DNA isn’t always passed down from parent to offspring through reproduction. In some instances, genetic material can also be passed from one individual organism to another through a process called horizontal genome transfer. If, for example, you graft two different plants together, then it’s possible for these separate lifeforms to mingle and exchange DNA.
While this process has loosely been understood for centuries, scientists have not always been certain about how the whole genome travels from another cell - until now. In a recent study, reported in the journal Science Advances last month, scientists at the Max Planck Institute of Molecular Plant Physiology in Potsdam say they’ve found a pathway through which plants can swap over their entire genomes via horizontal gene transfer.
Unusually, the process involves the cell wall undergoing changes, after which the genetic transfer occurs by cells transporting whole organelles to neighboring cells. The organelles in question are plastids, a group of small organelles that includes chloroplasts, the plant cell "engines" that are responsible for carrying out photosynthesis. A bit like mitochondria found in eukaryotic cells, chloroplasts also contain genetic material.
For this new study, the researchers managed to use live-imaging of cells to show how a gene for antibiotic resistance can be passed between two grafted tobacco plants through the cell-to-cell travel of tiny differentiated forms of plastids.
"We were able to observe that genome transfer from cell to cell occurs in both directions with high frequency at this site," Dr Alexander Hertle, first author of the study from Max Planck Institute, said in a statement.
"The cell walls formed protrusions, creating junctions between the two partners. The size of those created pores allows the migration of an entire plastid. Therefore, the genome does not migrate freely, but encapsulated from cell to cell," Dr Hertle.
The choice of an antibiotic resistance gene was no coincidence; horizontal gene transfer is thought to be one of the main drivers of antibiotic resistance.
However, this process of horizontal gene transfer doesn’t just occur between plant-and-plants or bacteria-or-bacteria. Scientists have documented a whole range of examples where there's been peer-to-peer gene swapping between distinctly different branches of life. A 2015 study found the genomes of dozens of animals (including primates, worms, and insects) all contained genes that had been picked up from bacteria and fungi by horizontal gene transfer. Stranger still, scientists have previously estimated that over 100 of our human genes were “stolen” from other organisms, such as bacteria or viruses, via horizontal gene transfer.
While there are plenty of strange examples of this process, it's uncertain how common cell-to-cell travel of whole organelles (the process seen in the new study) is in other lifeforms other than grafted tobacco plants. Nevertheless, the research does shed some light on a process that's had a profound effect on the story of life on Earth.