A graphic representation of the BovB element showing how it appears in species that are far apart on the evolutionary tree – for example sea urchins and elephants, cows and snakes. Credit: University of Adelaide

Far from being just the product of our parents, the University of Adelaide scientists have radically changed the genomes of today's mammals and has been an important motor of evolution for the widespread transfer of genes between species.

In the world's largest study on so-called "jumping genes," researchers have identified two specific jumping genes over 759 plant, animal, and fungal species. These jumping genes are actually small pieces of DNA that can copy through a genome and are known as transposable elements.

They have found that cross species transfers, even between plants and animals, have often occurred during evolution.

Both transposable elements that they tracked – L1 and BovB – entered mammalian DNA as foreign DNA. This is the first time anyone has shown that the L1 element, which is important in humans, has jumped between species.

"Jumping genes, properly referred to as retrotransposons, copy and encase themselves around genomes and into genomes of other species, as they do not yet know, though insects such as ticks or mosquitoes, or possibly viruses, may be involved – it's still one big puzzle, "says project leader Professor David Adelson, director of the Bioinformatics Hub of the University of Adelaide.

"This process is referred to as horizontal transfer, which is different from normal parent-child transfer and has a tremendous impact on the evolution of mammals."

For example, Professor Adelson says that 25% of the genome of cows and sheep are derived from jumping genes.

"Think of a jumping gene as a parasite," says Professor Adelson. "What's in DNA is not so important – it's the fact that they introduce themselves into other genomes and disrupt genes and how they are regulated."

Published today in the journal Genome BiologyIn collaboration with the South Australian Museum, the researchers found that horizontal gene transfer is much more widespread than previously thought.

"L1 elements were inherited only from parents to offspring," says lead author Dr. Atma Ivancevic, postdoctoral researcher at the University of Adelaide. "Most of the studies looked at just a handful of species and found no evidence of transmission, and we studied as many species as possible."

Human L1 elements have been implicated in cancer and neurological disorders. The researchers say that understanding the inheritance of this element is important for understanding the evolution of disease.

The researchers found that L1s are found in plants and animals, although they occur only sporadically in fungi. The most surprising result, however, was the absence of L1 in two major mammalian species – the Australian monotremes (platypus and echidna) – which show that the gene enters the mammalian evolutionary pathway after aberration of monotremes.

"We believe that the entry of L1s into the mammalian genome has been a key factor in the rapid evolution of mammals over the past 100 million years," says Professor Adelson.

The team also examined the transmission of BovB elements between species. BovB is a much younger jumping gene: it was first discovered in cows but has since been shown to jump between a bizarre set of animals including reptiles, elephants and marsupials. Previous research led by Professor Adelson found that ticks are the most likely mediators of cross-species BovB transfer.

The new study expanded the analysis to find that BovB has jumped even more than previously thought. BovB has transmitted at least twice between frogs and bats, and new potential vector species are bed bugs, leeches and grasshoppers.

The team believes that insect species research will help to find more evidence for cross-species transfer. They also aim to explore other jumping genes and explore the possibility of aquatic vectors, such as sea worms and nematodes.

"Although our recent work involved the analysis of genomes from more than 750 species, we have just begun to scratch the surface of horizontal gene transfer," says Professor Adelson. "There are many more species to study and other types of jumping genes."

Explore further:
Road rules for gene transfer are written in DNA

More information:
Atma M. Ivancevic et al., Horizontal transfer of BovB and L1 retrotransposons in eukaryotes, Genome Biology (2018). DOI: 10.1186 / s13059-018-1456-7

Journal Reference:
Genome Biology

Provided by:
University of Adelaide


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