Almost like a ritual before preparing to serve, a tennis player picks up three balls, looks at them once and rejects one. How efficiently could one reject a bad ball out of three with just a casual glance? Yet statistically this process makes sense. If not for one’s inability to grab more than three, the player has a better chance of ending up with the best two balls if there are more balls to reject. Anyway, this act passes off as a trivial routine – too mundane to register in our minds focussed on what is going to happen when the ball is in play. But this is exactly the process that mitochondria follow at a crucial stage in our lives and this has a huge impact on who we are.
A mitochondrion is technically an organelle – a tiny organ inside a cell. One of the curious features of mitochondrion is that, like a cell, it has its own DNA. This sure sets it apart from other functional organelles inside the cell and raises its status as something that might have its own life. Indeed it is now widely recognized that mitochondria were once free living bacteria that an ancient cell had engulfed. It has been living inside and producing offspring ever since. Mitochondria multiply and so do mitochondrial DNA. Cells too multiply and divide the mitochondrial population among themselves.
So far so good. But then something happens to certain cells as a result of organisms performing sex which disturbs this usual peaceful settlement process. These ‘sex-related’ cells are called germ cells. When an organism performs the act of sex, a germ cell from outside (sperm) fuses with the resident germ cell (egg) along with its genetic content. The two DNAs recombine and become DNA of the fused cell. Now think of the mitochondria inside each of the fusing cells. There are two sets of populations – one from Mars (sperm) and the other from Venus (egg). Too many individuals to accommodate in a restricted space. Some primitive form of sex had the two parties settle the score among themselves by trying to kill each other. Things are much streamlined in higher organisms like us humans. Not sure if this was a peaceful solution but one set of mitochondria completely vanishes from the scene and the individuals belonging to the other set occupy the newly formed cell. No prize for guessing which group sacrifices itself. The intruders – the mitochondria that previously belonged to the sperm cell – of course.
The offspring of mitochondria that belong to the mother organism’s germ cell occupy the newborn fused cell which eventually develops into a new individual organism. And life goes on. In all animals and most plants and other eukaryotes, only mother’s mitochondria are passed on to the next generation. The male’s only contribution is in providing sperm for sex. Everything else is a dead end for male. So much for the patriarchal society we are proud of!
Besides nurturing the egg and, in mammals, the pre-birth baby, the female body carries out some amazing processes inside the egg cell before and after sexual fusion that are beneficial to the offspring and to the living world in general. To be precise, to the mitochondria. One such process of great significance is called mitochondrial bottleneck. Let me explain.
The single biggest threat to life is damage to its DNA. Back in the times of early Earth, ultraviolet radiation from the Sun would do most of DNA damage. These days, a slow process of damage happens in the form of mutation during the process of creating copies. A few random mutations here and there are non-life-threatening. However, in bacteria such mutations tend to accumulate in the cells and over a period of time could potentially cause extinction of the species. The phenomenon has a popular scientific name – Muller’s ratchet. Mitochondrial bottleneck during the process of fertilization is a mechanism to mitigate the risk posed by accumulating mutations. Just like that tennis player, the egg cell sorts its mitochondrial DNA in clusters and then selects only a few to be passed along while rejecting the rest. Statistically, this process leads to reduced mutations in the mitochondrial DNA that is eventually passed on to the offspring thus ensuring a safer mitochondrial genome for the next generation.
You may be thinking, but that ‘purifies’ the mitochondrial DNA and not the DNA of the cell that in reality is the blueprint of an individual. How important are mitochondria to the overall functioning of the cell and eventually to the organism is a topic in itself – too big to cover here. From the obvious respiration which provides energy, to control of almost every aspect of the cell including sex, ageing and death, mitochondria have a critical role in our well-being. And in a yet another clever design by nature, a mother will pass on more mutational load to her male offspring (that cannot pass on the deleterious mutations further) than to her female progeny. This phenomenon – often referred to as mother’s curse – has been studied in the fly Drosophila – the most common guinea pig for genetics studies (Innocenti et al., 2011) and is likely to be present in all animals. If you are a male reading this, bad luck, but it’s just nature’s design to ensure the protection of what matters most – mitochondrial DNA!
Innocenti et al. (2011) Experimental Evidence Supports a Sex-Specific Selective Sieve in Mitochondrial Genome Evolution. Science, 332, 6031.