SummaryHow tissues acquire their characteristic shape is a fundamental unresolved question in biology. While genes have been characterized that control local mechanical forces to elongate epithelial tissues, genes controlling global forces in epithelia have yet to be identified. Here, we describe a genetic pathway that shapes appendages in Drosophila by defining the pattern of global tensile forces in the tissue. In the appendages, shape arises from tension generated by cell constriction and localized anchorage of the epithelium to the cuticle via the apical extracellular-matrix protein Dumpy (Dp). Altering Dp expression in the developing wing results in predictable changes in wing shape that can be simulated by a computational model that incorporates only tissue contraction and localized anchorage. Three other wing shape genes, narrow, tapered, and lanceolate, encode components of a pathway that modulates Dp distribution in the wing to refine the global force pattern and thus wing shape.

One of the aims of evolutionary developmental biology is to discover the developmental origins of morphological variation. The discipline has mainly focused on qualitative morphological differences (e.g., presence or absence of a structure) between species. Studies addressing subtle, quantitative variation are less common. The Drosophila wing is a model for the study of development and evolution, making it suitable to investigate the developmental mechanisms underlying the subtle quantitative morphological variation observed in nature. Previous reviews have focused on the processes involved in wing differentiation, patterning and growth. Here, we investigate what is known about how the wing achieves its final shape, and what variation in development is capable of generating the variation in wing shape observed in nature. Three major developmental stages need to be considered: larval development, pupariation, and pupal development. The major cellular processes involved in the determination of tissue size and shape are cell proliferation, cell death, oriented cell division and oriented cell intercalation. We review how variation in temporal and spatial distribution of growth and transcription factors affects these cellular mechanisms, which in turn affects wing shape. We then discuss which aspects of the wing morphological variation are predictable on the basis of these mechanisms.

Cell competition is a widespread process leading to the expansion of one cell population through the elimination and replacement of another. A large number of genetic alterations can lead to either competitive elimination of the mutated population or expansion of the mutated cells through the elimination of the neighbouring cells. Several processes have been proposed to participate in the preferential elimination of one cell population, including competition for limiting extracellular pro-survival factors, communication through direct cell-cell contact, or differential sensitivity to mechanical stress. Recent quantitative studies of cell competition have also demonstrated the strong impact of the shape of the interfaces between the two populations. Here, we discuss the direct and indirect contribution of mechanical cues to cell competition, where they act either as modulators of competitive interactions or as direct drivers of cell elimination. We first discuss how mechanics can regulate contact-dependent and diffusion-based competition by modulating the shape of the interface between the two populations. We then describe the direct contribution of mechanical stress to cell elimination and competition for space. Finally, we discuss how mechanical feedback also influences compensatory growth and triggers preferential expansion of one population.