Social divergence: molecular pathways underlying castes and longevity in a facultatively eusocial small carpenter bee

Exploring the genetics of social behaviour in C. calcarata

Studies investigating social evolution often focus on species that are obligately eusocial, where presumably all of the adaptive genetic changes associated with sociality have already been completed. To fully understand eusociality, we must study species with facultative social behaviour. The small carpenter bee Ceratina calcarata is an ideal model for studying the genetics and molecular biology of eusocial evolution as it can exhibit both subsocial behaviour with parental care and social behaviour facilitated by the altruistic dwarf eldest daughter. Here, we sequenced the genomes of subsocial and social C. calcarata to identify mutations and genes associated with social behaviour and used these data to test several hypotheses related to the evolution of eusociality. Many single nucleotide polymorphisms that had high levels of genetic differentiation (Fst) between social and subsocial C. calcarata were in or near genes or regions important for regulating gene expression. These results are consistent with the Genetic Toolkit Hypothesis of eusocial evolution. Our findings suggest that the low behavioural complexity observed in C. calcarata may involve modulation of existing regulatory genes and gene networks to generate phenotypes associated with social behaviour.

Genomic and transcriptomic analyses of a social hemipteran provide new insights into insect sociality

The origin of sociality represents one of the most important evolutionary transitions. Insect sociality evolved in some hemipteran aphids, which can produce soldiers and normal nymphs with distinct morphology and behaviour through parthenogenesis. The lack of genomic data resources has hindered the investigations into molecular mechanisms underlying their social evolution. Herein, we generated the first chromosomal-level genome of a social hemipteran (Pseudoregma bambucicola) with highly specialized soldiers and performed comparative genomic and transcriptomic analyses to elucidate the molecular signatures and regulatory mechanisms of caste differentiation. P. bambucicola has a larger known aphid genome of 582.2 Mb with an N50 length of 11.24 Mb, and about 99.6% of the assembly was anchored to six chromosomes with a scaffold N50 of 98.27 Mb. A total of 14,027 protein-coding genes were predicted and 37.33% of the assembly were identified as repeat sequences. The social evolution is accompanied by a variety of changes in genome organization, including expansion of gene families related to transcription factors, transposable elements, as well as species-specific expansions of certain sugar transporters and UGPases involved in carbohydrate metabolism. We also characterized large candidate gene sets linked to caste differentiation and found evidence of expression regulation and positive selection acting on energy metabolism and muscle structure, explaining the soldier-specific traits including morphological and behavioural specialization, developmental arrest and infertility. Overall, this study offers new insights into the molecular basis of social aphids and the evolution of insect sociality and also provides valuable data resources for further comparative and functional studies.

Adaptive evolution of A-to-I auto-editing site in Adar of eusocial insects

BACKGROUND

Adenosine-to-inosine (A-to-I) RNA editing is a co-/post-transcriptional modification introducing A-to-G variations in RNAs. There is extensive discussion on whether the flexibility of RNA editing exerts a proteomic diversification role, or it just acts like hardwired mutations to correct the genomic allele. Eusocial insects evolved the ability to generate phenotypically differentiated individuals with the same genome, indicating the involvement of epigenetic/transcriptomic regulation.

METHODS

We obtained the genomes of 104 Hymenoptera insects and the transcriptomes of representative species. Comparative genomic analysis was performed to parse the evolutionary trajectory of a regulatory Ile > Met auto-recoding site in Adar gene.

RESULTS

At genome level, the pre-editing Ile codon is conserved across a node containing all eusocial hymenopterans. At RNA level, the editing events are confirmed in representative species and shows considerable condition-specificity. Compared to random expectation, the editable Ile codon avoids genomic substitutions to Met or to uneditable Ile codons, but does not avoid mutations to other unrelated amino acids.

CONCLUSIONS

The flexibility of Adar auto-recoding site in Hymenoptera is selectively maintained, supporting the flexible RNA editing hypothesis. We proposed a new angle to view the adaptation of RNA editing, providing another layer to explain the great phenotypical plasticity of eusocial insects.

Gene regulation supporting sociality shared across lineages and variation in complexity

Across evolutionary lineages, insects vary in social complexity, from those that exhibit extended parental care to those with elaborate divisions of labor. Here, we synthesize the sociogenomic resources from hundreds of species to describe common gene regulatory mechanisms in insects that regulate social organization across phylogeny and levels of social complexity. Different social phenotypes expressed by insects can be linked to the organization of co-expressing gene networks and features of the epigenetic landscape. Insect sociality also stems from processes like the emergence of parental care and the decoupling of ancestral genetic programs. One underexplored avenue is how variation in a group's social environment affects the gene expression of individuals. Additionally, an experimental reduction of gene expression would demonstrate how the activity of specific genes contributes to insect social phenotypes. While tissue specificity provides greater localization of the gene expression underlying social complexity, emerging transcriptomic analysis of insect brains at the cellular level provides even greater resolution to understand the molecular basis of social insect evolution.