by Nick Lane
So where and how does selection act to ensure harmony between nuclear and mitochondrial genes? The probable answer is during the development of the female embryo, when the overwhelming majority of egg cells, or oocytes, die by apoptosis. The fittest cells apparently pass through a bottleneck, which selects for mitochondrial function. While little is known about how such a bottleneck works—and some dispute its very existence—the broad outlines conform wholly to the expectations of the dual control hypothesis. It seems that oocytes are selected on the basis of how well their mitochondria function against the nuclear background.
The mitochondrial bottleneck
The fertilized egg cell (the zygote) contains about 100 000 mitochondria, 99.99 per cent of which come from the mother. During the first two weeks of embryonic development, the zygote divides a number of times to form the embryo. Each time the mitochondria are partitioned among the daughter cells, but they don’t actively divide themselves: they remain quiescent. So for the first two weeks of pregnancy, the developing embryo has to make do with the 100 000 mitochondria it inherited from the zygote. By the time the mitochondria finally begin to divide, most cells are down to a couple of hundred each. If their function is not sufficient to support development, the embryo dies. The proportion of early miscarriages caused by energetic failures is unknown, but energetic insufficiency certainly causes many failures of chromosomes to separate properly during cell division, giving rise to anomalies in the number of chromosomes such as trisomy (three, rather than two, copies of a chromosome). Virtually all of these anomalies are incompatible with full-term development; indeed only Trisomy 21 (three copies of chromosome 21) is mild enough to deliver a live birth; and even so, babies born with this anomaly have Down syndrome.
In a female embryo, the earliest recognizable egg cells (the primordial oocytes) first appear after two to three weeks of development. Exactly how many mitochondria these cells contain is controversial, and estimates range from less than 10 to more than 200. The most authoritative survey, by Australian fertility expert Robert Jansen, is at the low end of this range. Either way, this is the start of the mitochondrial bottleneck, through which selection for the best mitochondria takes place. If we persist in clinging to the idea that all the mitochondria inherited from our mother are exactly the same, then this step might seem inexplicable, but in fact there is a surprising variety of mitochondrial sequences in different oocytes taken from the same ovary. One study by Jason Barritt and his colleagues at the St Barnabus Medical Center in New Jersey, showed that more than half the immature oocytes of a normal woman contain alterations in their mitochondrial DNA. This variation is mostly inherited, and so must also have been present in the immature ovaries of the developing female embryo. What’s more, this degree of variation is what remains after selection, so presumably the mitochondrial sequences are even more variable in the developing female embryo, where the selection takes place.
How does this selection work? The bottleneck means that there are only a few mitochondria in each cell, making it more likely that all of them will share the same mitochondrial gene sequence. Not only are there few mitochondria, but each mitochondrion has only one copy of its chromosome, rather than the usual five or six. Such restriction precludes compensation for poor function: any mitochondrial deficits are effectively paraded naked, and their inadequacies can be magnified to the point that they are detected and eliminated. The next stage is amplification—rushing out of the constraints of the bottleneck. Having established a direct match between a single clone of mitochondria and the nuclear genes, it is necessary to test how well they work together. To do so, the cells and their mitochondria must divide, and this relies on both mitochondrial and nuclear genes. The behaviour of the mitochondria is striking when examined down the electron microscope—they encircle the nucleus like a bead necklace. This remarkable configuration surely betokens some kind of dialogue between the mitochondria and the nucleus, but at present we know next to nothing about how it may work.
The replication of oocytes in the embryo over the first half of pregnancy takes their number from around 100 after 3 weeks, to 7 million after 5 months (a rise of about 218). The number of mitochondria climbs to some 10 000 per cell, or a total of about 35 billion in all the germ cells combined (a rise of 229), a massive amplification of the mitochondrial genome. Then follows some kind of selection. How this selection works is quite unknown, but by the time of birth the number of oocytes has fallen from 7 million to about 2 million, an extraordinary wastage of 5 million oocytes, or nearly three quarters of the total. The rate of loss abates after birth, but by the onset of menstruation there are only about 300 000 oocytes left; and by the age of 40, when there is a steep decline in oocyte fertility, just 25 000. After that, decline is exponential into menopause. Of the millions of oocytes in the embryo, only about 200 ovulate during a woman’s entire reproductive life. It’s hard not to believe that some form of competition is going on—that only the best cells win out to become mature oocytes.
There are indeed suggestions of purifying selection at work. I mentioned that half of all immature oocytes in the ovaries of a normal woman have errors in their mitochondrial sequence. Only a tiny fraction of these immature eggs mature, and only a few mature eggs are successfully fertilized to create an embryo. What selects for the best eggs is unknown, but the proportion of mitochondrial errors is known to fall to about 25 per cent in early embryos. Half of the mitochondrial error has been eliminated, implying that some kind of selection has taken place. Of course, most embryos also fail to mature (the great majority die in the first few weeks of pregnancy), and again the reasons are unknown. Nonetheless, it is known that the incidence of mitochondrial mutations in newborn babies is a tiny fraction of that in early embryos, implying that a purge of mitochondrial errors really has taken place. There is other indirect evidence of mitochondrial selection. For example, if the selection of oocytes acts as a proxy for natural selection in adults, avoiding all the costly investment to produce an adult, then species that invest their resources most heavily in a small number of offspring might be expected to have the best ‘filter’ for quality oocytes—they have the most to lose from getting it wrong. This does actually seem to be the case. The species that have the smallest litters also have the tightest mitochondrial bottleneck (the smallest number of mitochondria per immature oocyte), and the greatest cull of oocytes during development.
Although we don’t know how such selection acts, it is plain that failing oocytes die by apoptosis, and the mechanism certainly involves the mitochondria. It is possible to preserve an oocyte otherwise destined to die simply by injecting a few more mitochondria—this is the basis of ooplasmic transfer, the technique we mentioned on page 240. The fact that such a crude manoeuvre actually does protect against apoptosis suggests that the fate of the oocyte really does depend on energy availability; and indeed there is a general correlation between ATP levels and the potential for full-term development. If energy levels are insufficient, cytochrome c is released from the mitochondria, and the oocyte commits apoptosis.
All in all, there are many tantalizing hints that selection is taking place in oocytes for the dual control system of mitochondrial and nuclear genes, although there is as yet little direct evidence. This, truly, is twenty-first-century science. But if it is shown that oocytes are the testing ground for mitochondrial performance against the nuclear genes then this would be good evidence that two sexes exist to ensure a perfect match between the nucleus and the mitochondria. Having now selected an oocyte on the basis of its mitochondrial performance, the last thing we need is this special relationship to be messed up by a big injection of sperm mitochondria adapted to a different nuclear background.1
We have much to learn about the relationship between the mitochondrial and nuclear genes in oocytes, but we know rather more about this relationship in other, older cells. In ageing cells, mitochondrial genes accumulate new mutations and the dual genomic control begins to break down. Respiratory fu
nction declines, free-radical leakage rises, and the mitochondria begin to promote apoptosis. These microscopic changes are writ large as we age. Our energy diminishes, we become far more vulnerable to all kinds of diseases, and our organs shrink and wither. In Part 7, we’ll see that the mitochondria are central not only to the beginning of our lives, but also to their end.
PART 7
Clock of Life
Why Mitochondria Kill us in the End
Animals with a fast metabolic rate tend to age quickly and succumb to degenerative diseases such as cancer. Birds are an exception because they combine a fast metabolic rate with a long lifespan, and a low risk of disease. They achieve this by leaking fewer free radicals from their mitochondria. But why does free-radical leakage affect our vulnerability to degenerative diseases that on the face of it have little to do with mitochondria? A dynamic new picture is emerging, in which signalling between damaged mitochondria and the nucleus plays a pivotal role in the cell’s fate, and our own.
Ageing and death—mitochondria divide or die, depending on their interactions with the nucleus
The immortal elves in Tolkein’s immortal epic are as mortal as the next man. They die in droves on the battlefield. What they don’t do is age, or at least not much. Elrond, Lord of Rivendell in The Lord of the Rings, was thousands of years old, dwarfing even biblical lifespans. Tolkein described his face as ‘ageless, neither old nor young, though in it was written the memory of many things, both glad and sorrowful. His hair was dark as the shadows of twilight…’
Is this just the whimsy of an imaginative mind? Not necessarily. While ageing and the degenerative diseases it carries with it are the bane of the western world, they are not a universal currency throughout nature. Many giant trees, for example, live for thousands of years. Admittedly, trees are a long way removed from ourselves, and in any case much of the tree is just dead structural support. Better examples, far closer to home, are many birds. Parrots can live for over a hundred years, the albatross for more than a hundred and fifty. Many gulls live for seven or eight decades and show few overt signs of ageing in a way that we can recognize. A famous pair of photographs depicts the Scottish zoologist George Dunnet with a fulmar petrel that he had captured and ringed in Orkney. The first photograph shows Professor Dunnet as a handsome young man with a handsome young bird in 1952. The second was taken in 1982, and shows Dunnet with the same ringed fulmar, which he fortuitously recaptured thirty years later, again in Orkney. Dunnet is by now betraying the ravages of age, but the bird has aged not a jot, at least to the naked eye. A third photograph, which I have never managed to see, apparently pictures Dunnet with the same fulmar in 1992, just a couple of years before the death, after protracted illness, of one of them. Rest in peace, Professor Dunnet.
Yes, I hear you say, but we too may live for a hundred years or more; what is so special about a bird that does the same? The answer is that birds live far longer than they ‘ought’ to on the basis of their metabolic rate. If we lived as long as a lowly pigeon, relative to our own metabolic rate, we’d live happily, without much illness, for perhaps a few hundred years. So why not? Why not indeed! Given the political will to overcome the ethical dilemmas, there may be no biological reason why not. Over six million years of evolution, since we split off from the apes, we have already extended our own maximum lifespan by 5- or sixfold, from 20 or 30 years to about 120 years.1 As depicted in the familiar evolutionary succession, from the stooped knuckle-dragging ape to the erect homo sapiens, we have grown in weight as well as stature, and have a lower metabolic rate. These changes were wrought by natural selection—tampering with the genes—which if we were to apply them to ourselves would be called genetic modification. But even if we lack the stomach to meddle with our genes in the interests of a vainglorious immortality, still the best way to counter the desperate degenerative diseases of old age, which debilitate an ever-growing proportion of the population, is by applying the lessons of evolution in an ethically acceptable way.
I say ‘relative to metabolic rate’. Recall from Part 4 that in mammals and birds, body mass corresponds to metabolic rate: in general, the larger a species the slower its metabolic rate. For example, the cells of a rat have a metabolic rate that is seven times faster than our own. It’s no coincidence that the rat also lives for a fraction of the time. The relationship between metabolic rate and lifespan can be perceived more directly in insects such as the fruit fly Drosophila. In this case, the metabolic rate depends on the ambient temperature, and roughly doubles for every 10°C rise in temperature; and with it, their lifespan falls from a month or more to less than a couple of weeks.
Among the warm-blooded mammals, which are relatively immune to the vicissitudes of weather, there is a broad correlation between body mass, metabolic rate, and lifespan—the larger the animal, the slower the metabolic rate, and again, the longer the life. A similar relationship holds true if we plot out the birds, but now, intriguingly, there is a gap (Figure 14). On average, if a bird and a mammal are paired so their resting metabolic rate is similar—we might say their pace of life is similar—then the bird lives three or four times longer than the mammal. In some cases the discrepancy is even greater. Thus the resting metabolic rate of a pigeon and a rat are similar, yet the pigeon lives for 35 years while the rat lives barely three or four, an order of magnitude difference. We, too, live longer than we ‘should’ if our lifespan is plotted against our metabolic rate—like many birds, and indeed bats, we live three or four times longer than other mammals with similar resting metabolic rates. When I say that we could extend our lifespan to perhaps several hundred years, I am comparing us with the pigeon, which lives two or three times longer than us, relative to its own metabolic rate. Put another way, a pigeon doesn’t live so much longer than a rat because it has slowed down its pace of life. Rather, a pigeon lives ten times longer than a rat while maintaining exactly the same pace of living. There are, apparently, no strings attached.
14 Graph showing lifespan against body weight in birds and mammals. Large animals have a slower metabolic rate, and live longer. This is true of both birds and mammals, and the slope of the lines on a log–log plot is very similar. However, there is a gap between the groups: birds live three to four times as long as mammals with a similar body weight and resting metabolic rate.
An important point is that ageing is usually, but not inevitably, linked to disease. The rat suffers similar diseases of old age to us. Rats become obese, get diabetes, cancer, heart disease, blindness, arthritis, stroke, dementia, you name it; but they develop these diseases within a two or three year timeframe, and not over decades. Many birds, too, suffer from equivalent diseases, but always towards the end of their lives. There is unquestionably a link between ageing and degenerative disease, but the nature of this link remains speculative and disputed. There are few things we can say for sure. One is that the link is not chronological—it does not depend on a fixed passage of time, but is relative to the lifespan of the creature concerned. It depends on age, not time; and the rate of ageing is broadly fixed for each species. While there is plenty of variation around the average, it is still not too far from the truth to say, with the bible, that our allotted time in this world is three score years and ten. This allotted time comes from within: it is controlled by our genes in some way, even though it can be modulated to a degree by diet and general health. When asked what finding would make him question his belief in evolution, J. B. S. Haldane answered: ‘a Precambrian rabbit’. Likewise, I will cast all my views on ageing through the window when I meet a centenarian rat. A rat may one day evolve to live for a hundred years but only after changing a good many of its genes. It will no longer really be a rat.
There is a second point about the link between ageing and disease that is even more pertinent to our own suffering: degenerative disease is not an inevitable aspect of ageing. Some seabirds, for example, seem to sidestep the diseases of old age altogether, and do not age as ‘pathologically’ as ourselves. Like t
he elves they appear to live long and healthy lives, and somehow avoid many of the afflictions of old age. Exactly what they die of is not known with certainty, but it seems the incidence of crash landings rises with age; presumably, despite not succumbing to degenerative diseases, they begin to lose muscle power and coordination. There is a hint that the ‘oldest old’ among humans—those who live well past a hundred—are also less prone to degenerative diseases, and tend to die from muscle wastage rather than any specific illness.
There have been hundreds of theories of why we age. I discussed some of these in Oxygen, from a broad evolutionary point of view. Suffice to say here that many of the attributed causes of ageing fall prey to the traps of causality and circular argument. Some say, for example, that ageing is caused by a fall in the circulating levels of a hormone like growth hormone. Perhaps; but why do such hormone levels start to fall in the first place? Similarly, others hold that ageing stems from a decline in the function of our immune systems. Certainly this is a factor, but why does our immune function start to decline? One answer might be through an accumulation of wear and tear over many years, but this answer, albeit popular, will not do. Why do rats and humans accumulate wear and tear at such different rates? Could not a rat shielded from the slings and arrows of outrageous fortune live to a hundred? Absolutely not! Its rate of ageing is determined from within. We each hold within ourselves a ticking clock, and the speed at which the clock ticks is determined by our genes. In the jargon, ageing is endogenous and progressive: it comes from within, and it gets worse over time. Any explanation must account for these traits.