Most creatures that carry a Y chromosome turn out to be males: but this is not necessarily the case in African pygmy mice (Mus minutoides). Instead of depending on simple combinations of X and Y chromosomes to produce males carrying an X and a Y chromosome and females carrying two X chromosomes, the tiny rodents can carry a third chromosome, X*, with a mysterious sex-changing power. Samuel Ginot and colleagues from the University of Montpellier, France, explain that instead of turning into males, X*Y pygmy mice develop into females, and even though the X*Y females are physically indistinguishable from regular XX females, their behaviour is markedly different. ‘X*Y females have higher reproductive success and are much more aggressive’, says the team. Could the X* chromosome be contributing to the X*Y female's dominance?
Working with the University of Montpellier's colony of pygmy mice, which were established by Frederic Veyrunes in 2010, Ginot and Julien Claude measured the bite force generated by male and female mice to find out whether the exotic X* feminising chromosome might impact on aspects of the rodents’ performance. Offering a small force transducer to the animals to bite on, Ginot explains that the tiny mice were usually keen to cooperate. ‘The bite is a reflex defence mechanism, and the animals usually bite anything that comes within “teeth's reach”’, he says, adding that this included his own finger on some occasions: ‘It's the equivalent of being pricked by a needle’, he chuckles.
However, as the females carrying Y and X* chromosomes are physically indistinguishable from the XX and XX* females, Julie Perez had to search the genes of every female for evidence of the Sry (sex determining) gene, which provides incontrovertible evidence of the presence of a Y chromosome. Having identified which of the biters were double-X females (XX and X*X) and which were feminised males (X*Y), and then recorded their bite forces – which ranged from ∼2.5 to 16 N – Ginot and his colleagues were impressed to see that the X*Y females out-bit all of the other mice, males included. And, when Ginot and Claude analysed the head structures of the mice to identify the cause of the X*Y females’ ferocious bites, they discovered that they had larger skulls.
But why is a harder bite advantageous for the X*Y females? Explaining that it should allow the X*Y females to eat harder food than other pygmy mice while allowing the tougher females to mate with more males, Veyrunes says, ‘The effect of the X* chromosome goes well beyond feminisation’. The team adds, ‘These X*Y females are characterised as “super females”’, and they hope that the animals’ enhanced aggression, larger skulls and stronger bite forces will help us to learn more about the impact of sex chromosomes.