An international team of scientists has challenged one of the key assumptions about how the body-shapes of the world’s animals evolved.
New research into the genes that define the body plans of animals both simple and complex has found that corals and their relatives (sea anemones, jellyfish and and hydras) do not use Hox genes – generally assumed to be a defining characteristic of animals – to determine body shape.
Expression of Hox-like genes in the medusa (jellyfish) stage of the hydrozoan Eleutheria. Cnox-3 is expressed around the mouth (gene expression visualised using a purple pigment)
The findings throw new light on our understanding of one of the central problems in animal biology, the question of what distinguishes “simple” animals such as corals, from “higher” animals, such as the mouse and man , says Professor David Miller of the ARC Centre of Excellence for Coral Reef Studies at James Cook University.
They are also an important clue in understanding what caused the explosion of animal life some 540 million years ago, which enabled animals to colonise virtually the entire Earth.
From the paired wings of a fruitfly to the paired hands of a human, the basic body plans for most animals are decreed by the Hox genes, clusters of special genes which instruct cells in the developing body to build the appropriate organs (wings, ribs, legs, eyes and so on) based on their position along the central body axis.
It was this ability to shape the body so it suited different environments which was the springboard for the evolution of higher animals, including humans.
“When Hox clusters were first discovered about 20 years ago, it was like finding the Rosetta Stone of developmental biology,” Prof. Miller explains.
“It looked as if they held the key to all body plans. That the Hox system defined animal biology.”
“We’ve now found that Cnidaria – the simple animals which include corals, anemones and hydras – don’t use Hox genes at all, yet manage to shape themselves perfectly well. Which suggests the Rosetta Stone is in need of a bit of editing, because the fundamental assumption simply isn’t true.”
Corals and humans share a surprisingly large number of genes, but not the Hox cluster. Corals use a system that is at once simpler and much more laborious for determining their body plan.
“The obvious inference of this is that the Hox genes evolved after the ancestor of corals split with the ancestor of all the bilateral animals.
“The corals and their relatives appear to use other kinds of genes to define relatively simple body plans, but bilateral animals inherited a kind of short-hand version of the rule book – the Hox clusters are an elegant and flexible solution to the enormously complex problem of telling cells where they are along the axis. Tinkering with the Hox system seems to have been a major factor in enabling the huge array of body shapes we see today and in the fossil record.
“Once you have a Hox cluster, you can generate a vast range of different basic body plans – all of those that we see today, and a whole swag more that are known only from fossils. It’s a major step-jump in evolution, and it probably occurred right at the base of the Cambrian, about 540 million years ago.”
“But even without one, you can still do a lot of things. The Cnidaria are still among the most species-rich and diverse of all phyla.”
Kamm, K, Schierwater, B, Jakob, W, Dellaporta, SL and Miller, DJ (2006). “Axial Patterning and Diversification in the Cnidaria Predate the Hox System.” Current Biology 16(9): 920-926.
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Professor David Miller, CoECRS and James Cook University, +61 7 4781 4473
Professor Terry Hughes, Director, CoECRS, +61 7 4781 4000
Jenny Lappin, CoECRS, +61 7 4781 4222
Jim O’Brien, James Cook University Media Office, +61 7 4781 4822