sex by conjugation, such as ciliated protozoa and mushrooms, there are many different genders: In those species with sex by fusion, there are almost invariably two genders. In one especially satisfying case there is a 'hypotrich' ciliate that can have sex in either fashion.
When it has fusion sex, it behaves as if it had two genders: When it has conjugation sex, there are many genders: In 1991, just as he was putting the finishing touches on this tidy story, Hurst came across a case that seemed to contradict it: a form of slime-mold that has thirteen genders and fusion sex: But he delved deeper and discovered that the thirteen genders were arranged in a hierarchy. Gender thirteen always contributes the organelles, whomever it mates with. Gender twelve contributes them only if it mates with gender eleven and downward. And so on: This works just as well as having two genders but is a great deal more complicated: 2'
GENETIC MUTINY AND GENDER
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SAFE SEX TIPS FOR SPERM
Along with most of the animal and plant kingdoms, we practice fusion sex and we have two genders. But it is a much modified form of fusion sex: Males do not submit their organelles to be slaughtered; they leave them behind at the border: The sperm carries just a nucleus cargo, a mitochondrial engine, and a flagellum propeller. The sperm-making cells go to great lengths to strip off the rest of the cytoplasm before the sperm is complete and redigest it at some expense. Even the propeller and engine are jettisoned when the sperm meets the egg; only the nucleus travels farther.
Hurst explains this by raising once again the matter of disease. 2$ Organelles are not the only genetic rebels inside cells; bacteria and viruses are there as well. And exactly the same logic applies to them as to organelles. When cells fuse, the rival bacteria in each engage in a struggle to the death. If a bacterium living happily inside an egg suddenly finds its patch invaded by a rival carried by a sperm, it will have to compete, and that might well mean abandoning its latency and manifesting itself as disease. There is ample evidence that diseases are reawakened by other 'rival' infections.
For example, the virus that causes AIDS, known as HIV, infects human brain cells but lies dormant there: If, however, cytomegalovirus, an entirely different kind of virus, infects a brain cell already infected with HIV, then the effect is to reawaken the HIV virus, which proliferates rapidly. This is one of the reasons HIV seems more likely to go on to cause AIDS if the infected person gets a second, complicating infection: Also, one of the features of AIDS is that all sorts of normally innocuous bacteria and viruses, such as Pneumocystis, or cytomegalovirus or herpes, which live calmly inside many of our bodies, can suddenly become virulent and aggressive during the progression of AIDS. This is partly because AIDS is a disease of the immune system, and immune sur-veillance of these diseases is therefore lifted, but it also makes evolutionary sense. If your host is going to die, you had better multiply as fast as possible. So-called opportunist infections there-
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fore hit you when you are down: Incidentally, one scientist has suggested that the cross-reactivity of the immune system (infection with one strain causes immune resistance to another strain of the same species of parasite) might be the parasite's way of slamming the door on rival members of its species once it is inside. 36
If it pays a parasite to go for broke when a rival appears, then it pays a host to prevent cross-infection with two strains of parasite. And nowhere is the risk of cross-infection greater than during sex. A sperm fusing with an egg risks bringing its cargo of bacteria and viruses as well; their arrival would awaken the egg 's own parasites and cause a battle for possession that would leave the egg sick or dead: To avoid this, therefore, the sperm tries to avoid bringing into the egg material that might harbor bacteria or viruses. It passes just the nucleus into the egg: Safe sex indeed: Proof of this theory will be hard to come by, but suggestive support comes from
DECISION TIME
Gender, then, was invented as a means of resolving the conflict between the cytoplasmic genes of the two parents. Rather than let such conflict destroy the offspring, a sensible agreement was reached: All the cytoplasmic genes would come from the mother, none from the father. Since this made the father 's gametes smaller, they could specialize in being more numerous and mobile the better to find eggs. Gender is a bureaucratic solution to an antisocial habit.
GENETIC MUTINY AND GENDER
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This explains why there are two genders, one with small gametes, the other with large ones: But it does not explain why every creature cannot have both genders on board: Why are people not hermaphrodites? Were I a plant, the question might not arise: Most plants are hermaphrodites: There is a general pattern for mobile creatures to be ' dioecious ' (with separate genders) and ses-sile creatures, such as plants and barnacles, to be hermaphroditic: This makes a sort of ecological sense. Given that pollen is lighter than seed, a flower that produces only seed can have only local offspring: One that also produces pollen can generate plants that spread far and wide: A la* of diminishing returns applies to seed but not to pollen.
But it does not explain why animals took a different route: The answer lies in those muttering organelles left behind at the gate when the sperm entered the egg: In a male any gene in an organelle is in a cul-de-sac because it will be left behind by the sperm. All of the organelles in your body and all of the genes in them came from your mother; none came from your father: This is bad news for the genes, whose life 's work, remember, is to pass into the next generation: Every man is a dead end for organelle genes.
Not surprisingly, there is a 'temptation ' for such genes to invent solutions to their difficulty (that is, those that do solve the problem spread at the expense of those that do not): The most attractive solution for an organelle gene in a hermaphrodite is to divert all of the owner 's resources into female and away from male reproduction.
This is not pure fantasy. Hermaphrodites are in a state of constant battle against rebellious organelle genes trying to destroy their male parts: Male-killer genes have been found in more than 140 species of plant: They grow flowers, but the male anthers are stunted or withered: Seed but no pollen is produced: Invariably the cause of this sterility is a gene that lies inside an organelle, not a nuclear gene: By killing the anthers, the rebellious gene diverts more of the plant' s resources into female seed, through which it can be inherited. The nucleus has no such bias toward females; indeed, if the rebels are achieving their aims in many members of the species, the nucleus would benefit greatly from being the only
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