Different temporal requirements for tartan and wingless in the formation of contractile interfaces at compartmental boundaries

Abstract

Compartmental boundaries physically separate developing tissues into distinct regions, which is fundamental for the organization of the body plan in both insects and vertebrates. In many examples, this physical segregation is caused by a regulated increase in contractility of the actomyosin cortex at boundary cell-cell interfaces, a property important in developmental morphogenesis beyond compartmental boundary formation. We performed an unbiased screening approach to identify cell surface receptors required for actomyosin enrichment and polarisation at parasegmental boundaries (PSBs) in early Drosophila embryos, from the start of germband extension at gastrulation and throughout the germband extended stages (stages 6 to 11). First, we find that Tartan is required during germband extension for actomyosin enrichment at PSBs, confirming an earlier report. Next, by following in real time the dynamics of loss of boundary straightness in tartan mutant embryos compared to wildtype and ftz mutant embryos, we show that Tartan is required during germband extension but not beyond. We identify candidate genes that could take over from Tartan at PSBs and confirm that at germband extended stages, actomyosin enrichment at PSBs requires Wingless signalling.

Polarised cell intercalation during Drosophila axis extension is robust to an orthogonal pull by the invaginating mesoderm

“…While there is some de-repression of both sim and ind in the non-invaginated mesoderm and in the mesectoderm in twist mutants, the expression of all 3 genes is similar in the ectoderm ( S2 Fig ). We also checked the expression patterns of the Leucine-rich repeat (LRR) cell surface receptors Tolls 2, 6, 8, and Tartan, which are required for the polarised distribution of Myosin II during GBE downstream of the AP patterning genes [ 22 , 44 – 47 ]. We confirmed that these genes are expressed with the same patterns in twist mutants as in wild type ( S2 Fig ).…”

“…To compare averaged metrics between wild type and twist mutants, we synchronised the movies in time, using the start of tissue extension along the AP axis (measured as AP tissue strain rate) in each movie as time zero as before ( Methods ) ( S3A and S3B Fig ) [ 3 , 4 , 26 , 47 ]. To check that timelines of wild-type and twist mutant movies were comparable once synchronised, we checked the timings of 2 developmental events: (i) when Myosin II becomes detectable apically in the ectoderm; and (ii) when the first cell divisions occur ( Methods ).…”

“…This could be explained by the rapidly increasing Myosin II planar polarisation of the ectoderm during this period. We and others have shown that Myosin II–enriched interfaces form supracellular cable-like structures that straighten throughout the course of GBE [ 24 , 26 , 47 ]. Thus, this tissue-autonomous straightening might explain why the orientation of shortening junctions is mostly unaffected by mesoderm invagination.…”

“…In situ hybridisation chain reaction (HCR) and immunostaining methods are as described previously [ 47 ]. Stage 5 to 8 embryos were collected from twi 1 /CyO stocks.…”

Striped Expression of Leucine-Rich Repeat Proteins Coordinates Cell Intercalation and Compartment Boundary Formation in the Early Drosophila Embryo

“…In tartan mutant embryos, polarity is strongly decreased at cell-cell contacts between column 4 (Tartan+) and column 5 (Tartan−) as well as between column 8 (Tartan−) and column 1 (Tartan+), whereas polarity is not significantly affected at cell-cell contacts inside the Tartan stripe [ 39 ]. There is at least one stripe border at every position of the LRR module ( Figure 2B ), and evidence suggests that these four receptors account for the vast majority of planar polarity in the Drosophila neuroectoderm [ 39 , 42 , 43 ]. Of course, parasegments are not perfectly rectangular grids of hexagons, and this digital model of receptor expression is a necessary simplification of reality.…”