Sex Wars: Genes, Bacteria, and Biased Sex Ratios

  • Michael E. N. Majerus
Princeton University Press: 2003. 280 pp. $45, £29.95

In the first edition of On the Origin of Species, Charles Darwin made a brave and essentially correct attempt to explain the striking rule that males and females occur in equal numbers. In later editions, however, he described the problem of the even sex ratio as being “so intricate that it is safer to leave its solution to the future”. Ronald Fisher took up this challenge in 1930, noting that every offspring has one mother and one father. Therefore, the sexes on average fare equally well in passing their genes on to the next generation under an even sex ratio. Fisher realized that a mother that produces an excess of the rarer sex (whichever this may be) will be favoured by natural selection. So, like a pendulum in motion that eventually comes to rest, the sex ratio will stabilize at equal numbers of the sexes over evolutionary time. But there are many exceptions to this rule, and understanding them has proved to be a challenge for evolutionary biologists.

These exceptions form the centre of gravity of Sex Wars. Michael Majerus of the University of Cambridge starts off by walking us through the background and some of the basics: asexual versus sexual reproduction, sexual selection, the sex-determining mechanisms, the sex ratio and the various 'economic' factors that can lead to uneven sex ratios are all covered in the opening sections. But the bulk of the book is about the various genetic conflicts that knock the sex ratio off its evolutionary equilibrium.

Sex to die for: females of the ladybird Adalia 10-punctata distort the sex ratio by killing males. Credit: K. PRESTON-MAFHAM/PREMAPHOTOS WILDLIFE

For a gene located on a nuclear autosome — any chromosome except the sex chromosomes — life is as good in a male as it is in a female, in terms of the probability of being transmitted to the next generation. For extranuclear genes, however, males are a genetic dead-end because the sperm contributes nothing but its nuclear genes to the fertilized zygote. Cytoplasmic genes located in mitochondria or in endocellular microorganisms are expected to favour the female line, through which they are transmitted to the next generation.

This sets the stage for a battle between autosomal genes on one hand, and genes with unequal transmission rates in the two sexes on the other. I found the portrayal of these opponents by Majerus very exciting, and his account of them offers many insights. These evolutionary law-breakers use a variety of intrusive tactics (including killing sperm containing Y chromosomes, transforming genetic males into functional females, or simply killing males), which all bias the sex ratio towards females, sometimes even to the point when the 'host' is driven close to the brink of extinction because of a lack of males in the population. The autosomal genes fight back, however. Majerus describes several cases of autosomal 'rescue' genes that counter the effects of the law-breakers. The natural history of sex-ratio distortion is the definite strong point of this book — Majerus tells many intriguing and entertaining tales about various reproductive curiosities.

No book can be written for everyone, but every book should be written for someone. This book is aimed at “a wide audience”. Any reader of this book will find much of interest, but will also be frustrated. Like the indecisive Buridan's ass who could not choose between two equal bales of hay and therefore starved to death, Majerus roams the treacherous no-man's-land that lies between the general reader and the specialist, unable to quite reach either. General readers will stumble over many of the terms and turns in the more detailed and complicated sections. There is a glossary at the end of the book, but it is incomplete and partly confused.

Specialist readers, on the other hand, will find the lack of reference to many case studies and ideas, as well as some more technical problems, quite annoying at times. They might also find that the book loses the battle with a more exhaustive book on sex ratios — Sex Ratios, edited by I. C. W. Hardy (Cambridge University Press, 2002) — that recently reached the bookshelves. If Sex Wars were less technical and more 'popular', it might have found more enthusiastic readers.