Happy to present the peer-reviewed version of our Hydractinia Cell Atlas paper out today in Nature Communications! As I presented this back when we were in “that other” social network, let me recap the basic findings and revision items in this thread: www.nature.com/articles/s41...

So, Hydractinia: Not the hermit crab, nor the dead mollusc that once occupied the shell, but the colonial cnidarian growing on top of it. Can we learn more about how this association evolved using single-cell analysis?

Hydractinia symbiolongicarpus is a colonial cnidarian. The whole mat with hundreds of polyps is only one organism. Only one larva settles in the shell, and then it grows a structure made of stolons and polyps, that is intercommunicated and shares food, cells, etc.

Coloniality is a very interesting and enigmatic process. It challenges our views of individuality. You could think of the individual polyps as clones, but they are not: they are the same organism indeed.

But: what are the cellular bases of coloniality? In other words: are the different colony parts (polyps, stolons) made of the same cell types, different cell types, or a mixture? And how did Hydractinia evolve to adapt to this niche of cohabiting mollusc shells?

We decided to explore this using single cell analysis (ACME and SPLiT-seq). Enter the Hydractinia Cell Atlas: 200K cells uncovering ~38 cell types of isolated Hydractinia body parts, including feeding polyps, sexual polyps and stolons.

To answer the question: most cell types are present in both stolons and polyps. Both have neurons, nematocytes, i-cells, etc. But in different proportions. Hydractinia uses a mix-and-match model to create different colony parts.

This is new in the revision! When two Hydractinia colonies contact each other in nature, how do they know if they are the same colony or genetically distinct organisms? It was known that they have “allorecognition genes” which dictate which colonies are histocompatible.

We now show that these genes are expressed in a subcluster of what we had called conodipine+ cells, the Allorecognition cells, which also express venom proteins. These are found in the epidermis of the polyps and in the stolons.

There are also cell types that are highly enriched in certain colony parts. For instance, cluster 11 is highly enriched in the stolons: we called it the Prisilkin-like+ cells. They express an array of Prisilkin-like or Shematrin-like genes

But what are Prisilkin-like genes? They code for highly repetitive proteins. They have a signal peptide, suggesting they are secreted. They are expressed in the stolon enriched Prisilkin-like+ cells.

Prisilkin-like genes in Hydractinia are similar to those of molluscs (and other biomineralizing organisms), which are key components of the molluscan shell. So it turns out that Hydractinia might be making shell-like material to adhere to the mollusc shell.

And indeed, when we checked the localisation of these cell types (new data!) we see that they are in the stolon but right at the base of the polyps, just where they adhere to the substrate

So, in summary, our data reveals that Hydractinia colony parts are made of similar cell types at different proportions, with some part specific cell types. These are key players in their coloniality, including allorecognition and biomineralization.

This story has been spearheaded by @drsalamander.bsky.social in our group and @hrhorkan.bsky.social from Uri Frank’s lab @thecocodium.bsky.social . Together we have generated, analysed and interpreted the data. It has taken hours of joint discussion and several visits of Helen to our laboratory!