Adelaide scientists have published research challenging conventional thinking that DNA in complex organisms is only passed from parent to offspring.

The study by the Adelaide and Flinders Universities and the South Australian Museum suggests a quarter of a cow's genetic makeup originated in reptiles.

The peer-reviewed research has been published in the Proceedings of the National Academy of Sciences in the USA.

The head of Molecular and Biomedical Science at the University of Adelaide, Professor David Adelson, led the project and says it involved comparing dozens of DNA sequences from different species.

"There was an observation backs in the '80s that snakes and cows shared a segment of DNA that looked to be quite similar in sequence and that was found to be a repetitive sequence," he said.

"We basically went and scoured all of the databases for sequence, and did sequencing of our own across some species where there was no available information, and put all of that together."

Professor Adelson says the DNA sequence shared by cows and reptiles is known as Bov-B.

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He says it works as a retrotransposon - a genetic element with the ability to replicate itself.

"It cuts and pastes itself within the genome and amplifies itself," he said.

"The difference between a retrotransposon and a retrovirus is a retrotransposon has no way of making an infectious particle, so it's a bit of a mystery how it gets from species to another unless it's hitchhiking as part of another virus, and that's a very difficult thing to actually catch it in the act of doing."

He says the Bov-B sequence could have been carried over by ticks but the precise mechanism remains a mystery.

"We have very clear evidence ticks that feed on reptiles and mammals are very similar to each other," he said.

"It's pretty clear that nature has been shuffling bits of DNA between species for the last of couple of hundred million years at least to the extent that the cow, 25 per cent of its genome we can show that it's there only because of an element that was transmitted in, probably from a reptile.

"It's naturally genetically modified."

Paradigm shift

Professor Adelson says the research contradicts the traditional understanding of how genes are inherited.

"We tend to think of genetic material being transmitted vertically from parent to offspring or in the case of things like bacteria there can be lateral transfer," he said.

"There's potential to see there's a lot more room for horizontal transfer than we previously supposed. We already knew that viruses could do this. At some point HIV jumped from other primates into humans so it jumped species.

"But in higher organisms, vertebrates, mammals and so on we tend to believe that later or horizontal transfer of genetic material just doesn't really happen, but what we've shown is that there are DNA segments which are essentially jumping, what are called jumping genes, retrotransposons, which are able to jump between species."

Professor Adelson says his next project is to see whether DNA sequences in humans may also have originated in other species.

"Our DNA is about 50 per cent what we call repetitive," he said.

"Of that about 10 per cent is very small repetitive elements... and the rest is L1 elements which are sort of like Bov-B. They encode their own machinery like Bov-B does.

"But what we haven't seen and what no-one's really looked at yet is whether or not L1s can also jump species."