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genes trying to destroy their male parts. Male-killer genes have been found in more than a hundred and forty 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 towards females; indeed if the rebels are achieving their aims in many members of the species, the nucleus would benefit greatly from being the only plant on the block capable of producing pollen. So wherever they appear, male-sterility genes are soon blocked by nuclear fertility-restorers. In maize, for example, there are two male-sterility organelle genes, each suppressed by a separate nuclear-restorer. In tobacco there are no less than eight such pairs of genes. By hybridizing different strains of maize, plant breeders can release the male-sterile genes from nuclear suppression, because the suppressor from one parent no longer recognizes the rebel from the other. They wish to do this because a field of male-sterile maize cannot fertilize itself. By planting a different, male-fertile strain among it, the breeders can collect hybrid seed. And hybrid seed, benefiting from the mysterious boost known as hybrid vigour, yields more than both its parents. Male-sterile/female-fertile strains of sunflower, sorghum, cabbage, tomato, maize and other crops are a mainstay of farmers all over the world.27

It is easy to spot if male-sterile genes are at work. The plants have two types: hermaphrodite and female. Such populations of plants are known as gynodioecious; androdioecious plants, with males and hermaphrodites only, are almost unknown. In wild thyme, for example, about half the plants are usually female, the rest hermaphrodites. The only way to explain the fact that they have stopped half way along the one-way street is to posit a continuing battle between the organelles’ male-killer genes and nuclear fertility-restorer genes. Under certain conditions the battle will reach stalemate, such that any further advance by one side gives the other an advantage and the ability to force it back. The commoner male-killers get, the more restorer genes will be favoured and vice versa.28

The same logic does not apply to animals, many of which are not hermaphrodites. It pays an organelle gene to kill males only if by doing so some energy or resource is diverted to the sisters of the killed males, and hence male-killing is rarer. In hermaphroditic plants, if the male function dies, the female function of the plant grows more vigorously or produces more seed. But a male-killer gene in, say, a mouse, by killing the males in a brood, benefits those mice’s sisters not at all. Killing males because they are evolutionary culs-de-sac for organelles would be pure spite.29

Consequently, the battle is resolved rather differently in animals. Imagine a population of happy hermaphroditic mice. There arrives in its midst a mutation, which happens to kill male gonads (testes). It spreads because female individuals that have the gene do rather well: they have twice as many babies because they put no effort into making sperm. Soon, the population consists of hermaphrodites and females, the latter possessing the male-killing gene. It is possible for the species to escape back to hermaphroditism by suppressing the male-killer gene, as many plants have obviously done. But it is just as likely that something else will happen before a mutation that causes the suppression can appear and take effect.

Maleness is a rather rare commodity at this stage. The few remaining hermaphroditic mice are at a premium because only they can produce the sperm that the all-female mice still need. The rarer they get, the better they do. No longer does it pay to have the male-killing mutation. Rather the reverse; what would really pay the nuclear genes would be a female-killer gene so that one of the hermaphrodites