Correlations between spatiotemporal changes in gene expression and apoptosis underlie wing polyphenism in the ant Pheidole morrisi.

Wing polyphenism, which is the ability of a single genome to produce winged and wingless castes in a colony in response to environmental cues, evolved just once and is a universal feature of ants. The gene network underlying wing polyphenism, however, is conserved in the winged castes of different ant species, but is interrupted at different points in the network in the wingless castes of these species. We previously constructed a mathematical model, which predicts that a key gene brinker (brk) mediates the development and evolution of these different "interruption points" in wingless castes of different ant species. According to this model, brk is upregulated throughout the vestigial wing discs of wingless ant castes to reduce growth and induce apoptosis. Here, we tested these predictions by examining the expression of brk, as well as three other genes up- and downstream of brk-decapentaplegic (dpp), spalt (sal), and engrailed (en)-in the winged reproductive and wingless soldier castes in the ant Pheidole morrisi. We show that expression of these genes is conserved in the wing disc of winged castes. Surprisingly, however, we found that brk expression is absent throughout development of the vestigial soldier forewing disc. This absence is correlated with abnormal growth of the soldier forewing disc as revealed by En expression and morphometric analyses. We also discovered that dpp and sal expression change dynamically during the transition from larval-to-prepupal development, and is spatiotemporally correlated with the induction of apoptosis in soldier forewing disc. Our results suggest that, contrary to our predictions, brk may not be a key gene in the network for suppressing wings in soldiers, and its absence may function to disrupt the normal growth of the soldier forewing disc. Furthermore, the dynamic changes in network interruptions we discovered may be important for the induction of apoptosis, and may be a general feature of gene networks that underlie polyphenism.