitions.

Insects are one of the most successful classes on Earth, reflected in an enormous species richness and diversity. Arguably, this success is partly due to the high degree to which polyphenism, where one genotype gives rise to more than one phenotype, is exploited by many of its species. In social insects, for instance, larval diet influences the development into distinct castes; and locust polyphenism has tricked researchers for years into believing that the drastically different solitarious and gregarious phases might be different species. Solitarious locusts behave much as common grasshoppers. However, they are notorious for forming vast, devastating swarms upon crowding. These gregarious animals are shorter lived, less fecund and transmit their phase characteristics to their offspring. The behavioural gregarisation occurs within hours, yet the full display of gregarious characters takes several generations, as does the reversal to the solitarious phase. Hormones, neuropeptides and neurotransmitters influence some of the phase traits; however, none of the suggested mechanisms can account for all the observed differences, notably imprinting effects on longevity and fecundity. This is why, more recently, epigenetics has caught the interest of the polyphenism field. Accumulating evidence points towards a role for epigenetic regulation in locust phase polyphenism. This is corroborated in the economically important locust species Locusta migratoria and Schistocerca gregaria. Here, we review the key elements involved in phase transition in locusts and possible epigenetic regulation. We discuss the relative role of DNA methylation, histone modification and small RNA molecules, and suggest future research directions.

Royalactin extends lifespan of Caenorhabditis elegans through epidermal growth factor signaling.

Royalactin is a glycoprotein essential for the development of long-lived queen honeybees. Only larvae fed with royal jelly, containing royalactin, develop into queens. Royalactin plays a central role in this process by switching on the epidermal growth factor (EGF) receptor signaling pathway which ultimately leads to epigenetic changes and a long-lived queen phenotype. Recently it was shown that royalactin by itself also extends lifespan in Drosophila melanogaster. Yet, the mechanism by which royalactin promotes longevity remains largely unknown. We set out to characterize the effects of royalactin on Caenorhabditis elegans lifespan, and clarify the possible involvement of EGF signaling in this process. We demonstrate that royalactin extends lifespan of this nematode and that both EGF (LIN-3) and its receptor (LET-23) are essential to this process. To our knowledge, this is the first report of royalactin-mediated lifespan extension in a non-insect species. Additionally, we show that royalactin enhances locomotion in adult nematodes, implying that royalactin also influences healthspan. Our results suggest that royalactin is an important lifespan-extending factor in royal jelly and acts by promoting EGF signaling in C. elegans. Further work will now be needed to clarify which (secondary) signaling pathways are activated by royalactin, and how this ultimately translates into an extended health- and lifespan.

Life-prolonging measures for a dead theory?

In a recent review article, Selman and colleagues (Trends Ecol Evol 27:570-577, 2012) discuss the status quo of the oxidative stress theory of aging (OSTA) and how it links to life history evolution. They suggest that the OSTA should be tested in wild populations which might show effects masked in laboratory settings. We disagree with their propositions for several reasons. We argue that there is increasing evidence that reactive oxygen species (ROS) are not causally linked with aging and that ROS do not play a straightforward role in shaping life history evolution. We propose that laboratory animals and semi-wild populations rather than wild animals are suited best to test any hypothesized effect of reactive oxygen species. This is because data from controlled manipulative experiments rather than observational correlations are preferred to solve this issue. In addition, nonconventional model organisms will be useful in answering the question how relevant the OSTA could be for life history evolution.

Reproduction of honeybee workers is regulated by epidermal growth factor receptor signaling.

Eusocial insect societies display a remarkable reproductive division of labor between a single fertile queen and thousands of largely sterile workers. In most species, however, the workers retain the capacity to reproduce, particularly in queenless colonies where typically many workers lay eggs. As yet, the molecular determinants that initiate this shift in worker fertility are still poorly documented. By using RNA interference we here demonstrate that the knockdown of epidermal growth factor receptor, a gene which was previously shown to be involved in queen-worker caste differentiation, also induces reproduction in worker honeybees (Apis mellifera). These data show that worker fertility and queen-worker caste determination partly rely on the same gene regulatory networks, thereby providing a major breakthrough in our understanding of the molecular determinants of the social insects' spectacular reproductive division of labor.

The mode of action of juvenile hormone and ecdysone: towards an epi-endocrinological paradigm?

In some insect species, two sites of juvenile hormone (JH) synthesis have been reported: the very well documented corpora allata that secrete JH for "general use", and the reproductive system, in particular the male accessory glands, in which the function of the sometimes huge amounts of JH (e.g. in Hyalophora cecropia) remains to be clarified. A recent finding in Schistocerca gregaria, namely that suppression of the ecdysteroid peak preceding a molt by RNAi of the Halloween genes spook, phantom and shade does not impede normal molting, challenges the (never experimentally proven) classical concept that such a peak is causally linked to a molt. Recent developments in epigenetic control of gene expression in both the honey bee and in locusts suggest that, in addition to the classical scheme of hormone-receptor (membrane- and/or nuclear) mode of action, there may be a third way. Upon combining these and other orphan data that do not fit in the commonly accepted textbook schemes, we here advance the working hypothesis that both JH and ecdysone might be important but overlooked players in epigenetic control of gene expression, in particular at extreme concentrations (peak values or total absence). In this review, we put forward how epi-endocrinology can complement classical arthropod endocrinology.

Worker honeybee sterility: a proteomic analysis of suppressed ovary activation.

Eusocial behavior is extensively studied in the honeybee, Apis mellifera, as it displays an extreme form of altruism. Honeybee workers are generally obligatory sterile in a bee colony headed by a queen, but the inhibition of ovary activation is lifted upon the absence of queen and larvae. Worker bees are then able to develop mature, viable eggs. The detailed repressive physiological mechanisms that are responsible for this remarkable phenomenon are as of yet largely unknown. Physiological studies today mainly focus on the transcriptome, while the proteome stays rather unexplored. Here, we present a quantitative 2-dimensional differential gel electrophoresis comparison between activated and inactivated worker ovaries and brains of reproductive and sterile worker bees, including a spot map of ovaries, containing 197 identified spots. Our findings suggest that suppression of ovary activation might involve a constant interplay between primordial oogenesis and subsequent degradation, which is probably mediated through steroid and neuropeptide hormone signaling. Additionally, the observation of higher viral protein loads in both the brains and ovaries of sterile workers is particularly noteworthy. This data set will be of great value for future research unraveling the physiological mechanisms underlying the altruistic sterility in honeybee workers.

Genome-wide analysis of alternative reproductive phenotypes in honeybee workers.

A defining feature of social insects is the reproductive division of labour, in which workers usually forego all reproduction to help their mother queen to reproduce. However, little is known about the molecular basis of this spectacular form of altruism. Here, we compared gene expression patterns between nonreproductive, altruistic workers and reproductive, non-altruistic workers in queenless honeybee colonies using a whole-genome microarray analysis. Our results demonstrate massive differences in gene expression patterns between these two sets of workers, with a total of 1292 genes being differentially expressed. In nonreproductive workers, genes associated with energy metabolism and respiration, flight and foraging behaviour, detection of visible light, flight and heart muscle contraction and synaptic transmission were overexpressed relative to reproductive workers. This implies they probably had a higher whole-body energy metabolism and activity rate and were most likely actively foraging, whereas same-aged reproductive workers were not. This pattern is predicted from evolutionary theory, given that reproductive workers should be less willing to compromise their reproductive futures by carrying out high-risk tasks such as foraging or other energetically expensive tasks. By contrast, reproductive workers mainly overexpressed oogenesis-related genes compared to nonreproductive ones. With respect to key switches for ovary activation, several genes involved in steroid biosynthesis were upregulated in reproductive workers, as well as genes known to respond to queen and brood pheromones, genes involved in TOR and insulin signalling pathways and genes located within quantitative trait loci associated with reproductive capacity in honeybees. Overall, our results provide unique insight into the molecular mechanisms underlying alternative reproductive phenotypes in honeybee workers.

Locust phase polyphenism: Does epigenetic precede endocrine regulation?

The morphological, physiological and behavioural differences between solitarious and gregarious desert locusts are so pronounced that one could easily mistake the two phases as belonging to different species, if one has no knowledge of the phenomenon of phenotypic plasticity. A number of phase-specific features are hormonally controlled. Juvenile hormone promotes several solitarious features, the green cuticular colour being the most obvious one. The neuropeptide corazonin elicits the dark cuticular colour that is typical for the gregarious phase, as well as particular gregarious behavioural characteristics. However, it had to be concluded, for multiple reasons, that the endocrine system is not the primary phase-determining system. Our observation that longevity gets imprinted in very early life by crowding of the young hatchlings, and that it cannot be changed thereafter, made us consider the possibility that, perhaps, epigenetic control of gene expression might be, if not the missing, a primary phase-determining mechanism. Imprinting is likely to involve DNA methylation and histone modification. Analysis of a Schistocerca EST database of nervous tissue identified the presence of several candidate genes that may be involved in epigenetic control, including two DNA methyltransferases (Dnmts). Dnmt1 and Dnmt2 are phase-specifically expressed in certain tissues. In the metathoracic ganglion, important in the serotonin pathway for sensing mechanostimulation, their expression is clearly affected by crowding. Our data urge for reconsidering the role of the endocrine system as being sandwiched in between genetics and epigenetics, involving complementary modes of action.

Differential proteomics in dequeened honeybee colonies reveals lower viral load in hemolymph of fertile worker bees.

The eusocial societies of honeybees, where the queen is the only fertile female among tens of thousands sterile worker bees, have intrigued scientists for centuries. The proximate factors, which cause the inhibition of worker bee ovaries, remain largely unknown; as are the factors which cause the activation of worker ovaries upon the loss of queen and brood in the colony. In an attempt to reveal key players in the regulatory network, we made a proteomic comparison of hemolymph profiles of workers with completely activated ovaries vs. rudimentary ovaries. An unexpected finding of this study is the correlation between age matched worker sterility and the enrichment of Picorna-like virus proteins. Fertile workers, on the other hand, show the upregulation of potential components of the immune system. It remains to be investigated whether viral infections contribute to worker sterility directly or are the result of a weaker immune system of sterile workers.