Developmental bias of body size and caste by the fire ant social supergene

Supergenes are clusters of linked genes that drive complex phenotypic divergence. In the fire ant Solenopsis invicta, a supergene determines polymorphic social organization, and its derived (b) haplotype is over-represented among large workers and gynes relative to the ancestral (B) haplotype, acting as a selfish genetic element. We tested whether this caste bias reflects altered juvenile hormone (JH) mediated growth by treating colonies with a JH analogue. We show that BB and Bb pupae reach similar maximal sizes and exhibit a comparable JH-mediated growth response despite consistent Bb over-representation in the upper size classes. We also used developmental gene expression data from another ant to reveal significantly enriched overlap between genes with differential expression by supergene genotype and orthologues with differential expression between gyne- and worker-destined larvae. This overlap could be shaped in part by observed cis-regulatory evolution of the supergene. Overall, our results are consistent with incompletely penetrant effects of a supergene on developmental trajectories linked to size-based caste allocation in a highly social insect.