You can find the paper here and, if you are interested, read the thread below www.biorxiv.org/content/10.6...

While I was a post-doc with Claude Desplan, I studied how neuronal types are specified in the Drosophila visual brain through a process called temporal patterning, which is the successive expression of temporal transcription factors in neuroblasts that change their ability to generate cell types.

This is a widespread phenomenon in nervous systems, however the "actors" (the temporal transcription factors - tTFs) differ. We asked "how did these temporal programs evolve?". Konstantina Filippopoulou collected eight different insect species that span almost the entire insect phylogenetic tree.

She performed single-cell mRNA sequencing in their developing optic lobes and used trajectory inference to predict their tTFs. What we saw was pretty stunning: the insect ancestor of hemi- and holo-metabolous insects 400 million years ago already had a very complex series.

Konstantina didn't only rely on the sequencing data; she performed more than 100 HCR-FISH experiments to confirm the in silico analysis, but also to visualise them in situ in 8 different developing insect visual brains!

Of course, insects have conquered the world and adapted in very different environments; their nervous system has to change. Indeed, we found a number of species-specific modifications and we reconstructed the evolutionary history of the temporal series in these insects.

We grouped these events in four distinct categories: what I find quite remarkable is that each of these mechanisms can be expected to affect neuronal diversity to a different extent.

It could change neuronal identity fundamentally (e.g. the absence of a temporal window or the introduction of a new tTF from a different developmental context) or to a smaller extent (e.g. the substitution of a Sox TF by another Sox TF or the gain of a new tTF in an existing temporal window)

Come to think about it, this graded spectrum of effects could provide evolution with a flexible means to modulate neuronal diversity without destabilizing developmental programs. Personally, I find this hypothesis super-interesting!

Of course, these changes in temporal programs are not only "cosmetic". They translate directly into changes in neuronal identity. We looked in a bit more detail in the effects of these different changes in Musca and saw differences in neuronal type diversity.

We believe that our results reveal both a deep conservation of developmental logic, as well as a remarkable evolutionary plasticity and they establish the temporal transcription factor series as a powerful, tractable substrate for the evolution of neuronal diversity.

Here, I really want to highlight Konstantina's huge work that involved rearing and establishing protocols for 8(!) different insects in our lab (which when she joined was a collection of empty benches) - a massive achievement. I was extremely lucky to have her as my first recruitment in the lab.

I would love to thank everyone that believed in this project and, in particular, the @ijmonod.bsky.social that provided an ideal, supportive environment for this work, and the @erc.europa.eu that funded this work.