Diversity of epithelial morphogenesis during eggshell formation in drosophilids

Abstract

The eggshells of drosophilid species provide a powerful model for studying the origins of morphological diversity. The dorsal appendages, or respiratory filaments, of these eggshells display a remarkable interspecies variation in number and shape, and the epithelial patterning underlying the formation of these structures is an area of active research. To extend the analysis of dorsal appendage formation to include morphogenesis, we developed an improved 3D image reconstruction approach. This approach revealed considerable interspecies variation in the cell shape changes and neighbor exchanges underlying appendage formation. Specifically, although the appendage floor in Drosophila melanogaster is formed through spatially ordered neighbor exchanges, the same structure in Scaptodrosophila pattersoni is formed through extreme changes in cell shape, whereas Drosophila funebris appears to display a combination of both cellular mechanisms. Furthermore, localization patterns of Par3/Bazooka suggest a self-organized, cell polaritybased origin for the variability of appendage number in S. pattersoni. Our results suggest that species deploy different combinations of apically and basally driven mechanisms to convert a two-dimensional primordium into a three-dimensional structure, and provide new directions for exploring the molecular origins of interspecies morphological variation.

Epithelial Patterning, Morphogenesis, and Evolution: Drosophila Eggshell as a Model

“…Likewise, an examination of apical cell shapes in Scaptodrosophila pattersoni , which produces a large and variable number of dorsal appendages from a single appendage primordium (Figure 4B), reveals that this species generates appendages without the floor-floor zippering seen in D. melanogaster (Osterfield et al, 2015). Instead, the floor cells undergo an extreme cell-shape change involving the lengthening of alternate floor-floor edges (Figure 4C).…”

“…Defects in the subcellular localization of integrins in Dynamin mutants, combined with the observation that both knockdown and overexpression of integrins result in shorter dorsal appendages, suggest that Dynamin-mediated integrin turnover on the basal surface is involved in tube elongation (Peters and Berg, 2016b). Open questions include whether basal crawling plays any role in specifying appendage shape, and whether it might even drive appendage formation in the absence of patterned apical tension, as suggested by work in other species (Osterfield et al, 2015). It is worth noting that the elongation of epithelial tubes via cell crawling occurs in other systems, including Drosophila trachea and mouse mammary ducts (Andrew and Ewald, 2010).…”

“…Roof-cell behavior, including both apical constriction and convergent extension, is similar to that seen in other species, but it is not obvious whether these processes would be sufficient to drive formation of multiple, regularly spaced tubes. One proposed source for the physical force needed to drive appendage formation in this species is basal crawling; in other words, the basal pulling thought to drive appendage elongation in D. melanogaster might be used earlier in S. pattersoni for appendage formation (Osterfield et al, 2015). …”

“…E) Schematic of an eggshell (lateral view), color-coded to show which eggshell structures are formed by which follicle-cell type in (D). Figures C-E are adapted from (Osterfield et al, 2015). …”

Gene regulation during Drosophila eggshell patterning

“…In particular, the expression of brL should be expanded in the lateral direction, to generate the "handle" characteristic of the wild-type pattern of BR in D. virilis. This handle is responsible for defining the anteriormost pair of appendages (39,44).…”