Sex Wars: Genes, bacteria and biased sex ratios by Michael Majerus, Princeton University Press, £29.95, ISBN 0691009813
WANTING a male child to carry on the family name (and inherit all the property) is a common enough ambition in many male-dominated human societies. Yet few people have taken as bizarre a step to achieve that goal as the French aristocrats described in Majerus’s book, Sex Wars.
They believed the theory of Anaxagoras, a Greek philosopher in the 5th century BC, who claimed that male babies were produced by sperm generated only in the right testicle. So, to increase their chances of fathering a male heir the aristocrats had their left testicle surgically removed.
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A roughly equal mix of males to females is normal for most species that reproduce by sexual means. As the great population geneticist Ronald Fisher argued in the 1930s, this ratio is inherently stable. Any reduction in the numbers of males, for example, will create opportunities for parents that tend to produce offspring of that sex. The relative numbers of males will increase but the selective advantage will then disappear as the numbers of the two sexes approach parity.
But elsewhere in the natural world, there are reliable ways of manipulating the sex ratio of the next generation – usually in favour of the female line. The most closely studied and accomplished meddler is the bacterium Wolbachia. It is an intracellular parasite of insects and other invertebrates that is transmitted vertically to the next generation in the egg. Since males play no part in passing on the parasite’s genes, the bacteria have devised a number of ingenious ways to ensure that their host only rears female offspring.
The existence of organisms such as Wolbachia was only suspected after populations of insects were found in which males were extremely rare. Subsequent studies have shown that between 10 and 20 per cent of all insect species harbour Wolbachia. In these species the parasite has upped the ante by killing off male gametes or causing male embryos to develop into females. It is even possible that in some cases the parasites have forced the host to give up sex completely and begin reproducing by parthenogenesis in which unmated females produce clones of themselves.
Nevertheless, the relationship between Wolbachia and its host is an uneasy one in which the parasite effectively forms part of the genetic complement of an organism while seeking to further its own interests at the expense of the rest of the genome. Unsurprisingly, the host doesn’t take this lying down. Insects have developed many different ways of fighting back, with some males apparently able to assess the health status of potential mates and choosing to breed only with uninfected females. In other cases they have evolved genes that block the parasite’s ability to kill male embryos.
Ultimately, it may benefit the parasite to develop ways in which its presence no longer harms the host and may even be of practical benefit. There is some evidence that this has occurred with Wolbachia. Michael Majerus suggests in Sex Wars that the organism may prove a useful model for studying virulence factors in host/parasite relationships. And there are potentially sound economic reasons for the increasing scientific interest in these organisms. Manipulating sex ratios could be a new method for controlling agricultural pests and the insect vectors that transmit human diseases such as river blindness.
Majerus’s account is rich in examples from newly published studies and offers an excellent introduction to a rapidly developing discipline for undergraduate university students. But this fascinating area should also be of interest to a general readership, so there is another book just waiting to be written. From the extrapolations and asides that appear in this volume it is clear that author is quite capable of producing a more accessible account. So how about it?