Developmental- and food-dependent foraging transcript levels in the desert locust

Worker Reproduction and Caste Polymorphism Impact Genome Evolution and Social Genes Across the Ants

Eusocial insects are characterized by several traits, including reproductive division of labor and caste polymorphisms, which likely modulate genome evolution. Concomitantly, evolution may act on specific genes and pathways underlying these novel, sociality-associated phenotypes. Reproductive division of labor should increase the magnitude of genetic drift and reduce the efficacy of selection by reducing effective population size. Caste polymorphism has been associated with relaxed selection and may facilitate directional selection on caste-specific genes. Here, we use comparative analyses of 22 ant genomes to test how reproductive division of labor and worker polymorphism influence positive selection and selection intensity across the genome. Our results demonstrate that worker reproductive capacity is associated with a reduction in the degree of relaxed selection but is not associated with any significant change to positive selection. We find decreases in positive selection in species with polymorphic workers, but no increase in the degree of relaxed selection. Finally, we explore evolutionary patterns in specific candidate genes associated with our focal traits in eusocial insects. Two oocyte patterning genes previously implicated in worker sterility evolve under intensified selection in species with reproductive workers. Behavioral caste genes generally experience relaxed selection associated with worker polymorphism, whereas vestigial and spalt, both associated with soldier development in Pheidole ants, experience intensified selection in worker polymorphic species. These findings expand our understanding of the genetic mechanisms underlying elaborations of sociality. The impacts of reproductive division of labor and caste polymorphisms on specific genes illuminate those genes' roles in generating complex eusocial phenotypes.

Role of the foraging gene in worker behavioral transition in the red imported fire ant, Solenopsis invicta (Hymenoptera: Formicidae)

BACKGROUND

Worker division of labor is predominant in social insects. The foraging (for) gene, which encodes cGMP-dependent protein kinase (PKG), has been implicated in the regulation of behavioral transitions in honeybees, but information regarding its function in other social insects is scarce.

RESULTS

We investigated the role of the for (Sifor) gene in the red imported fire ant, Solenopsis invicta, and found that Sifor and PKG exhibited different expression patterns in different castes, body sizes, ages and tissues of fire ants, especially in foragers and nurses. Foragers displayed greater locomotor activity but showed no preference for larval or adult odors, whereas nurses showed lesser locomotor activity but had a strong preference for larval odors. We found that the expression of Sifor was significantly higher in the heads of foragers (compared to nurses). RNA interference-mediated Sifor knockdown in foraging workers induced behavioral transition of foragers toward the nurse phenotype characterized by reduced locomotor activity and a stronger preference for larval odors. By contrast, treating nurses with 8-Br-cGMP, an activator of PKG, resulted in behavioral transition toward the forager phenotype characterized by higher locomotor activity but reduced preference for larval odors.

CONCLUSION

Our results suggest that Sifor plays a critical role in the behavioral transition between foragers and nurses of workers, which may be a promising target for RNAi-based management of worker caste organization in S. invicta. © 2022 Society of Chemical Industry.

The foraging gene as a modulator of division of labour in social insects

The social ants, bees, wasps, and termites include some of the most ecologically-successful groups of animal species. Their dominance in most terrestrial environments is attributed to their social lifestyle, which enable their colonies to exploit environmental resources with remarkable efficiency. One key attribute of social insect colonies is the division of labour that emerges among the sterile workers, which represent the majority of colony members. Studies of the mechanisms that drive division of labour systems across diverse social species have provided fundamental insights into the developmental, physiological, molecular, and genomic processes that regulate sociality, and the possible genetic routes that may have led to its evolution from a solitary ancestor. Here we specifically discuss the conserved role of the foraging gene, which encodes a cGMP-dependent protein kinase (PKG). Originally identified as a behaviourally polymorphic gene that drives alternative foraging strategies in the fruit fly Drosophila melanogaster, changes in foraging expression and kinase activity were later shown to play a key role in the division of labour in diverse social insect species as well. In particular, foraging appears to regulate worker transitions between behavioural tasks and specific behavioural traits associated with morphological castes. Although the specific neuroethological role of foraging in the insect brain remains mostly unknown, studies in genetically tractable insect species indicate that PKG signalling plays a conserved role in the neuronal plasticity of sensory, cognitive and motor functions, which underlie behaviours relevant to division of labour, including appetitive learning, aggression, stress response, phototaxis, and the response to pheromones.

Expression Characterization and Localization of the foraging Gene in the Chinese Bee, Apis cerana cerana (Hymenoptera: Apidae)

In social insects, the foraging gene (for) regulates insect age- and task-based foraging behaviors. We studied the expression and localization of the for gene (Acfor) in Apis cerana cerana workers to explore whether the differential regulation of this gene is associated with the behaviors of nurses and foragers. The expression profiles of Acfor in different tissues and at different ages were examined using real-time quantitative reverse transcription polymerase chain reaction. Cellular localization in the brain was detected using in situ hybridization. Acfor transcripts in different ages workers showed that Acfor expression was detected in all the heads of 1- to 30-d-old worker bees. Acfor expression reached a peak at 25 d of age, and then declined with increasing age. The results showed that Acfor gene expression in five tissues was respectively significantly higher in foragers than in nurses. In nurses, the relative expression of Acfor was the highest in the antennae. There was a highly significant difference in expression between antennae, legs, and the other three tissues. In foragers, Acfor expression was the highest in the thorax, which was significantly different from all other tissues. In situ hybridization showed that Acfor was highly expressed in the lamina of the optic lobes, in a central column of Kenyon cells in the mushroom bodies of the brain of workers, and in the antennal lobes. This suggested that Acfor expression affects age-related foraging behavior in Apis cerana cerana, and that it may be related to flight activity.

An ant-plant mutualism through the lens of cGMP-dependent kinase genes

In plant-animal mutualisms, how an animal forages often determines how much benefit its plant partner receives. In many animals, foraging behaviour changes in response to foraging gene expression or activation of the cGMP-dependent protein kinase (PKG) that foraging encodes. Here, we show that this highly conserved molecular mechanism affects the outcome of a plant-animal mutualism. We studied the two PKG genes of Allomerus octoarticulatus, an Amazonian ant that defends the ant-plant Cordia nodosa against herbivores. Some ant colonies are better 'bodyguards' than others. Working in the field in Peru, we found that colonies fed with a PKG activator recruited more workers to attack herbivores than control colonies. This resulted in less herbivore damage. PKG gene expression in ant workers correlated with whether an ant colony discovered an herbivore and how much damage herbivores inflicted on leaves in a complex way; natural variation in expression levels of the two genes had significant interaction effects on ant behaviour and herbivory. Our results suggest a molecular basis for ant protection of plants in this mutualism.

Food searching behaviour of a Lepidoptera pest species is modulated by the foraging gene polymorphism

The extent of damage to crop plants from pest insects depends on the foraging behaviour of the insect's feeding stage. Little is known, however, about the genetic and molecular bases of foraging behaviour in phytophagous pest insects. The foraging gene (for), a candidate gene encoding a PKG-I, has an evolutionarily conserved function in feeding strategies. Until now, for had never been studied in Lepidoptera, which includes major pest species. The cereal stem borer Sesamia nonagrioides is therefore a relevant species within this order with which to study conservation of and polymorphism in the for gene, and its role in foraging - a behavioural trait that is directly associated with plant injuries. Full sequencing of for cDNA in S. nonagrioides revealed a high degree of conservation with other insect taxa. Activation of PKG by a cGMP analogue increased larval foraging activity, measured by how frequently larvae moved between food patches in an actimeter. We found one non-synonymous allelic variation in a natural population that defined two allelic variants. These variants presented significantly different levels of foraging activity, and the behaviour was positively correlated to gene expression levels. Our results show that for gene function is conserved in this species of Lepidoptera, and describe an original case of a single nucleotide polymorphism associated with foraging behaviour variation in a pest insect. By illustrating how variation in this single gene can predict phenotype, this work opens new perspectives into the evolutionary context of insect adaptation to plants, as well as pest management.

Gene-environment interplay in Drosophila melanogaster: chronic nutritional deprivation in larval life affects adult fecal output

Life history consequences of stress in early life are varied and known to have lasting impacts on the fitness of an organism. Gene-environment interactions play a large role in how phenotypic differences are mediated by stressful conditions during development. Here we use natural allelic 'rover/sitter' variants of the foraging (for) gene and chronic early life nutrient deprivation to investigate gene-environment interactions on excretion phenotypes. Excretion assay analysis and a fully factorial nutritional regimen encompassing the larval and adult life cycle of Drosophila melanogaster were used to assess the effects of larval and adult nutritional stress on adult excretion phenotypes. Natural allelic variants of for exhibited differences in the number of fecal spots when they were nutritionally deprived as larvae and well fed as adults. for mediates the excretion response to chronic early-life nutritional stress in mated female, virgin female, and male rovers and sitters. Transgenic manipulations of for in a sitter genetic background under larval but not adult food deprivation increases the number of fecal spots. Our study shows that food deprivation early in life affects adult excretion phenotypes and these excretion differences are mediated by for.

Examining the role of foraging and malvolio in host-finding behavior in the honey bee parasite, Varroa destructor (Anderson & Trueman)

When a female varroa mite, Varroa destructor (Anderson & Trueman), invades a honey bee brood cell, the physiology rapidly changes from feeding phoretic to reproductive. Changes in foraging and malvolio transcript levels in the brain have been associated with modulated intra-specific food searching behaviors in insects and other invertebrates. Transcription profiles for both genes were examined during and immediately following brood cell invasion to assess their role as potential control elements. Vdfor and Vdmvl transcripts were found in all organs of varroa mites with the highest Vdfor transcript levels in ovary-lyrate organs and the highest Vdmvl in Malpighian tubules. Changes in transcript levels of Vdfor and Vdmvl in synganglia were not associated with the cell invasion process, remaining comparable between early reproductive mites (collected from the pre-capping brood cells) and phoretic mites. However, Vdfor and Vdmvl transcript levels were lowered by 37 and 53%, respectively, in synganglia from reproductive mites compared to early reproductive mites, but not significantly different to levels in synganglia from phoretic mites. On the other hand, in whole body preparations the Vdfor and Vdmvl had significantly higher levels of transcript in reproductive mites compared to phoretic and early reproductive, mainly due to the presence of both transcripts accumulating in the eggs carried by the ovipositing mite. Varroa mites are a critical component for honey bee population decline and finding varroa mite genes associated with brood cell invasion, reproduction, ion balance and other physiological processes will facilitate development of novel control avenues for this honey bee parasite.