How the Starfish Lost Its Way

Forest and Bird, Issue 302, 1 November 2001, Page 34

How the Starfish Lost Its Way

ANN GRAEME

investigates a hox;

hree billion instructions tell the human body how to develop. 3,000,000,000! It makes the mind boggle thinking about it. Yet that’s how many pairs of links there are in the DNA code that represents the genetic make-up of humans. So you may be relieved to

hear about the master genes, the ‘hox’ genes. These organise the multi-millions into comprehensible groups. The hox (from homeotic) genes are the eight master genes which determine what each part of a developing animal ought to be. Each hox gene governs a territory, and each territory is based on body sections first delineated in the early stages of embryonic development. One hox gene oversees the genes that create the first body section, the second hox gene oversees the second section, and so on. Such a sequence lends itself to the evolution of the head, with the senses and the nervous tissues concentrated in the front segments. Then the paired limbs follow, regulated by the hox genes of the following body sections. And so a bilaterally symmetrical animal develops, with a head end and a tail end. A bit far-fetched, do you think? Just look at yourself. And the truly astounding thing is that the same eight hox genes that master-minded you, also regulate all animal life from the humble worm to the insects, fish, reptiles and mammals. This is why all we

animals share a be basic body plan. We are all running on the same genetic software! But there is an ancient group of marine animals which seems to buck this trend. They are the members of the Phylum Echinodermata. Echinoderm larvae are perfectly orthodox, being bilaterally symmetrical with a head and a tail. Their early form reveals that they are of the same lineage as the chordates, amongst whom we ultimately find our place. But from the young echinoderm grows an adult which has neither head nor tail, and whose body is organised in a circle of radial symmetry. The most ancient echinoderms grew on stalks, as modern sea lilies still do, with the mouth pointed upwards. More modern groups like the starfish, the brittle star and the sea urchin have flipped over so that the mouth faces down. Lacking a head, echinoderms lack a brain too, or rather they lack a distinct organising centre, but they get along with a network of nerves linked to a ring around oe the mouth.

To read more about echinoderms, and hox genes, try The Variety of Life: a survey and a celebration of all the creatures that have ever lived, by Colin Tudge (Oxford University Press). ‘It’s very readable — but huge and pretty dense, according to Ann Graeme.

It seems that echinoderms are swimming against the tide of animal evolution, and yet they are an ecological success. They are major players in many marine ecosystems and sometimes even the boss — remember the oa

depredations of the Crown-of-thorns starfish on the Great Barrier Reef? There are about 7000 species of echinoderms living today, and 13,000 in the fossil records stretching back more than 500 million years. But what about the hox genes? Have they abdicated their responsibilities in the echinoderms? If those eight hox genes are in charge, how did they permit a proper,

bilaterally symmetrical young echinoderm to grow up into a radially symmetrical adult starfish? The answer was discovered by the molecular biologist,

Greg Wray. In an experiment with a brittle star, he attached markers to a gene accompanying a hox gene. Then he could trace where the hox gene was being expressed in the growing brittle star. Wray found that the gene was expressed along each arm in the same way, as if each arm was a separate

animal. In other words, the arms are like five complete animals, joined at the heads. The apparently radial echinoderm is in fact made up of independent symmetries — so the hox genes are at work, and the mystery of the radial echinoderm has an elegant solution.