This marine Hydra is little more than a two-layered cylinder, a tenth the size of a household tap. In evolutionary terms it sits near the base of the animal tree of life, and last shared a common ancestor with humans over 600 million years ago. And yet humans and Hydra have much in common. All animals, regardless of size and complexity, must challenge invading microorganisms and decide whether they are ‘friend’ or ‘foe’. Most, if not all, use proteins called Toll-like receptors to do the job. Toll-like receptors have many functions, but importantly they act as ‘first responders’ that recognise molecular patterns on invading microbes and can turn on a cascading immune response. Here, scientists introduced a green fluorescent protein into the Hydra (right-hand image) and turned off a protein in the immune cascade, helping to confirm that some of Earth’s very first animals used Toll-like receptors to sense bacteria.
Written by Caroline Cross
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