Patterned embryonic invagination evolved in response to mechanical instability

Abstract

The cephalic furrow is a deep epithelial fold that demarcates the head–trunk boundary of fly embryos during gastrulation. It forms under strict genetic control using active cellular mechanisms and follows an invariant morphogenetic sequence. But unlike other embryonic invaginations, the cells that invaginate in the cephalic furrow do not give rise to any precursor tissues or larval traits. The cephalic furrow is transient and unfolds leaving no trace. For these reasons, its function during development has remained elusive. Here we show that the cephalic furrow plays a mechanical role duringDrosophilagastrulation. By live-imaging mutant embryos, we find that without the cephalic furrow, ectopic folds appear around the head–trunk interface indicating that the epithelial stability has been compromised. Usingin vivoperturbations andin silicosimulations, we demonstrate that ectopic folding in cephalic furrow mutants occurs due to the concomitant formation of mitotic domains and extension of the germ band. These events increase the tissue strain in the head–trunk interface giving rise to mechanical instabilities. Further, we show by simulations that an early pre-patterned invagination can effectively prevent the build-up of compressive stresses by retaining epithelial tissue out-of-plane before other morphogenetic movements take place. Our findings suggest the cephalic furrow absorbs compressive stresses at the head–trunk boundary during fly gastrulation, and raise the hypothesis that mechanical forces may have played a role in the evolution of the cephalic furrow.

Divergent evolutionary strategies preempt tissue collision in fly gastrulation

“…2e, f, i; Fig. 2 Movie S2), confirming previous reports 9,10 and the independent study by Vellutini et al 18 , thus validating the use of eve or btd mutants, or global RNAi knockdown (see below), in generating the head-trunk buckles.…”

“…2 Movies S4, S5), while class II shows complete lack of buckling ( stg(II) ; Fig. 2k-n), similar to the phenotype reported in the independent study by Vellutini et al 18 . These data thus support the hypothesis that head expansion contributes to buckling, presumably by driving the posterior-ward flow of the head that collides with the trunk.…”

“…1f; Fig. 1 Supplement 1), and, as demonstrated in an independent study by Vellutini et al (footnote 1) , shows no overlapping expression between btd and eve 18 . These results suggest that the evolutionary origin of the CF is associated with a change of the expression domain of btd and the gain of its overlap with the first stripe of eve (called eve1 hereafter) in the stem group of Cyclorrhapha.…”

“…Given the pliability of invagination depth and persistence, we conclude that the CF has the capacity to ‘absorb’ the tissue surfaces of the expanding neighbors, acting as a bona fide , genetically patterned mechanical sink that guides the tissue flow, thereby preemptively preventing tissue collision and buckling at the head-trunk interface. This interpretation is further corroborated by the in silico simulation performed in the independent study by Vellutini et al 18 .…”

“…It would thus be worthwhile employing methods established in our work and the independent study by Vellutini et al . 18 to further explore the possibility that the CF carries out additional functions in Drosophila , Megaselia , and potentially other species representing the basal branches of Cyclorrhapha.…”