Hydra vulgaris is a member of the phylum Cnidaria. It appears to be a relatively simple animal and has a small number of cell types (you can read more about its basic biology here). Yet, as we have seen, it turns out that cnidarians actually possess a rather complex genetic tool kit.

We have also seen that receptor tyrosine kinases (RTK) would play important roles in facilitating the evolution of multicellular life. Added to this is the recent discovery that one example of an RTK, the insulin receptor, plays an important role, along with its ligand insulin, in the development of the nervous system.

So let’s begin the process of tying this together.

Could it be possible that the protein hormone insulin, that is spread throughout the body of mammals via the circulatory system, would actually play a role in the development or life of Hydra? In 1996, Steele et al. (1) identified a gene for a receptor tyrosine kinase that was very similar to the insulin receptor in mammals, called HTK7. They found that is was expressed in ectodermal cells (the cell type that can generate nervous tissue) at both ends of Hydra’s tube.

But what is most striking of all is that they found insulin, obtained from cows, had the ability to induce both DNA replication and cell division in Hydra’s ectodermal cells.

Okay, from the perspective, there is nothing all that surprising about finding insulin receptors, and responsiveness to mammalian insulin, in cnidarians. This is just another example of deep homology that is consistent with such a system being in place with the last common ancestor of all animals.

What’s more interesting this time around is that we are talking about a hormone and its receptor. Here, the function is simple - BIND. What makes this interesting is that cows and Hydra last shared a common ancestor at least 600 million years ago. This in turn means there are 1.2 billion years of evolution that separate the Hydra insulin receptor and the bovine insulin.

Each lineage would possess an independent history of mutations in the receptor followed by secondary, suppressor mutations in the ligand. Each lineage would possess an independent history of mutations in the ligand followed by secondary, suppressor mutations in the receptor. Yet despite two separate spans of co-evolution between receptor and ligand, the ligand from cows retains the ability to function with the receptor from Hydra.

All that this indicates a fairly strong selective constraint on a seemingly simply biochemical function (BIND). So where do we go from here?

1. R. E. Steele, Pauline Lieu, Ninh H. Mai, M. Andrew Shenk and Michael P. Sarras Jr. 1996. Response to insulin and the expression pattern of a gene encoding an insulin receptor homologue suggest a role for an insulin-like molecule in regulating growth and patterning in Hydra Development Genes and Evolution 206:247-259