Variation in the Dorsal Gradient Distribution Is a Source for Modified Scaling of Germ Layers in Drosophila

SummarySpecification of germ layers along the dorsoventral axis by morphogenetic gradients is an ideal model to study scaling properties of gradients and cell fate changes during evolution. Classical anatomical studies in divergent insects (e.g., flies and grasshoppers) revealed that the neuroectodermal size is conserved and originates similar numbers of neuroblasts of homologous identity [1, 2 and 3]. In contrast, mesodermal domains vary significantly in closely related Drosophila species [ 4]. To further investigate the underlying mechanisms of scaling of germ layers across Drosophila species, we quantified the Dorsal (Dl)/NF-κB gradient, the main morphogenetic gradient that initiates separation of the mesoderm, neuroectoderm, and ectoderm [ 5, 6 and 7]. We discovered a variable range of Toll activation across species and found that Dl activates mesodermal genes at the same threshold levels in melanogaster sibling species. We also show that the Dl gradient distribution can be modulated by nuclear size and packing densities. We propose that variation in mesodermal size occurs at a fast evolutionary rate and is an important mechanism to define the ventral boundary of the neuroectoderm.

Graphical AbstractFigure optionsDownload high-quality image (196 K)Download as PowerPoint slideHighlights
► The germ layers of Drosophila species are unequally scaled to size during evolution
► Evolutionary changes in nuclear size and density affect the Dl gradient scaling
► Mesodermal variations rely on Dl distribution instead of regulation of target genes
► Dl gradient distortions allow new positioning of ventral neuroectodermal border