The influence of the social environment on larval development and resulting caste in Bombus impatiens

Abstract

The ability of a single genome to express multiple phenotypes is key to understanding social evolution, where individuals with different morphologies perform different tasks. In social insects, phenotypes are typically determined during larval development and depend on the social environment. Here, we used the bumble bee Bombus impatiens to examine the social regulation of body size variation and caste by manipulating the identity (queen/worker) and the number of caretakers tending for the brood. Eggs of females and males were kept in cages with (1) a single queen; (2) a single worker; (3) three workers; or (4) ten workers. We measured larval weight, developmental length, and the resulting caste (in females) throughout the brood development of >2000 individuals. We found differences in larval mass when reared by varying number of workers, but not when reared by a single worker compared to a queen. Additionally, in contrast with previous findings in B. terrestris, none of the female eggs reared by workers developed into gynes (new queens), indicating that the mechanisms regulating caste determination in B. impatiens is not solely dependent on the queen presence as in B. terrestris. Adult males were slightly larger than workers and developed for a longer period in the presence of the queen. Overall, we suggest that body size in B. impatiens is determined by the number of caretakers and is likely mediated by the amount of provision. The hypothesis that the queens presence manipulates female caste was not supported by our data.