Blogs by Frank P Ryan
symbiosis ain't about love
12/8/2015 3:53:45 AM
a brief explanation of the difference between symbiosis in nature and love or sharing between humans.
Symbiosis, and its evolutionary implications, which is called "symbiogenesis", isn't quite the same as non-scientists tend to imagine. It isn't about being friendly, or boy and girl meet and all is hunkey-dorey from then on. It's about need and survival in the maw of nature. Only one partner needs to benefit, so it includes parasitism. Indeed many symbioses begin as parasitism. That's how the viruses in the human genome began. All symbioses fit somewhere into a spectrum from outright parasitism at one end and mutualism at the other end. Mutualism, in which two or more partners benefit from the association, is closer to what non-scientists imagine symbiosis to be. Virulent parasitism can end up as mutualism.
It's a very interesting aspect to life. You can breathe oxygen because something like two billion years ago a virulently parasitic bacterium that happened to be oxygen-breathing invaded an amoeba-like nucleated organism. The amoeba-like organism probably ingested the bacterium as food. The bacterium would very likely have killed most of the population of the amoeba, but not all. We know this because similar symbioses are being observed even today in nature. The amoeba was unable to clear the bacterium from its tissues. So they arrived at a kind of stalemate. Soon - probably within a year - they exchanged genes. A new symbiotic organism emerged - a nucleated single celled amoeba-like organism that could now breathe oxygen. The bacterium continued to reproduce within the amoebal cytoplasm through budding. That's what our mitochondria still do today. They retain some 37 bacterial genes, all linked to oxygen-breathing, while some 300 of the original bacterial genes have moved into our nuclear genome. Many of the latter are still linked to the biochemistry and physiology of the mitochondria. Understanding the evolutionary biology is key to understanding mitochondrial disease.
The inheritance of mitochondria, including diseases linked to them, is utterly different from our vertebrate type of inheritance. It is non-nuclear, doesn't follow the Mendelian laws of inheritance linked to our 46 chromosomes, and is entirely maternally derived. The latter comes about because the mother gives a living cell, the ovum, to our offspring whereas the father only gives his genome. Thus the "cytoplasmic" inheritance of all of us is maternally derived. The maternal cytoplasm includes her mitochondria. That means that part of your (and my) inheritance is identical to your mother's, grandmother's, and so on back indefinitely right up to that original amoeba-type/ bacterial genetic symbiosis two billion years ago.
If you wanted to play about with ideas, a million years ago a female human ancestor would be readily identifiable as your, or my, ancestor if we compared mitochondrial genomes. In fact the mitochondrial genomes would be very close to identical.
Now there's a thing!
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