Inheritance is inescapable for us animals after all
The topic of lateral gene transfer came up on Tuesday during questions at a Miller lunch talk by Mohamed Noor about speciation in animals and the role of genetic recombination. I remember reading a few years ago that Tardigrades, cute little microscopic critters with the appropriate nickname of “water bear”, may have about 15% of their genome acquired from prokaryotic donors via lateral gene transfer (LGT).
That’s a huge percentage.
Tardigrades are weird for a lot of reasons, but this kind of genomic promiscuity is unheard of in eukaryotes. Eukaryotes are organisms whose cells have a nucleus: plants, fungi, animals–including tardigrades and you–all fall in this category. In eukaryotes, mixing up a species’ genetic make-up happens through sexual or asexual reproduction–from parent to offspring. Vertical gene transfer, if you will.
Lateral (or horizontal) gene transfer, on the other hand, plays a big role in transferring genes between single-celled bacteria (prokaryotes). They have several tools to facilitate this process. But a spate of recent studies have suggested a surprising number of cases in which prokaryotes have donated genetic material to eukaryotes.
Turns out a healthy dose of skepticism is warranted after all.
A recent review suggests that LGT transfer events are actually very rare. From the accompanying commentary on the article:
[Ku and Martin’s] idea is that if LGT from prokaryotes to eukaryotes is continuous and prevalent, traces of recent LGT must be detectable in eukaryote genomes. To assess this, they have re-analyzed their 2015 dataset made up of ~2600 phylogenetic trees encompassing 55 eukaryotes from diverse lineages and ~2000 prokaryote species. While they identify many prokaryote to prokaryote LGT candidates with high similarity between donor and receiver genes, indicative of recent transfer, they found a paucity of highly similar prokaryote to eukaryote LGT candidates.
The authors argue that studies finding such a high percentage of apparently foreign genetic material actually suffer from bacterial contamination. This had recently been found to be the case in tardigrades. Helpfully, the authors also propose a test for future studies to determine whether the contamination is an issue.
Since recent transfers are rare,
- closely related eukaryotic species should share a common history of prokaryotic gene acquisitions,
- a certain amount of change to the borrowed genes should have happened in the intervening time after a gene transfer event.
If the genetic material looks too similar to that found in the prokaryote donor, then it’s a red flag that what’s being measured is a contaminant from a prokaryote interloper.
Because these few recent candidates [of prokaryote to eukaryote LGT] are specific to one or a few species and highly similar to their prokaryotic candidate donors, they cannot easily be distinguished from bacterial contamination. […] They further propose that any protein-coding gene in a eukaryotic genome with ≥70% identity to prokaryotic homologs should be first considered as likely contamination rather than candidate LGT.
So it’s unlikely after all that we have stolen many genetic snippets from bacteria, at least not recently, and certainly at nothing like the rate they swap amongst themselves. We’re a product of the tried-and-true eukaryotic way of gene mixing: inheriting from mom and pop.