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Press release from Trends in Genetics

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Trends in Genetics
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Date: 24 November 2005
Embargo: 1 December 2005

Simple animals are not so simple after all

Corals and sea anemones (the flowers of the sea), long regarded as merely simple sea-dwelling animals, turn out to be more genetically complex than first realised. They have just as many genes as most mammals, including humans, and many of the genes that were thought to have been “invented” in vertebrates are actually very old and are present in these “simple” animals. This turns on its head the idea that underlying the complexity of higher animals (e.g. humans) is genetic complexity. An international group of scientists, led by Ulrich Technau and David Miller (James Cook University, Queensland), report their surprising findings in the December issue of Trends in Genetics. Their discovery is already altering the way that scientists view the evolution of animal genomes and might even shed light on how corals deal with the effects of global warming.

“There is no correlation between the complexity of an animal, in terms of numbers of different cell and tissue types, and the number of genes that are present in its genome. It appears that the numbers and diversity of genes required to build a sea anemone or coral do not differ much from the number required to build a mouse or a human,” said David Miller.

Technau and colleagues examined over 26 000 DNA sequences of transcribed (or activated) genes from the coral Acropora millepora and the sea anemone Nematostella vectensis (which belong to a group of animals called the cnidarians) and compared them with DNA from more complex animals. To their surprise, they found that the coral and sea anemone had more genes in common with complex animals than they had expected. Most of the genes needed in higher animals to instruct cells and tissues to adopt a particular fate are present also in these simple organisms. “In fact, a substantial number of genes that were previously thought to be vertebrate-specific inventions were revealed in our analysis, which shifts the origin of these genes back to at least 500 million years ago,” said Technau.

Most surprisingly, these animals not only have the major part of the gene repertoire found in higher animals, they also have genes that are typical of plants and fungi and are normally absent from animal genomes. These genes might help the cnidarians to respond to specific environmental conditions in previously unsuspected ways. For example, some cnidarians appear to use cell-cell signaling systems known only otherwise from plants (e.g. those involving abscisic acid).

They also found Notch and Hedgehog genes in coral and sea anomone. “These genes play fundamental roles in the way that cells signal to each other during development and they are associated with important diseases such as skin cancer. Finding them in these animals means that almost all of the signalling pathway types that are involved in regulating the development of mammals and other animals were in place very early in animal evolution,” said Robert Saint, Australian National University, Canberra.

“Our results emphasize the role of gene loss during animal evolution. It seems that much of the animal gene complement was in place before the cnidarians branched off from the rest of the animal kingdom more than 500 million years ago, and that the best-studied invertebrates have simply lost a large number of these genes,” added Miller.

The study of coral genes has several practical applications in addition to increasing our knowledge of the evolution of genes and genomes. The coral sequences analysed by Technau and colleagues were collected as part of a larger study attempting to establish when specific genes are expressed during coral development. “This will hopefully allow us to identify genes critical to processes such as coral settlement, calcification and the uptake and loss of symbionts,” said Eldon Ball, Australian National University, Canberra. “This knowledge may help us to ameliorate some of the adverse effects of global warming, such as the loss of vital photosynthetic symbionts (coral bleaching) in response to temperature stress,” he concluded.

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