Fig. 5.

Developmental model and hypothesis about GRN evolution. (A) Summary of expression and functional data obtained in this work. Black connections indicate genetic interactions among genes. Gray connections represent observed phenotypic effects upon manipulation of gene expression. (B) Schematic representation of functional constraints on vertical bristle development. The genotype level (i) is depicted as GRN centered on Pnr (red node in i). Pnr regulates the development of the dorsal occipital cuticle (dc), the compound eye (eye), and vertical bristles (vb) (ii). Due to a functional constraint on vertical bristle development, only the effect of differences of pnr expression on dc and eye are phenotypically relevant during the evolution of head morphology (iii). We propose that the constraint effect of pnr expression differences between D. melanogaster and D. mauritiana is mediated by coevolution of ush expression (orange node in i). (C) The ratio of pnr and ush expression is stable at 72 and 96 h AEL when vertical bristles are defined. Later expression of both genes is not constraint anymore by a function in bristle development. (D) Schematic representation of structural (i, ii) and tuning nodes (iii) in developmental GRNs.