Jumping gene flash: horizontal transfer is a major evolution driver
Study of 759 species finds derided mechanism in fact exerts substantial influence. Stephen Fleischfresser reports.
It isn’t supposed to happen according to one of the central tenets of biology, but it does. Now, for the first time, researchers have tried to ascertain how often genes jump from one species to another and just how much of an impact this has had on evolutionary history.
The concept known as the Weismann Barrier in biology posits that genetic information only passes from sex cells (such as sperm and ova) to body cells. This means that genetic information only passes vertically from parent to offspring and genetic novelty is mostly created by recombination and mutation: the environment can’t create inheritable changes and genetic information can’t be introduced from another individual, let alone another species.
Those who held otherwise have been largely overlooked or dismissed, from the nineteenth century French proto-evolutionist Jean-Baptiste Lamarck to the contemporary Australian Neo-Lamarkian molecular immunologist Edward Steele.
Now, however, scientists are finding that genes are jumping around all over the place, in a phenomenon known as Horizontal Gene Transfer (HGT).
New research published in the journal Genome Biology has focussed on genes called retrotransposons, also known transposable elements (TEs), or, more colloquially, “jumping genes”. TEs are genes that can change position on the chromosome, and were first uncovered by the Nobel prize-winning cytogeneticist Barbara McClintock.
TEs, however, can jump a lot further and do so far more regularly than anyone imagined.
In the largest study of its kind, lead researcher David Adelson, Director of the University of Adelaide’s Bioinformatics Hub and a team of University of Adelaide scientists have sifted through the genomes of 759 species of plants, animals and fungi, tracking two jumping genes, known as L1 and BovB.
What they found is startling. The genes have jumped from species to species, even phylum to phylum, regularly throughout evolutionary history.
“Jumping genes … copy and paste themselves around genomes, and in genomes of other species,” says Adelson.
“How they do this is not yet known although insects like ticks or mosquitoes or possibly viruses may be involved – it’s still a big puzzle.”
One of the genes tracked, L1, is a TE long thought only to pass vertically from parent to offspring, but was found in abundance across animals and plants, in 74% of species studied.
Ubiquitous in so-called therian mammals – those which give birth to live young – L1 almost certainly entered the lineage in a horizontal gene transfer event not long after the group’s divergence from monotremes. (Egg-laying mammals, the platypus and echidna, from which L1 is utterly absent.)
The effect of the introduction of TEs into mammals was striking. “We think the entry of L1s into the mammalian genome was a key driver of the rapid evolution of mammals over the past 100 million years,” says Adelson.
The specific genes that jump are not so important, Adelson continues; rather “it’s the fact that they introduce themselves into other genomes and cause disruption of genes and how they are regulated.”
Despite being the largest study of HGT to date, Adelson believes they have “only begun to scratch the surface of horizontal gene transfer. There are many more species to investigate and other types of jumping genes.”