Size-dependent foraging gene expression and behavioral caste differentiation in Bombus ignitus

Background. In eusocial hymenopteran insects, foraging genes, members of the cGMP-dependent protein kinase family, are considered to contribute to division of labor through behavioral caste differentiation. However, the relationship between foraging gene expression and behavioral caste in honeybees is opposite to that observed in ants and wasps. In the previously examined eusocial Hymenoptera, workers behave as foragers or nurses depending on age. We reasoned that examination of a different system of behavioral caste determination might provide new insights into the relationship between foraging genes and division of labor, and accordingly focused on bumblebees, which exhibit size-dependent behavioral caste differentiation. We characterized a foraging gene (Bifor) in bumblebees (Bombus ignitus) and examined the relationship between Bifor expression and size-dependent behavioral caste differentiation. Findings. A putative open reading frame of the Bifor gene was 2004 bp in length. It encoded 668 aa residues and showed high identity to orthologous genes in other hymenopterans (85.3-99.0%). As in ants and wasps, Bifor expression levels were higher in nurses than in foragers. Bifor expression was negatively correlated with individual body size even within the same behavioral castes (regression coefficient = -0.376, P < 0.001, all individuals; -0.379, P = 0.018, within foragers). Conclusion. These findings indicate that Bifor expression is size dependent and support the idea that Bifor expression levels are related to behavioral caste differentiation in B. ignitus. Thus, the relationship between foraging gene expression and behavioral caste differentiation found in ants and wasps was identified in a different system of labor determination.