Sex creates a conflict of interests. Most organisms have elaborate defences against genetic invasion and disease, yet sex demands that the genes of two individuals mix. Why, then, isn鈥檛 the rejection of genetic material by sex cells more common? Studies of the sexual encounters of slime moulds and fungi are beginning to provide some clues. Researchers are finding that, when two sex cells fuse, the outcome may depend not just on the compatibility of the genes stowed in their nuclei, but on the behaviour of another group of organelles in cells, known as mitochondria.
Mitochondria have long been regarded as the powerhouses of the living cell. Yet although energy production is their forte, there is growing evidence that their influence in cellular affairs runs much deeper. Mitochondria contain their own sets of genes (genomes) and their own machinery for making proteins. More-over, because mitochondria lie at the hub of a wide variety of metabolic processes, any change in their behaviour can have far-reaching effects. In fungi, at least, mitochondria seem to influence not only sexual compatibility but the organism鈥檚 pattern of development.
Underlying these effects is a subtle relationship between mitochondria and the cell nucleus. The two types of organelle seem to be able to 鈥榗ommunicate鈥� through the cell鈥檚 cytoplasm. Some cells contain thousands of mitochondria, all capable of altering the chemical composition of the cytoplasm, which in turn can influence the behaviour of nuclei. Some types of cell nuclei and mitochondria seem more compatible than others. The most compelling evidence for this comes from studies of the fates of mitochondria after sexual union.
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Sex creates new opportunities, but it also brings the risk of destabilisation or infection by foreign DNA. Perhaps to resolve the conflict, many organisms limit sexual encounters to certain parts of the body-those made from specialized sex cells produced by 鈥榞erm line鈥� tissues. Even so, it is important to understand why sex does not result in a rejection response. An important clue may lie in the fact that sexual union often produces a fertilised egg containing nuclei from both parents, but mitochondria from only one parent-normally, but not inevitably, the female. This is the usual situation in multicellular plants and animals with specialised sex cells. It is also evident in some single-celled creatures.
When ciliates get together for sex, for example, only nuclei pass between the cells, not mitochondria. In the green alga Chlamydomonas, cells of opposite mating types (called plus and minus) unite for sex, but only the mitochondria of the minus type survive intact. Those of the plus type have their DNA eradicated. The slime mould Physarum polycephalum also loses one of the two sets of mitochondria that it inherits from its parents. Based on which sets survive in different crosses, the mitochondria of different mating types can be arranged in a kind of dominance hierarchy. Only in yeast do mixed populations of mitochondria persist after fertilisation. The genomes of yeast mitochondria are particularly malleable. They have the ability to fuse and exchange genes, such that the progeny of a fertilised cell may contain distinctive mitochondria carrying new combinations of genes.
Why should organisms inherit their mitochondria from only one parent? One answer could be that it is the mixing of mitochondria, rather than nuclei, that threatens the wellbeing of newly fertilised cells. Foreign mitochondria could interfere with the stable relationship between resident nuclei and mitochondria. Incoming nuclei, which are usually in a biochemically quiescent state, are probably much less disruptive than an invasion of foreign mitochondria.
The idea that a unique relationship may develop between particular types of nuclei and mitochondria has its roots in current theories on how the first complex cells evolved. These theories hold that mitochondria and similar organelles began as symbiotic microbes living inside other organisms. Since then the relationship between mitochondria and other cellular organelles, particularly cell nuclei, has become steadily more complex.
Mitochondria contain DNA but depend on genes located in the nucleus for much of their structure. There is evidence that during the course of evolution at least some mitochondrial genes have transferred to cell nuclei. The present-day mitochondria of humans, for instance, contain only 13 genes. In Oxford, Chris Lever and colleagues at the university鈥檚 Botany Department have been looking for signs of genetic flexibility in the mitochondria of certain plants. Their research shows that mitochondrial DNA can reorganise itself as organisms evolve.
Another key discovery is that nuclei contain genes that influence the ability of mitochondria to make certain proteins. This results from the fact that the genes produce scores of different regulator molecules that can act on mitochondrial enzymes. How much such interactions influence the compatibilities of different types of nuclei and mitochondria is unknown. Yet they could allow fine tuning of the relationship between a nucleus and its familiar mitochondria.
Further evidence of the special relationship has emerged from Germany, where Karl Esser and Ulrich Kuck at the Royal University, Bochum, are studying the fungus Podospora paucicete. Their research shows that when a circular piece of DNA, or plasmid, is released from the mitochondrial gene for cytochrome oxidase-a vital respiratory enzyme-the organism stops growing. Yet mitochondria are not in complete control of the process. The release is instead triggered by the action of nuclear genes controlling an enzyme called laccase. Laccase is a member of the phenoloxidase family of enzymes which some researchers think holds the key to understanding how mitochondria influence cell development. The effects of certain kinds of infectious genetic material on mitochondria can also alter the virulence of pathogenic fungi. The prime example is the so-called 鈥榙鈥� (for 鈥榙isease鈥�) factor in Ophiostoma ulmi, the fungus which causes Dutch elm disease.
Interactions between nuclei and mitochondria probably play a part in the mating of fungi, particularly among the basidiomyctes, the group that includes mushrooms and similar forms. Basidiomyctes, like most fungi, are made up of a mass, or 鈥榤ycelium鈥�, of branching filaments called hyphae. Most do not have sex organs, so mating depends on the fusion of ordinary hyphae from different mycelia. Such fusions create a unique natural laboratory in which to study the interplay of populations of nuclei and mitochondria.
Fungal hyphae come in two brands: homokaryons, which contain just one variety of nucleus, and heterokaryons, whose cells contain nuclei of two or more different genetic types. Sexual fusions can take place between two homokaryons (ho-ho fusions) or between heterokaryons and homokaryons (he-ho fusions).
The simplest case involves two homokaryons of different mating type. Once the two participating hyphae have established a link, each donates nuclei to the other. As a consequence, each former homokaryon becomes partially or wholly converted into a heterokaryon, containing two popu-lations of nuclei of different types. Research in several laboratories, including Lorna Casselton鈥檚 at Queen Mary College, London, shows that mitochondria do not mix except at the sites of fusion.
The process is a form of mating, but it is an unusual one. For instead of involving specialized sex cells, it is a mating of non-reproductive tissues, or soma. In most organisms such an encounter would lead to rejection not acceptance. Here, however, signs of rejection are limited. Where two homokaryons join, some degeneration is a common sight, but this usually does not persist once nuclei are exchanged. We think that such degeneration is caused by the mixing of mitochondria rather than by any direct effects of the nuclei. Once past the precarious fusion point, nuclei have every chance of completing their invasion. And, having gained access, they can set about establishing their own partnerships with mitochondria.
Yet not all matings between homokaryons have such a favourable outcome. If the par-ticipants are from populations that are geographically isolated, success is by no means assured for both partners. The exchange of nuclei often leads to stability on one side of the partnership, but to degeneration on the other. The underlying mechanisms for this imbalance are unknown. Nonetheless, the observation at least proves that the occupation of different cells by the same set of nuclei can have radically different results. The key to the phenomenon may well lie in the different compatibilities of various types of cell nuclei and mitochondria.
Flexible life style
The influence of mitochondria probably does not end with sexual compatibility. There is growing evidence that they can also have a profound effect on the growth and development of an organism. The clearest signs of this come from organisms whose development is shaped as much by their environment as by genetic programming, namely plants and fungi.
An unassailable truth about plants and fungi is that, given sufficient water and nutrients, they need never stop growing. Unlike animals, whose shapes and sizes are largely preordained by inheritance, they can continually adjust their pattern of growth in response to variable environmental conditions. Instead of becoming irreversibly specialized, their living tissues enjoy an open-ended, or in the language of physiology 鈥榠ndeterminate鈥�, form of development. The behaviour of their cells depends mainly on how their cytoplasm responds to environmental signals. The result is an extraordinary degree of flexibility.
鈥楧eterminate鈥� development, the usual route followed by animals, is much less flexible. Here the nucleus is much more in control of decision-making processes. Any disturbance of the relationship between the nucleus and its surrounding cytoplasm results in death or disease. Animal cells can of course respond to signals from their surroundings but development proceeds along a largely pre-set course. As a result, determinate organisms lack the versatility to change their pattern of development. Adaptation occurs only over several generations through the operation of natural selection.
In indeterminate organisms, on the other hand, versatility is the birthright of each individual. Here, part of the body continues to grow indefinitely, although cells can switch to determinate development at any stage. If the resulting determinate offshoots stay attached to the main structure (in some instances they do not), the cellular materials they contain can be redistributed when they die. Death of parts is a way of life in indeterminate organisms. In plants the switch to determinate development produces a multitude of specialized cells and organs; in fungi it results in the growth of structures bearing spores or sex cells. In animals, including humans, the converse switch-from determinate to indeterminate development-usually leads to cancers.
What influences these developmental switches? One factor, not surprising in view of its large gene bank, is the activity of the nucleus. But mitochondria may be important too. Take fungi. The production of specialized structures in these organisms requires that a new set of genes be activated deep inside the nuclei belonging to their soma. Yet the stimulus for the change is likely to be environmental. The complex mechanisms by which physical or chemical stimuli activate genes in cells are only partially understood. Enough is known, however, for us to be reasonably certain that mitochondria are involved.
What probably happens in fungi is that the environmental signal activates receptor molecules in the tips of growing hyphae. This would allow the signal to pass through the cell membrane into the cytoplasm, where it could conceivably trigger a cascade of biochemical events ultimately capable of influencing the nucleus. By analogy with the effects of hormones and other substances that activate receptors on cell surfaces, the cascade is probably triggered by the release of potent messenger molecules such as cyclic adenosine monophosphate (cyclic AMP) and by calcium. Cyclic AMP has the ability to switch on various enzymes, while calcium triggers metabolic changes when it binds to a protein called calmodulin.
The passage of an environmental signal from a cell membrane to the nucleus could be helped or hindered by a number of factors. To produce cyclic AMP, for instance, cells need an adequate supply of ATP (adenosine triphosphate), their energy currency. In the absence of sufficient ATP an environmental stimulus might fail to release the cyclic AMP needed for signal propagation. Too much calcium could also dampen the response to a signal.
So the chemical composition of the cytoplasm when the signal is received is crucial. This is where mitochondria could exert their effects. For they not only generate ATP, but can also either mop up calcium ions themselves or cause them to be stowed in other locations in the cell. In principle both effects could be important in switching the cell from an indeterminate to determinate course of development.
Sometimes the net effect of ATP is to suppress the activity of a biochemical pathway. There are signs that this too could influence development. Researchers have found, for example, that in some cells a drop in ATP production by mitochondria (sometimes seen as a result of starvation) coincides with the start, or onset, of secondary metabolism. This involves a series of specialized chemical reactions that are not directly required for growth. In some organisms the onset of secondary metabolism is accompanied by a switch in cell development.
The products of secondary metabolism include a variety of aromatic compounds synthesized by a chemical route unique to plants and microbes called the shikimic pathway. It is here that the phenoloxidase enzymes could play an important part. For their targets are aromatic compounds, which they convert into a series of highly reactive intermediates and, finally, the hydrophobic pigment melanin. In fungi, we know that the switch to determinate development coincides with the production of aromatic compounds and an upturn in phenol-oxidase activity. The complexity of the processes, however, has so far made it difficult for researchers to establish a firm causal relationship between them.
Mitochondria may also influence the form adopted by indeterminate organisms as they grow. Plants and fungal mycelia, for example, sometimes show two distinct patterns of branching. One produces dense, slowly expanding structures and the other results in a sparse, rapidly extending structure. One theory is that these patterns reflect variations in the flows of materials within and between cells, which in turn are caused by gradients of hydrostatic pressure. The steeper the gradient, the more polarised the pattern. Mitochondria could affect these gradients in two ways. First, they produce the ATP that fuels the pumping of solutes through cell membranes, a process that leads to the development of hydrostatic pressure by osmosis. Second, by influencing the onset of secondary metabolism, mitochondria could affect a cell鈥檚 production of aromatic products. Being water-repellent, these could conceivably be important in controlling hydrostatic pressure.
Researchers will undoubtedly be working out the details of the above-mentioned phenomena for some time yet. But even at this stage we can be sure about one thing: the mitochondrion can no longer be regarded simply as a source of energy. It has its own genes and it enjoys a delicately poised relationship with other organelles inside the cell. Instead of viewing the mitochondrion as controlled and enslaved by the nucleus, we need to learn to see it as an active partner in one of life鈥檚 most important interactions.
Alan Rayner lectures in the School of Biological Sciences at the University of Bath. Ian Ross lectures in the Department of Biological Sciences at the University of California, Santa Barbara.
