The Red Queen_ Sex and the Evolution of Human Nature - Matt Ridley [50]
As the prince soon discovers, even this severe sentence is insufficient to suppress the quarrel. Had he followed the example of the nuclear genes, he would have killed all the Montagues. The nuclear genes of both father and mother between them arrange that the organelles of the male are slaughtered. It is an advantage (to the male nucleus, not the male organelles) to be of the type that allows its organelles to be killed, so that a viable offspring results. So owners of docile, suicidal organelles (in the minus gender) would proliferate. Soon, any deviation from a ratio of fifty-fifty killers and victims would benefit the rarer type and cause the ratio to correct itself. Two genders have been invented, killer which provides the organelles, and victim which does not.
Laurence Hurst of Oxford uses these arguments to predict that two genders are a consequence of sex-by-fusion. That is, where sex consists of the fusing of two cells, as in Chlamydomonas and most animals and plants, you find two genders. Where it consists of conjugation – the formation of a pipe between the two cells and the transfer of a nucleus of genes down the pipe – and there is no fusion of cells, then there is no conflict and no need for killer and victim genders. Sure enough, in those species with sex-by-conjugation, such as ciliated protozoa and mushrooms, there are many tens of 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 to this tidy story, Hurst came across a case that seemed to contradict it: a form of slime mould that has 13 genders and fusion sex. But he delved deeper and discovered that the 13 genders were arranged in a hierarchy. Gender 13 always contributes the organelles, whomever it mates with. Gender 12 only contributes them if it mates with gender 11 and downwards. And so on. This works just as well as having two genders, but is a great deal more complicated.23
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Along with most of the animal and plant kingdoms, we practise 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 further.
Hurst explains this by raising once again the matter of disease.24 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 therein. 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 that HIV seems to be 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