Immigration is much in the news these days. But, go back to history, and we find that early humans started migrating ‘Out of Africa’ since about 3 million years ago. As territories, communities and nations became established, movement from a ‘foreign’ place or group into such ‘nations’ became the basis of accepting or denying entry. This depended on whether the migrants added ‘value’ to the locals or otherwise.
In biology, this process has been on even at the single-cellular levels, over 2.5 to 3 billion years ago — and continues even today. Leave alone infection by pathogens; there have been helpful ones too. Two outstanding examples of helpful immigration that happened during those early years are chloroplasts and mitochondria. The chloroplasts are neatly packaged mini-cells which come with their own genetic make- up, and they have the ability to absorb sunlight and use it to convert atmospheric carbon dioxide and water to produce the sugar glucose and the gas oxygen. They appear to have arisen from even more ancient cells called ‘cyanobacteria’ (3.5 billion years ago), and have migrated from there to plant cells. This immigration led to what is called the ‘oxygen revolution’, through which the air surrounding the earth became over 20% rich in oxygen ( pranavayu - a gas without which we cannot live).
Powerhouses and solar panels
At about the same time, or a bit later, another ancient life form, derived from ‘the purple bacterium’, migrated to both plant and animal cells. This is the mitochondrion. Mitochondria do the reverse; they use oxygen and enhance the metabolic energy production of their ‘host’ cells by as much as tenfold. (For example, when you exercise rapidly and are short of breath, each molecule of glucose in your cells generates three molecules of lactic acid, and produce three units of energy in the process. But when you now take a deep breath and inhale oxygen, the immigrant mitochondria in your body cells break down the accumulated lactic acid to produce carbon dioxide, water and 30 units of energy). Mitochondria are thus power houses in cells, as chloroplasts are solar panels of energy in plants.
Cellular immigrants such as these two are welcome in cells and have been given permanent residence permits therein. But they bring their own genomes through which they produce progeny, and live in ghettos called organelles in the cells, offering power and prosperity to their hosts. All animals, plants and fungi have accommodated mitochondria in their cells. The number of mitochondria in a cell varies depending on the role of the cell. Muscle cells, which have high energy needs have large numbers of mitochondria in them, while red blood cells whose job is just to transport oxygen have none.
Given all this importance of mitochondria, it comes as a surprise to learn that we humans inherit our mitochondria only from the mother and none at all from the father. In other words, it is the mother who provides her progeny the Power-Pack that her children’s body cells need. So it is in plants too; it is the female that provides the chloroplasts. This too is a process that has been conserved evolutionarily from worms, fruit flies, animals and humans, and is referred to as ‘uniparental inheritance’.
But how and why does this happen? After all the egg cell is fertilized by the sperm cell, and both of them carry their own mitochondria. And as the sperm cell enters the egg cell, its mitochondria are eliminated, and why? This is a puzzle that has bothered scientists, and several suggestions have come about recently. Some have proposed that mitochondrial DNA is inherently more prone to damage than nuclear DNA, and that if the introduced mitochondria are avoided or deleted, one can make do with the maternal mitochondria, which can be multiplied as the embryo forms and develops. Dr. William Bridy of Ohio State University, USA, who has long studied this problem, suggests that such uniparental inheritance of mitochondria (and chloroplasts too) reduces the spread of parasites that lurk around in the cytoplasm, and also errors through what is termed as ‘selfish’ DNA (keep on making more copies of its segments, see his review in PNAS 92, 11331-38, 1995). Likewise, Dr. J.M. Cummins of Murdoch University in Australia suggests that doing away with the mitochondrial DNA contained in the sperm helps in preventing the inheritance of damaged or mutated DNA, occurring due to free-radical based damage (Hum. Reprod. 2000; July 15, Suppl. 2:92-101). And the review by Greiner et al. ( Bioessays 2015. 37(1) p.80-94) posits that such removal of sperm-based DNA helps in avoiding competition between organelles, and also in avoiding negative interactions between the organelle and nuclear genomes).
The most recent paper by Dr. Ding Xue of the University of Colorado, USA and colleagues ( Science 2016 July 22; 353 (6297) p.394-399) uses the transparent worm C. elegans and shows that the paternal mitochondria rapidly lose their inner membrane when entering the egg cell, releasing an enzyme that chops up the DNA therein, and which also aids in the maternal garbage disposal (called autophagy and proteasome) machinery. The last word is not said yet, but it does appear that the mother cell decides that it is best to make do with what it has, and not seek the aid of damage-prone external mitochondria for the job ahead. Mother knows best!
dbala@lvpei.org
