NEVER write off a rank outsider. A female mammal with a 鈥渕ale鈥� chromosome should struggle to reproduce, but for female African pygmy mice, a male chromosome spells more offspring.
Mus minutoides has three sex chromosomes. On top of the usual X and Y seen in other mammals, there is a modified X called X*. In 2010, Fr茅d茅ric Veyrunes of the Institute of Evolutionary Sciences at the University of Montpellier in France realised the X* is 鈥渟uper-feminising鈥�: it blocks the masculinising effect of the Y chromosome. So females can be XX, XX* or X*Y. Males are all XY.
Now Veyrunes has found that X*Y females reproduce more than XX or XX*. They were more likely to have a litter during a six-month spell with a male, and had larger litters (Evolution, ).
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Yet one-quarter of a Y-female鈥檚 offspring die before birth 鈥� they inherit two Y chromosomes, one from each parent, so lack vital genes from the X chromosome.
So why does a Y chromosome help a female reproduce? Part of the answer may be that males and Y-females are more likely to breed, because the females behave in a more 鈥渕ale鈥� way. Lab studies offer some support for this.
In that case, the Y-females鈥� secret may be being tomboys.
This article appeared in print under the headline 鈥淭omboy mice have the most pups鈥�