Transcriptomic correlates of nutritional manipulation in a facultatively social bee

Subsocial behaviour in insects consists of extended parental care and may set the stage for the evolution of cooperation through manipulation of offspring. Manipulation of brood nutrition may produce differences in developmental or adult gene regulation, but it also produces smaller offspring which may be coerced into cooperation. The eastern small carpenter bee Ceratina calcarata frequently produces a smaller under-provisioned dwarf eldest daughter (DED). These DEDs are the only offspring to forage and feed siblings. To test whether nutritional manipulation of DEDs alters gene expression, inducing cooperative sibling care, we conducted a transcriptomic study, using whole heads, to assess differences in brain gene expression among naturally provisioned regular daughters and DEDs, experimentally under-provisioned regular daughters, and experimentally supplemented DEDs, prior to social interaction. Differences in gene expression were minimal among groups but were dramatic as a function of body size as a continuous variable, suggesting that differences in gene expression are more associated with absolute differences in body size, not discrete castes or order of eclosion. Enrichment for GO terms related to hormonal regulation in small bees points to hormonal regulation of transcription factors in behavioural differences that emerge in DEDs. Subordinate behaviours thus likely involve experience and social environment, though other developmental mechanisms, such as parental care, and later adult social interactions after eclosion, may act on differences in body size and gene expression to produce the distinct behaviour of DEDs.