I published a cephalic furrow thread about our paper on Twitter and Mastodon. We recently updated the manuscript on bioRxiv. In addition to the tissue mechanics, this new version includes gene expression data comparing Drosophila with Clogmia, a fly that has no cephalic furrow… It gives us some hints about the patterning changes associated with the evolution of this novel invagination.
From the labs of Pavel Tomancak and Carl Modes:
What can a fold tell us about the interplay between genetic patterning, tissue mechanics, and the evolution of morphogenesis?
2. What’s the role?
The cephalic furrow is a puzzling fold.
While its formation is under strict genetic control, the invagination is transient and doesn’t give rise to specific tissues. It simply unfolds, leaving no trace.
It has no obvious function. Or does it?
3. Instability in mutants
To investigate, we analyzed the tissue dynamics in cephalic furrow mutants.
We found that without the invagination, the head-trunk epithelium becomes unstable and buckles, forming what we call ectopic folds (see bottom embryo).
What’s causing this instability?
4. Sources of stress
We identified two potential sources of mechanical stress…
Mitotic domains (groups of dividing cells in the head) and germ band extension (tissue movement towards the head).
…and analyzed their contribution using biophysical modeling and experimental perturbations.
5. Biophysical model
The model developed by Abhijeet Krishna and Alicja Szałapak from the Carl Modes lab allowed us to simulate epithelial dynamics in different conditions, and make predictions for what’s happening in vivo.
For example, would ectopic folds appear if there was no germ band extension?
6. Germ band cauterization
To test this, Marina Cuenca mechanically blocked the germ band extension by laser cauterization in cephalic furrow mutants. No ectopic folds appear.
This indicates that mitotic domains alone cannot induce buckling, only when combined with the germ band push.
7. One double mutant
Similarly, inhibiting mitotic domains in cephalic furrow mutants (double mutant btd–stg) also abolishes ectopic folds.
This indicates that germ band extension alone cannot induce buckling, only when combined with mitotic domains.
8. Head–trunk under stress
These data suggests that, without the cephalic furrow, the simultaneous formation of mitotic domains and germ band extension generates mechanical instability.
Could the cephalic furrow be counteracting these increased compressive stresses during gastrulation?
9. Early fold prevents buckling
Our simulations point in this direction, showing that an early head invagination can effectively absorb these stresses and prevent epithelial instabilities at the head-trunk boundary.
10. A mechanical role
Putting all the data together, we propose the cephalic furrow has a function, it plays a mechanical role during Drosophila gastrulation.
This raised an intriguing evolutionary question for us: Could the cephalic furrow have evolved in response to these mechanical stresses?
11. Evolutionary novelty
We were lucky that Steffen Lemke and Yu-Chiun Wang had been working on the cephalic furrow from an evolutionary perspective because they reveal a crucial insight:
The cephalic furrow is an evolutionary novelty limited to a derived group of dipteran flies.
12. Instability impacts development
They also provide evidence that other flies experience increased head-trunk stresses, and that this instability may impact developmental robustness, supporting the idea that the cephalic furrow evolved in response to mechanical conflict. Their thread:
13. Patterning evolution
Now, we wondered.
The cephalic furrow is a patterned fold. The initiator cells driving the invagination are specified genetically by the narrow overlap between btd and eve domains.
So, which genetic changes are associated with the evolution of this novel invagination?
14. Novel head-trunk domain of btd
We compared gene expression between Drosophila and Clogmia, a fly that has no cephalic furrow.
Clogmia lacks btd expression at the head-trunk, a domain crucial for cephalic furrow formation, suggesting the appearance of this domain was a key event in cephalic furrow evolution.
15. Evolutionary scenario
Altogether, the integrative scenario we are establishing is that mechanical instability acted as a selective pressure for the evolution of the cephalic furrow, and that this occurred through the cooption of a new molecular player at the head-trunk boundary.
To us, the cephalic furrow case illustrates an example of a possibly more widespread mechanism on how mechanical forces can influence the evolution of patterned morphogenetic innovations in early development.
17. Thank yous
I’d like to thank Pavel Tomancak, Marina Cuenca, Abhijeet Krishna, Alicja Szałapak, Carl Modes for their crucial contributions, the MPI-CBG facilities for support, and the Steffen Lemke and Yu-Chiun Wang teams for this unique collective effort on such ephemeral but remarkable invagination ;)
18. Data availability
For more the details: https://www.biorxiv.org/content/10.1101/2023.03.30.534554v2
For high-res figures/videos: https://zenodo.org/record/7781916
For main data/analyses: https://zenodo.org/record/7781947
For model/simulations: https://zenodo.org/record/7784906