The study was the result of more than four years of work by an interdisciplinary group made up of physicists, computational biologists, and cell biologists from Argentina, Israel, Sweden, Uruguay, and Switzerland.
In dialogue with Télam, the biochemist Pablo Aguilar, one of the directors of the study and a Conicet researcher at the Institute of Physiology, Molecular Biology and Neurosciences (Ifibyne-Conicet), explained from basic concepts of biology to the implications of the finding.
Télam: What specifically did you discover in this investigation?
Pablo Aguilar: We find in very simple and ancient organisms called archaea proteins that allow two sexual cells to fuse.
Archaea, along with bacteria, are the microorganisms that gave rise to the beginning of life on planet earth, 3.5 billion years ago. They are very simple microorganisms that we call prokaryotes because their genetic material is not in a nucleus.
Both are still found in nature. Archaea are famous for inhabiting extreme environments (deserts, salt flats, geysers) and also, like bacteria, they are in many everyday environments, including on our skin and in our intestines. Unlike bacteria, we do not know of diseases caused by archaea.
T: Why is this finding important? What was believed before?
PA: About 1,500 million years after the beginning of life (that is, 2,000 million years ago) with the prokaryotes, the eukaryotes appear, which are organisms made up of more complex cells where the genetic material is enclosed in a nucleus, and from Almost all the living beings we know come from there: dinosaurs, plants, insects, parasites, fungi and animals.
With eukaryotes sexual reproduction also appears, this is a type of reproduction in which a new organism is generated from the fusion of two cells that we call gametes (such as the egg and the sperm in the case of mammals). Bacteria and archaea don’t do that.
Now, for two gametes to fuse, there are proteins that are essential. In plants, invertebrate animals and many parasites we call these proteins “fusexins” and we think that they appeared (like sex) together with the first eukaryotes, that is, 2,000 million years ago.
Interestingly, there are also fusexins in some viruses (such as Dengue or Zika). Viruses appeared as long as cells existed and do not reproduce by themselves, but to do so they need a host cell. So viruses like Dengue or Zika use fusexins to fuse with cells, enter them and use them to multiply.
So, this “double belonging” of the fusexins, being both in viruses and in gametes, led us to a “chicken or the egg” type of question: what appeared first, a viral fusexin or a eukaryote?
T: And your finding adds a new variable to that question…
PA: Exactly. The answer is ‘neither one nor the other’ because we found fusexins in archaea. So, are these proteins older than eukaryotes? Are they 2 billion and not 3.5 billion years old? what do fusexins do in organisms that do not reproduce sexually? Is a protein essential for sexual reproduction older than sex itself?
It seems paradoxical but it ceases to be so if one thinks that these organisms, the archaea, may have some kind of primitive sexuality, prior to sex.
T: What is the importance of this discovery?
PA: On the one hand, new theories appear that help us understand the origin of sexual reproduction and of eukaryotic organisms themselves. Does the sex of the eukaryotes come from a primitive sex of the archaea? Did archaeal fusexins help transform an archaeal cell into a eukaryotic cell?
On the other hand, we believe that today archaeal fusexins, fusing cells, allow gene exchange. If this is so, then it is a way, hitherto unknown, to transfer genes between cells.
So far we know three mechanisms that bacteria and archaea use to transfer genes: transformation (which consists of introducing free DNA that is in the environment, coming from cells that break or release it), conjugation (which is a transfer mechanism by a ‘syringe’ that goes from one microorganism to another through which the DNA passes), and the third is transduction and is the passage of DNA between cells through a virus that acts as an intermediary. If the fusexins fuse the cells, it would be a fourth way.
From this we can think if this mechanism also exists in bacteria and perhaps relate it to a very urgent health problem: bacteria resistant to antibiotics.
In this context, a bacterium that receives more and more antibiotic resistance genes from its neighbors becomes a “super bacterium” because there is no antibiotic to give it. If bacteria have fusexins (we don’t know yet) or a type of “primitive sexuality” similar to what we think archaea have, then it will be interesting to know if this can also generate “super bacteria”.
These will be the next investigations that we will have to face.