Variation in a pleiotropic regulatory module drives evolution of head shape and eye size in Drosophila

The size and shape of an organism is tightly controlled during embryonic and postembryonic development to ensure proper functionality. However, in the light of the breath-taking diversity of body forms observed in nature, developmental processes must have evolved to allow evolutionary changes in adult morphology. Therefore, gene regulatory networks (GRNs) that orchestrate organ development are mostly constrained, but nodes and edges within such networks must change to give rise to morphological divergence. Identifying such tuning nodes remains a major challenge in evolutionary developmental biology. Here, we combined comparative transcriptomics and chromatin accessibility data to study developmental differences leading to natural variation in compound eye size and head shape in the two closely related Drosophila species D. melanogaster and D. mauritiana. We show that variation in expression of the GATA transcription factor Pannier (Pnr) is associated with extensive remodeling of the transcriptomic landscape during head development. Since U-shaped (Ush), a co-factor of Pnr, is involved in the same regulatory context, we argue that variation in expression of both factors may be a driver of divergence in head morphology. Applying functional genetics and geometric morphometrics we confirmed that manipulation of pnr expression in D. melanogaster largely phenocopies D. mauritiana dorsal head shape and ommatidia number. Therefore, we propose that the regulatory module composed of Pnr and Ush represents a tuning node within the otherwise highly conserved GRN underlying head development in Drosophila.