It’s something of a platitude that men and women want different things, but it seems that just such a battle is fought out on the sex chromosomes. Genes that are mainly expressed in only one of the sexes – for example, those that have important roles in the ovary or testis – are distributed unequally: even though the X chromosome is found in both sexes, it contains more female-biased than male-biased genes. A common explanation for this difference is that the mammalian X chromosome is inactivated early in spermatogenesis, making it a poor location for genes necessary for sperm production.

However, according to a new study in BMC Biology, this won’t work for Drosophila. Fruit fly X chromosomes don’t seem to be inactivated during sperm production, though their genes show the same biased distribution as everyone else’s. So if it’s not meiotic X inactivation causing the difference, what is it? The authors suggest an intriguing possibility by showing that many tissue-specific genes – even those which have identical expression in each sex – tend not to be found on the X chromosome. Consequently, they hypothesize that the X chromosome is simply a bad location for genes which need to be expressed in particular tissues, and that sex differences aren’t as important as we thought.

Xuemei Lu and Chung-I Wu, discussing this mystery in an accompanying commentary, bring back the battle of the sexes. Evolutionarily, the X chromosome has spent two-thirds of its time in females rather than males, so they suggest that where genes have conflicting male and female roles – for example, if they’re used in both testes and ovaries, but have a different function in each – the female role will tend to win out. Whatever the explanation, the battle of the sex chromosomes seems set to continue.