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Last Updated: Wednesday, 21 July, 2004, 08:20 GMT 09:20 UK
Big genome can up extinction risk
Coast Horned Lizard    US Fish & Wildlife Service
The effect can be observed within families and orders of reptiles
Scientists have shown that - within groups of reptiles and birds at least - the bigger an animal's collection of DNA, the greater its risk of extinction

But the picture is more complicated in amphibians, fish and mammals.

Those are the findings of Russian biologist Alexander Vinogradov, who compared genome size and conservation status in almost 2,000 animal species.

In mammals, genome size does not affect extinction risk, Dr Vinogradov reports in Proceedings of the Royal Society B.

The question is how you interpret all these correlations. There are real things going on above the species level, but it's not clear what those things are yet
Professor Scott Edwards, Harvard University
When the raw data is analysed for fish, reptiles and amphibians, threatened species do, on average, tend to have larger genomes.

But Vinogradov broke these big groups down into families and orders to see if the relationship still held true within even smaller groupings of animals.

Genome size had no effect at either the family or order level in amphibians and fish. But it still held true at lower "taxonomic levels" in reptiles and birds.

In mammals, no significant effect was seen at either the higher or lower taxonomic levels.

'Selfish DNA'

The effect of large genomes on extinction threat was first noted in plants.

According to some scientists, this relationship fits the so-called selfish DNA hypothesis in which DNA continues to propagate despite serving no purpose other than its own "survival".

The accumulation of this "Junk DNA" is said to be maladaptive - that is to say it adversely affects the fitness of the organism. However, the new data suggests that the clear effect seen in plants does not hold true in all organisms.

WHAT'S IN A GENOME?
A genome is the name given to an organism's collection of "life code"
Genomes comprise long strands of DNA, into which are written genes
Genomes vary in size between organisms. Human have about 30,000 genes, rice contains 60,000 genes, and some bacteria contain less than 1,000 genes
Scott Edwards, professor of organismal and evolutionary biology at Harvard University, suggested the more complicated picture in animals might be caused by the influence of another variable.

"For example, frogs and salamanders are very sensitive to environmental change, maybe because of the way their absorbent skin interacts with the environment," Professor Edwards told BBC News Online.

Dr Vinogradov also proposes that for some animals, the cost of accumulating non-coding DNA in their genomes is probably balanced by other benefits. These may include having a low metabolic rate in ecosystems where energy is in short supply.

He says that having a relatively small genome may influence the strong effect recorded in reptiles and birds: "It is interesting that birds and reptiles have the smallest genomes among tetrapods, which suggests that selection against the redundant DNA is generally stronger in them."

Professor Edwards commented: "The question is how you interpret all these correlations. There are real things going on above the species level, but it's not clear what those things are yet."




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