I was alerted to the following research by another science forum I subscribe to: https://www.science.org/doi/10.1126/science.adk1075 It is well-known that various plants, e.g. clover, peas and beans, make use of a symbiosis with nitrogen-fixing bacteria in their root nodules. This new research has discovered a marine alga that can fix nitrogen by means of an organelle actually within its own cells. The researchers dub this the "nitroplast", by analogy with the chloroplasts that enable photosynthesis. It looks as if what has happened is that a nitrogen-fixing bacterium has first become an endosymbiont, living within the alga, and then has become incorporated into the actual machinery of the alga's own cells. This may shed some light on very early evolutionary processes by which other organelles may have arisen, by being first endosymbionts and then getting integrated into the cell. While the nitroplast still has DNA of its own, the template for some of the proteins that the former endosymbiont needs is now in the cell nucleus. When these proteins are manufactured by the cell, a label is attached to them which gets them picked up by the "nitroplast". The cycle of cellular division of the nitroplast has also become harmonised with that of the cell, so that when one divides the other one does too. A key feature of the change seems to be this progressive migration of at least parts of the genetic coding needed for replication, from the endosymbiont to the nucleus of the host cell. Amazing! I understand the prevailing explanation for the evolution of the mitochondrion is that it was a bacterium that somehow became assimilated by the cells of eukaryotes. They too still retain some of their own DNA, separate from the cell nucleus. Apparently, mitochondrial DNA resembles bacterial DNA. But this "nitroplast" seems to have evolved much more recently, as the process of integration within the cell is not so pronounced. Perhaps investigation of this will help us understand how eukaryotes acquired other organelles in the long distant past.
This may be of interest Introducing the “nitroplast” — The first nitrogen-fixing organelle Please Register or Log in to view the hidden image! Significance of the nitroplast discovery Nitroplast evolution from symbiosis to organelle more... Introducing the “nitroplast” — The first nitrogen-fixing organelle (msn.com)
While endosymbiosis is pretty clearly (in my estimation anyway) a major part of the story of the origin of eukaryotic cells, it isn't the whole story. It accounts for mitochondria, chloroplasts and some other rarer occurring organelles (the origin of the cell nucleus is still a matter of conjecture) but some of the more ubiquitous eukaryotic organelles don't seem to be explained that way. And there are many genes peculiar only to eukaryotes that don't seem to be found in any known prokaryotes at all. I think that it's clear that eukaryotes have an evolutionary origin in the preexisting prokaryotes, but I'm not convinced that eukaryote origins can be entirely explained by prokaryotes swallowing each other.
I know one thing that Eukaryotes have, that had a precursor in Prokaryotes. Origin and Evolution of the Self-Organizing Cytoskeleton in the Network of Eukaryotic Organelles Abstract more... https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4142967/
This book gives a nice treatment although many important papers on abiogenesis have been published since. since https://press.uchicago.edu/ucp/books/book/chicago/I/bo18692281.html A nice read around for a budding biology student. Mitochondria have their own genome which is a give away for that distant prokaryotic purloining. Lateral gene transfer, plasmids viroids and RNA world all part of the puzzle. I will have to read Exchemists paper in paper, there was something similar on phys.,org. I will cite it if relevant.
