Science and Reason: Comb jellies, again

Comb jellies were in the news back in April, and I discussed that here. They're a lot like jellyfish, only different. In particular, they are now thought to be the closest living relatives of the earliest animals on this planet.

The most recent news is that, although the bodies of these animals appear primitive, their genetic machinery has interesting resemblances to that of more modern animals, including humans.

Genes key to the development of modern animals' body plans show up in primitive-looking comb jellies (6/6/08)

No one suspected that the primitive comb jellies — watery, rotund and nearly invisible sea creatures — would rely on an intricate interplay of genes to design their rudimentary bodies. Yet researchers got a surprise when they looked at the comb jelly’s genes. Scientists found pattern-making genes that, in most animals, plot out the position of the head, brain, limbs and rear ends during development. These “homeobox” genes turned on in a specific pattern in the comb jellies, even though these ancient sea creatures are headless, brainless, limbless and rear end–less, scientists show in the June Development Genes and Evolution.

Homeobox genes are genes that have specific genetic code sequences. The sequences code for portions of proteins (called homeodomains) that are in turn able to bind to DNA, so that the proteins act as transcription factors. These transcription factors are especially important in the process of embryonic development, as they determine the overall body plan of the organism. Homeobox genes aren't unique to animals. They're also found in plants and fungi.

However, although comb jelly homeobox genes are similar to those of other animals, and they have functions in comb jellies that are vaguely similar to their functions in other animals, there are important differences too:

Certain genes expressed in the mouths of comb jellies and in the heads of other animals could indicate that the comb jellies’ mouths correspond to the front-end of all animals (except for amorphous sponges), Martindale says. And it implies that the mouth region of an ancestral headless animal is in the same area where the first head eventually arose. Although comb jellies are using the same basic toolkit as other animals, they might be doing so in an entirely different way, Martindale says. Genes involved with limb formation in other animals were expressed at seemingly random points along the comb jellies’ throat-like pharynx, for example.

“Mice, flies and even cnidarians [jellyfish and sea anenomes, mainly] seem to be built on the same basic plan, but the sponges and comb jellies don’t fall into that mold,” Martindale says.