[Albert de Roos is a cell biologist from Amsterdam, who has previously graced Telic Thoughts with this guest post about applying engineering principles to evolution. We've invited him to write about his latest article, published in the journal Artificial Life. Not because we agree with everything he has to say (nor vice versa), but because we find it to be an interesting approach, which may jolt researchers into thinking about evolution in new terms. Don't forget to check out Albert de Roos' blog]
A design hypothesis for the evolution of the nucleus
By Albert de Roos, Ph.D. Cell Biology
Recently, I published an article about the origin of the nucleus. Basically, I pose that the nucleus arose in evolution when a nucleus-like cell generated an extra plasma membrane around itself. Or in other words, when we take the current nucleus, we are looking at the direct descendent of a free living ancestor cell. Genetic material that is wrapped in a double membrane with large simple pores in them that keeps macromolecules such as DNA, RNA and proteins inside, while nutrients and waste is free to diffuse in and out.
This article is not 'just another theory' about the origin of the nucleus, but it is derived from an engineering framework named 'design by contract'. This concept is used in the development of software where components of systems communicate according to defined interfaces or contracts. As long as you don't change existing interfaces, you can extend the system. You can directly apply that to evolution: you can add new functionality, as long as you keep existing interfaces intact. The conserved mechanisms for translation, transcription, splicing etc. can be considered to reflect these constant interfaces in this engineering view. Conservation is thus regarded as an inevitable consequence of extension since any evolutionary process that would require extensive rework in critical systems would never survive because of the direct fitness costs.
Looking at the origin of the eukaryotic cell, the only scenario that would fulfil a conservation of existing interfaces for transcription and translation is when the nucleus evolved first in evolution and a plasma membrane was wrapped around it. Everything could remain intact and the new cytoplasm could work initially as a controlled microenvironment buffering external influences and eventually giving the possibility for multicellularity. In line with a model of evolution that sees evolution as an expanding set of functionalities, the cell wrapped itself with another layer of functionality. This is common practice in software engineering when legacy systems are wrapped in new technology and many nuclear systems can thus be regarded as biological legacy systems.
Two main assumptions are held in the view where the nucleus represents the ancestor cell. One is the functioning of the nucleus as an independent cell or unit, and the other is that the resulting evolutionary scenario is feasible. It is very easy to seen the nucleus as an independent unit because it contains all the DNA, transcription and translation machinery. It also can divide itself into two, for instance in closed mitosis. The syncytial blastoderm in Drosophila show that the nuclei can independently divide and later form a multicellular embryo. Also in plants, the nucleus and its surrounding cytoskeleton seems to be the basic building block.
The evolutionary scenario is also relatively easy. Starting with DNA surrounded by lipid vesicles and DNA binding proteins, a double membrane may spontaneously self-assemble around the DNA, a process which happens at every division in telophase. The generation of a surrounding plasma membrane may have occurred through the ER as an intermediate. The nuclear membrane starts to form additional stack-like extensions when certain proteins are added. If vesicles are then generated from this proto-ER/Golgi, a plasma membrane can be formed on an existing cytoskeleton, a process similar to the formation of the cellular blastoderm in Drosophila and similar to the addition of vesicle to the current plasma membrane. It is interesting to note that certain algae can regenerate their own plasma membrane. The cytoskeleton may have formed when the translation machinery moved out of the nucleus through the pores, or maybe have extended out of the nucleus through the pores.
My design approach is based on a careful study of the existing functional modules and their interaction, while paying special attention to the conserved interfaces. After such an analysis, the pieces of the evolutionary puzzle are assembled into a logical scenario. Apart from this theory on the origin of the nucleus, I have published an article on the origin of introns and have a manuscript for the origin of Life based on the same design paradigm. When this approach proves to be fruitful for evolution in general, it may be the first design hypothesis for evolution.