Introduction on the evolution of the nucleus

Evolutionary questions are often left to the biologists and geologists, but are rarely considered by other scientists, such as chemists. However, studying evolution is key to understanding how present day organisms of interest function. One research area that has microbiologists, cell biologists, biochemists and evolutionary biologists equally puzzled is the evolution of the nucleus.

Why is this so important? As multicellular eukaryotes, all of our cells have a nucleus (except for red blood cells), which is the decision-making centre of the cell. Our genetic information is stored there and thus the function of all the biochemical processes that can take place are regulated from that compartment.

I have already mentioned the nucleus in a previous article, in reference to protein transport. It allows a barrier to form between transcription and translation, to ensure the proper formation of proteins and maximise efficiency within the cell. If you would like to take a look at that before continuing, click on this link.

What happens if there is no nucleus in a cell? Nothing happens per se, but it does limit the way in which the cell can function. The name for these cells without a nucleus is prokaryote. While prokaryotes can perform relatively complex functions, they cannot regulate the expression of certain genes as well as eukaryotes.

In today’s article, you will learn about the three main hypotheses of the origin of the nucleus that are still being researched. Bear in mind that science is always changing and that throughout the next years, one of these hypotheses may be proven or disproven.

Three hypothesis of nucleus evolution

Bacteria which exhibit complex membrane systems resembling nucleus

The first hypothesis was proposed by Fuerst and Webb (1991). Their evidence is based on some bacteria, like the Planctomycetes (see below), which exhibit a membrane system resembling the eukaryotic nuclear membrane. They suggest that this structure could indicate the nucleus evolved gradually from a simpler membrane system.

Evolution of nucleus resembling structure in bacteria — Electron micrograph and diagrammatic representation of nuclear envelope in bacteria https://www.nature.com/articles/nrmicro2578

The Planctomycetes belong to a recently discovered superphylum called PVC (Planctomycetes, Verrucomicrobiaand Chlamydiae). Although the group is still largely unexplored, the cells exhibit a high degree of compartmentalisation compared to our current definition of prokaryotes.

There are both arguments in favour and against this hypothesis. On the one hand, the existence of these cells may link the bacterial domain to the eukaryote domain. If this is the case, a common ancestor of the PVC superphylum had to exist as well, tracing eukaryote-like features further back in evolutionary history.

However, these cells share many features with eukaryotes, and while one would presume that strengthens this hypothesis, it actually raises the following questions: what if the PVC superphylum is not made out of bacteria? What if the differences are so radical between PVC members and other prokaryotes that they should be considered separately? What consequences would this have on the hypothesis?

There is still ongoing debate about this group of mysterious bacteria. If you would like to read more about it, look at this article Fuerst (2014).

Viral eukaryogenesis

The next hypothesis, by Villarreal and DeFilippis (2000), posited that the nucleus could have arisen from DNA viruses that were incorporated into bacteria or archaea. This shocking idea was based on the similarities between modern eukaryotic nuclei and some aspects of viruses, such as their inability to produce proteins and lipids before transcription. Additionally, both nuclei and viruses have linear chromosomes, while prokaryotes often have circular DNA. Finally, during replication, both disassemble their membrane. Viruses do this because they must hijack their host and Eukaryotes do it to carry out mitosis.

It is thought that the DNA from the prokaryote was incorporated into the viral DNA and the redundant genes were slowly inactivated and eliminated. This pattern of genetic elimination has already been observed with chloroplast DNA and host DNA in organisms who have acquired photosynthesis by endosymbiosis.

Viral eukaryogenesis
https://genomebiology.biomedcentral.com/articles/10.1186/gb-2006-7-6-110

Another controversial hypothesis, but not completely impossible. While it may sound like science fiction, we must remember that all living organisms arose from a single protocell almost 3.5 billion years ago. There must have been exchanges between this vesicle-like structure and its environment to obtain more genes, so endosymbiosis is not far-fetched at all.

Endosymbiosis between Archaea and Bacteria

The last hypothesis discussed in this article was developed by López-García and P. and D. Moreira (2006). It consists on the assumption of an endosymbiotic event between Archaea and Bacteria to produce a eukaryote. It is still unclear if a methanogenic bacteria would have engulfed an archaea or vice versa. In either case, the genetic material of the engulfing cell was lost, while that of the engulfed cell was maintained.

This would be a simple enough event to be true, but it is very hard to prove. Phylogenetic analysis is required from many strains of bacteria, and although we have the technology for it, we are outnumbered. Furthermore, it is very difficult to determine which genes are ancestral and which ones were acquired by horizontal gene transfer. It might be a few more years until we fully map the bacterial world.

Archaea have been mentioned throughout, but it is important to note that they only became a phylogenetic category in 1977, thanks to Carl Woese’s work. This renowned scientist is also credited for the RNA World Hypothesis, which will be discussed in a subsequent article.

Conclusions on evolution of the nucleus

To sum up, there are three main hypotheses on the evolution of the nucleus. The first hypothesis supports a more gradualistic approach, in which due to successive mutations and structural changes, the nucleus arose from complex inner membrane folds. The other two lean towards the idea of endosymbiosis: archaea which engulfed a bacterium (or vice versa) and viruses which were integrated as nuclei.

As I mentioned in the introduction, we should pay close attention to the evolution of life around us. We never know the hidden potential of basic science to discover techniques or drugs that can improve society in unimaginable ways unless we invest time (and money) in research. Plus, it makes for a fantastic ice breaker: “Do you know that we come from viruses?”