Social dominance alters nutrition-related gene expression immediately: transcriptomic evidence from a monomorphic queenless ant.

Queen-worker differentiation in eusocial organisms may have originated from decoupling of maternal care and reproductive behaviours. Recent advances in sequencing techniques have begun to elucidate the molecular basis of queen-worker differentiation. However, current knowledge of the molecular basis of caste differentiation is limited, especially to species with morphological castes. It seems likely that at the dawn of eusociality morphologically undifferentiated, monomorphic females underwent physiological differentiation that yielded egg-laying and caretaking castes. The molecular basis of such physiological differentiation may provide evolutionary insight into the emergent state of eusociality. In this study, we identify genes associated with monomorphic caste differentiation, specifically focusing on the onset of queen-worker differentiation, using a monomorphic queenless ant, Diacamma sp., that secondarily lost morphological castes. Using individuals experimentally manipulated to become sterile or reproductive, we identified 1546 caste-biased transcripts in brain and 10 in gaster. Because caste differentiation occurs in Diacamma soon after eclosion via behavioural dominance, identified transcripts are interpreted as molecular agents responding immediately to dominance rank formation. Among identified genes, expression levels of genes involved in nutrition processing and storage, such as insulin signalling genes and hexamerins, were strongly altered soon after dominance rank formation. We conclude that the rapid modification of nutrition-related genes in response to social rank may be the fundamental mechanism underlying caste differentiation in Diacamma. Together with functional evidence from the literature, we show that a specific set of genes frequently plays a role in reproductive differentiation across systems with and without morphological castes.