The role of epigenetic mechanisms in the regulation of division of labor in the ant Temnothorax longispinosus
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Abstract
Division of labor (DOL) is a key organizing principle of complex biological systems that arose repeatedly during the major transitions in evolution and contributes significantly to the ecological success of insect societies. Reproductive DOL results in one or a few individuals in a colony focusing on reproduction (queens/kings), while the remaining group members function as sterile, non-reproductive workers. In most insect societies, there is also a DOL among the workers for tasks important for colony growth and productivity. All members have a specific role, concentrating on intranidal tasks such as brood or nestmate care, and outside tasks such as foraging or colony defense. Although age plays an important role in DOL, the allocation of tasks remains dynamic, and workers can switch back and forth according to the needs of the colony. It is generally accepted in the field that DOL stems from individuals differing in their response to task-related chemical signals, leading to behavioral variation and task specialization. Therefore, a major question is to understand the regulatory processes that control the behavior of workers and their specialization on specific tasks via differential gene expression.
In this study, we investigated transcriptional differences between distinct behavioral phenotypes in the brain (i.e., the central nervous system) and in the antennae (i.e., the peripheral sensory organs) of the small acorn ant Temnothorax longispinosus. We found that i) behavioral variation was associated with significant transcriptomic changes in antennae, ii) these changes encompassed a large portion of the repertoire of odorant receptor (OR) genes, and iii) workers specialized on brood care overexpressed ORs of exon-9 gene family thought to be involved in social cue recognition. These results support our hypothesis that the peripheral nervous system plays an important role as a sensory filter in regulating behavioral differences between workers and thus in the DOL of social insects. Until now, it had been assumed that all workers perceive task-related cues in the same way, but only take on a task if the associated signal crosses the response threshold of the individual. However, our results suggest that not all workers perceive the signals similarly because a sensory filter in the antennae restricts the central nervous system's access to chemical information that is important for task performance.
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The epigenetic regulation of this sensory filter is still unknown, but previous experiments suggest a role of candidate genes such as Vg-like-A and epigenetic factors such as histone acetylation or miRNAs. Therefore, we proceeded to knocked down Vg- like-A via RNAi injection in the gaster of nurse ants to assess the function of this pathway in influencing worker behavior within 4-10 days. We documented a downregulation over 10 days, which also led to some behavioral changes, but in the final experiment, in which we aimed to investigate changes in gene expression in different tissues including the antenna, the downregulation of Vg-like-A failed. Finally, we developed a ChIP-seq protocol for brain samples and bioinformatic analytical pipeline to investigate differences in histone acetylation (H3K27ac) between nurses and foragers and discuss experimental design issues that may have influenced our results. In summary, this work has made an important contribution to the understanding of the molecular regulation of the division of labor in ant colonies. It has not only provided evidence for the function of a sensory filter, but also approaches for further analysis of regulatory elements, including new candidate genes, histone modifications and miRNAs, which deserve further investigation.