Specific gene-expression patterns define morphogenetic movements during development. Emmanuel Farge now reports, in Current Biology, that the reverse is also true — gene expression can be modulated by the mechanical strains of morphogenetic movements.

To study the effects of mechanical stress on gene regulation, Farge used micromanipulation to induce a unilateral 10% deformation for 5 minutes in the early Drosophila embryo and then tested the expression of early patterning genes. Expression of the dorsal–ventral gene Twist was induced around the dorsal–ventral axis, which resulted in the ventralization of the embryo.

Farge then showed that mechanical deformation caused the translocation of Armadillo to the nucleus. This dual-function protein — a transcription factor and a component of the cadherin adhesion complex at the plasma membrane — was a possible candidate for mediating Twist expression. Indeed, the expression of a dominant-negative form of dTCF/Pangolin (a co-factor that is necessary for Armadillo-dependent transcription) prevented expression of Twist in response to the shape change. So, Armadillo somehow mediates the mechanical induction of Twist expression.

When cells of the stomodeal primordium are compressed during the first phase of germ-band extension, which leads to anterior-gut formation, Twist expression increases eightfold. To test whether this effect is the result of mechanical strain caused by germ-band extension, Farge analysed mutant embryos that did not undergo this morphogenetic process. The level of Twist expression in the mutants was similar to that before germ-band extension, but manual mechanical compression could rescue Twist expression in stomodeal cells.

Mutant embryos also had no Armadillo in the nucleus, but, again, this could be rescued by applying mechanical pressure to the stomodeal cells. So, the expression of Twist depends on the mechanically induced nuclear translocation of Armadillo.

The dorsal epithelium is normally subjected to pressure that is exerted by the extending germ band at the posterior pole and is transmitted to the anterior pole, thereby compressing the stomodeal cells against the invaginations of the mesoderm and foregut. Photoablation of the dorsal epithelium mechanically disconnects this tissue from the posterior pole and releases the compression of stomodeal cells, which results in a lack of Twist induction. This finding confirms that Twist expression is mechanically induced by stomodeal-cell compression as a result of germ-band extension.

Farge concludes that although the mechanism that triggers nuclear localization of Armadillo is not known, the fact that its homologue β-catenin translocates into the nuclei at the dorsal pole of early frog and fish embryos indicates that “...mechanical compression may reactivate a conserved and ancient pathway for dorsal-ventral axis formation” in the Drosophila embryo.