Blueprint for life not so clear cut

DNA is often referred to as a blueprint for life, or the instruction set for living organisms. We know that DNA is passed down through generations from one individual to their offspring. But did you know that DNA doesnt always follow this linear path, but instead jumps around between organisms? Professor David Adelson and his team in the School of Molecular and Biomedical Science at the University of Adelaide work on mapping the location of these genes, known as transposons, which jump between organisms to find out when and how this happens, and to determine their impact on the genome. In this post, Professor Adelson explains.

By Professor David Adelson, adapted from Jumping Genes, e-Science, October 2013

Jumping DNA For decades it was believed that all the genes that encoded an organism could only be transmitted vertically from ancestors to descendants. But the discovery of Avery, MacLeod and McCarty in 1944 showed that DNA could transfer the gene for virulence from one strain of bacteria to another, indicating that it could also be transmitted horizontally between unrelated organisms. While this research was the origin of modern molecular biology, its full significance was not understood until many years later.

And until the 1990s, horizontal transfer of genes was believed to be restricted to simple organisms such as bacteria. The first evidence that DNA could be horizontally transferred in animals came from studies of mobile genes called transposons that could move between fruit flies and even between different species of fruit flies. Transposons are perhaps uniquely suited to horizontal transfer, as they are stretches of DNA that encode a single protein whose only function is to cut them out of the region of DNA they occupy and paste them back in at a completely different position.

In the intervening years since this horizontal transfer between animals was first described transposons have been shown to jump not only between locations within a single genome, but between the genomes of a number of animals, including between insects, reptiles and mammals, even Tasmanian devils.

Retro genes While this has changed our understanding of the nature of horizontal transfer, transposons make up only a very small percentage of any species genome, so the impact of this type of transfer has not been viewed as very significant. However, genomes contain more than just genes and transposons; they also contain many repetitive sequences from retrotransposons. Retrotransposons are similar to transposons in that they can jump from location to location in a genome, but they differ in that they use a copy and paste mechanism rather than a cut and paste mechanism, so they can amplify their numbers significantly. In fact, while genes occupy about 2% of a typical mammalian genome, and transposons might occupy about 3%, retrotransposons can occupy about 40% of a genome.

Weve carried out an analysis of a particular retrotransposon called BovB in all available animal and insect genome sequence data to detect evidence of horizontal transfer for BovB. We found that BovB was only present in a number of unrelated species or groups of species, including reptiles (snakes and lizards), ruminants (cow and sheep), elephants, horses, platypus and wallaby, with no BovB in primates (humans, apes), carnivores (dogs, cats) or rodents (mice and rats), thus ruling out a common ancestor and vertical inheritance as an explanation for BovB distribution.

BovB was not just found in the species above, but was also found in two species of ticks known to feed on reptiles or mammals depending on the opportunity. The BovB sequences of cow, reptiles, marsupials and ticks are quite similar, indicating a likely horizontal transfer of BovB from reptiles to ticks to ruminants (cows). Using computational analyses we were able to determine that at least 9 horizontal transfers must have occurred to generate the current known patchy species distribution of BovB.

When is a cow not a cow? While this on its own is striking, BovB and derived sequences account for about 25% of the cow genome, implying that a horizontally transferred DNA sequence from reptiles (transferred through ticks) accounts for a quarter of the cow genome. BovB sequences are not without functions, they have been adapted to become part of genes and can regulate gene function, so the abundant BovB sequences in the cow genome are not just passive, but are integral, dynamic components that turn genes on or off and ultimately regulate what makes a cow, a cow.

Our perspective on horizontal transfer in animals has been revised by these results and it now appears that horizontal transfer is much more common that we previously expected and can have very significant effects on genome sequences.

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Blueprint for life not so clear cut