Differential targeting of the apical extracellular matrix is associated with extreme cell shapes accompanying morphological diversification in Drosophila genitalia

A major challenge in evolutionary and developmental biology is to map the genetic causes of morphological differences between species and determine how these genetic variants exert their effects on developing tissues. Classic case studies have demonstrated that morphological changes are often associated with variation in regulatory DNA that controls gene expression for transcription factors or signaling molecules, but it has proven challenging to dissect the genetic basis of interspecific differences in the size and shape of multicellular 3-dimensional organs. We used high-resolution imaging to investigate the cellular basis of morphological diversification in the posterior lobe, a genital structure specific to the Drosophila melanogaster clade that varies in size and shape between Drosophila simulans and Drosophila mauritiana, two species can generate viable hybrid offspring. By tracking the development of the pupal epithelium that forms the posterior lobe, we found that the increased area of the Drosophila simulans lobe compared to Drosophila mauritiana results from a larger number of cells committed to lobe fate, as well as a subsequent tapering process that occurs in Drosophila mauritiana. Cells in both species adopt extreme apical dimensions, with some cells Drosophila mauritiana reaching an apical surface area of 2-3 square microns. We used lectin staining to correlate some of these differences in epithelial remodeling with the apical extracellular matrix (ECM), which targets a larger number of cells in Drosophila simulans and hybrids compared with Drosophila melanogaster and has been previously shown to underlie lobe development in Drosophila melanogaster. Finally, we measured similar differences in the gene expression pattern of dumpy, an ECM component, which suggests that this diverged developmental process is at least partially controlled at the level of gene regulation. This work suggests that extracellular forces contribute to morphological diversification, specifically that changes in dumpy gene regulation may alter the number of cells committed to the posterior lobe between species.