I received this query from a high school student:
I currently am in high school at [a good school] and I’m very interested in science. In my biology class, I was learning about how chloroplasts and mitochondria have cell membranes, the only organelles to do so in the current eukaryotic cell. I was thinking about them, and had a thought: Mitochondria and chloroplasts, two organelles that each possess a cell membrane, got into current eukaryotic cells long ago and are still here today, according to the current theory. Could there have been more organelles than just these two that originally “migrated” into our larger cells, and through natural selection and evolution over the millions of years inside the cell’s cytoplasm they got naturally selected down to these two? Would there be enough evidence to prove it today, after hundreds of millions of years?
I asked my biology teacher, and she said that didn’t know the answer, so I’d like to ask you this as well.
Here is my reply:
That’s a great question!
Scientists originally suspected that mitochondria and chloroplasts were endosymbiotic organelles 1) because they are surrounded by membranes like a cell, 2) because they undergo division that looks very much like cell division, and 3) because if a cell ever loses them they can’t be restored. When people realized that they also have their own genomes the evidence for endosymbiosis became very strong (although organellar genomes are much smaller than the genomes of free-living organisms) . However, what really convinced people that those two organelles are endosymbionts was when scientists studied the DNA in the organelles using DNA forensic techniques. The genes in the mitochondrial and chloroplast genomes were so similar to genes from purple bacteria and cyanobacteria (respectively) that pretty much everyone was convinced. Although we still use the term “endosymbiotic hypothesis,” it is really a fact that those two are endosymbiotic organelles.
Now, a really interesting question is “why are organellar genomes so small?” In fact, if you look at all the proteins in a chloroplast there are many more proteins than there are genes to make them; there are around 1000 proteins, but only about 120 protein-coding genes! That’s weird. When scientists looked around to figure out where the extra proteins came from they discovered that many of the proteins that are found in the chloroplast are encoded in genes that are located in the nuclear genome of the cell. Those proteins are synthesized in the cytosol, then transported into the chloroplast. The same thing is true of mitochondria — they use many proteins that are encoded in the nuclear genome rather than in the organellar genome. One of the the things my lab does is to study how that evolved. The answer is very long and complex, but the bottom line is that it is possible for genes to move from the organellar genome to the nuclear genome (or vice versa). Lots of the proteins in organelles are from bacterial genes that now reside in the nuclear genome. So our nuclear genomes are a mixture of genes that are originally from the nuclear lineage, and genes that come from the bacterial (organellar) lineage (scientists use a technical term from Greek mythology, “chimera” for this mixture of parts with different ancestries).
Are you with me so far?
If I understand your question correctly, you are suggesting the hypothesis that at some point in the distant evolutionary past there were endosymbiotic organelles that were subsequently lost. This *is* possible, and evidence for it could come from the presence of “foreign” genes in the nuclear genome that couldn’t be explained by having come from the mitochondrion or chloroplast. I’m not aware of any good evidence for this having happened in the evolutionary history of animals, but if you look in other lineages there are some really interesting situations.
First of all, some eukaryotes have what are called “secondary” chloroplasts. What this means is that the endosymbiont wasn’t a cyanobacterium, but rather was a eukaryote that already had chloroplasts. It is like a Russian Metruska doll, with a cell inside of a cell inside of another cell:
Because the host cell has a nuclear and mitochondrial genome, and the endosymbiont has a nuclear, mitochondrial, and chloroplast genome, that organism has FIVE genomes that all might trade genes. In fact, in some organisms (like the one shown on the far right in the drawing above) the chloroplast is known to be a secondary chloroplast, but the nuclear and mitochondrial genomes of the endosymbiont have completely disappeared. We know they used to be there because we can find genes in the nucleus that came from these different sources (again, tracing their evolutionary history with DNA forensic methods).
In my opinion that is pretty cool, but it gets better. There are some eukaryotes that don’t have any mitochondria at all (for example some amoebae that live in oxygen-free mud, and some parasites, just to name two). Scientists used to think that these were very ancient lineages that arose before the endosymbiotic origin of mitochondria, but it turns out that these organisms have genes derived from mitochondria in their nuclear genomes! The best explanation for that observation is that they used to have mitochondria, but lost them. In fact, in some of these organisms, if you look at the “mitochondrial” genes in the nuclear genome, the proteins that they make are targeted into a membrane-bound organelle called a “hydrogenosome” (these don’t occur in plant or animal cells, so you have probably never heard of them). The scientists who work on this now think that the hydrogenosome is a kind of a mitochondrion, but that has completely lost its genome and is 100% dependent on genes encoded in the nuclear genome. So it is possible for an endosymbiotic organelle to completely lose its genome and yet still function.
Taken together, all these observations make me think that it is possible that there could have been other endosymbiotic organelles that have been completely lost, but have left their footprint in the genome. I am NOT aware of any clear examples of this, but it is certainly possible.
I hope that answers your question.
I love this Little student’s story that seems more likely to mine. at that age i was also looking to do some researches on anything that helps to little kids mind to think about the largest Projects.