When is a choanoflagellate not a flagellate? When it's an amoeba.

Check out our new preprint with @Choano_Lab in which we report a new, amoeboid cell phenotype in the closest living relatives of animals: https://www.biorxiv.org/content/10.1101/2020.06.26.171736v1

What induces the switch? At first, we didn't know - it was an accidental observation. After some trial and error, we found out that the signal seems to be cell confinement. Animal cells (and others) often actively react to confinement, but its role had not been studied in choanos

Once we started applying controlled confinement, we found that the switch was *fast* - only a few seconds for the cells to start extending protrusions, a minute or so until they retract their flagellum!

Importantly, the switch works both ways: it is reversed when releasing confinement.

How it this phenotype generated? Using @dsboothacosta's transfection method for choanos, we visualized the actin cytoskeleton of live amoeboid cells, and found they extend blebs and undergo retrograde F-actin flow - like amoeboid cells in animals (and some other eukaryotes).

What is the switch for? To see if it could allow an escape response, we put the cells in front of an interface between confined and non-confined space - and found that they reliably escaped, polarizing in the process!

Choanos in the wild have repeatedly been isolated from fine interstitial environments (such as silts) - suggesting confinement is a condition they experience in nature. It makes sense they'd respond to it and be able to escape overly narrow cracks. (pic https://en.wikipedia.org/wiki/Grain_size )

The evolutionary implications were potentially important - the switch between flagellate and amoeboid form parallels cell differentiation events known in many animals, not least in sponges. (from https://www.nature.com/articles/s41586-019-1290-4)

Could animal cell types have evolved from stress-induced phenotypes in single-celled ancestors? It's too early to tell, but we found an important piece of data: the amoeboid switch is very, very conserved in choanos. 6/7 species tested - widely spread across the tree - have it.

This reminds us of how much can be left to discover even in seemingly familiar organisms. Thanks to @Choano_Lab, William Roman and @DNAielleSpitzer for working on this and special thanks to @_malberto for image analysis (see automated recognition of blebs in some choanos below!)

- (oh, and) thanks as well to @diatomdeb for help with the figures - she produced some really nice cell cartoons