They discover in worms the process by which genes can be transferred from one species to another, in animals and plants

June 30 () –

Scientists have known for decades that genes can be transferred from one species to another, in both animals and plants, but the process was unknown. Now they have identified a vector for horizontal gene transfer (THG) in worms. The findings, published in the journal Science, could lead to the discovery of other TGH vectors in eukaryotes and find applications in pathogen control.

The transfer of genetic information between species, called horizontal gene transfer, is a phenomenon observed in eukaryotes, including some species of vertebrates. Researchers from the laboratory of Alejandro Burga, from the Institute for Molecular Biotechnology (IMBA) of the Austrian Academy of Sciences, have discovered one of the most sought-after vectors of horizontal gene transfer in nematodes, the ancient viral transposons called Mavericks.

According to the study, the Mavericks are responsible for a TGH event between two species of worms whose genomes are as divergent as those of humans and fish. The scientists predict that Mavericks and analogous elements could mediate TGH in a broader set of animal lineages, including vertebrates.

Fish living in the Arctic and Antarctic oceans have developed ingenious strategies to prevent their blood and tissue from freezing in the inhospitable polar waters. One such adaptive strategy is the evolution of genes that produce antifreeze proteins.

However, more than a decade ago, scientists were astonished to discover that herring and silverside –two completely different species– they have the exact same antifreeze protein encoded in their genomesindicating a gene transfer between them.

Using genetic detective work, Burga and his team demonstrated a TGH event between two reproductively isolated worm species that are as genetically different from each other as humans are from fish. And more importantly, they were able to identify the cause: a family of virus-like transposons called Mavericks.

“Mavericks were already known as a class of transposons, but our work links them to TGH for the first time.“, explains Alejandro Burga, author of the study. “We knew that TGH was produced between animal species, but we had no idea how. This is the first time that we can definitively identify a culprit,” adds Sonya Widen, a co-author of the study and a postdoctoral fellow in Burga’s lab.

When Mavericks were discovered in the mid-2000s, they were initially thought to be large transposons, selfish genetic elements that jump around and self-propagate in the genome at the expense of their host. Mavericks soon appeared in most branches of eukaryotes, including humans, thus establishing that they originated a long time ago.

Evidence soon began to appear that the Mavericks contained genes that coded for viral elements, such as a capsid and DNA polymerase. “The evolution of transposons and viruses is closely intertwined“, says Burga. However, the capsid and DNA polymerase are not enough for a transposon to jump from its host’s genome and infect the cells of a completely different host.

Now, the IMBA researchers have found the missing link: The transposons of the worm genomes have acquired the so-called fusogenic protein, a transmembrane protein that mediates the fusion of membranes between different cells.

By acquiring a fusogen, the authors hypothesize that the Mavericks worms became capable of forming virus-like particles that can fuse with the cell membranes of another organism and infect it.

“To the best of our knowledge, no fusogens have been described in Mavericks. We therefore thought that Mavericks worms might have acquired their sequence from a virus,” Widen said. Transposons and viruses can be considered as nature’s melting pot. Their union may have unpredictable repercussions.”

In the present study, the IMBA team led by Alejandro Burga and co-first authors Sonya Widen and Israel Campo Bes, a former master’s student in Burga’s lab, stumbled upon HGT “entirely by chance,” Widen acknowledges.

In fact, the team was studying the evolutionary origin of a selfish element in the nematode ‘Caenorhabditis briggsae’. Doing some detective work, they were able to trace the sequence of this selfish gene to another nematode, ‘C. plicata’, who carried an almost identical copy.

This finding is surprising because ‘C. briggsae’ and ‘C. plicata’ are two reproductively isolated species. “Their genomes are as divergent as those of humans and fish, yet both have a nearly identical gene that clearly shows features of an evolutionarily recent HGT event.says Camp Bes.

“By looking carefully at the C. plicata genome, we discovered that the ancestral sequence that gave rise to the selfish gene in C. briggsae was embedded within a Maverick in C. plicata,” Widen explains. That this newly introduced gene subsequently evolved into a new selfish gene in C. briggsae demonstrates the impact of TGH on genome evolution.”

The IMBA team then demonstrated that Mavericks are responsible for dozens of independent TGH transfers between worm species belonging to different genera and found all over the planet.

The IMBA scientists argue that the binding between transposons and viruses is a key factor in mediating TGH. Although they still find it hard to believe their success, they recognize the impact their findings could have in unraveling the mysteries of HRT.

“I was convinced that we were looking at a case of TGH when we first saw these results in the lab, but I was also sure that we would never find out how it happened. The stars did align, though,” says Burga, who also predicts that Mavericks and virus-like transposable elements could mediate TGH in vertebrates and other eukaryotes.

Finally, the team envisions possible applications both in the laboratory and in the fight against parasitic worm species.

“If Maverick-mediated TGH is shown to be broadly applicable to any nematode species, it has the potential to become an invaluable resource. –continues–. Beyond strict laboratory and research applications, such as the genetic manipulation of non-model nematodes, such a resource could allow us, in the future, to genetically modify species of parasitic nematodes that could have agricultural or medical relevance,” Burga concludes.