Well, shuffle my genes (Image: Hugo Willcox/Foto Natura/Minden/Getty)

Species: Sorex araneus

Habitat: throughout northern Europe and northern Asia, thumbing its pointy nose at convention

At first glance the common shrew is unremarkable. Between 5 and 8 centimetres long, it looks like a mouse with a pointed nose, or perhaps a miniature mole. If you saw it scurry through a field, you would barely give it a second glance.


But in fact the common shrew is still another animal that makes humans look like wimps. While other hardened creatures can survive extreme temperatures or toxic chemicals, the shrew copes with dramatic rearrangements of its genes.

They are divided into at least 68 different races, which look alike but are distinguished by wholesale genetic rearrangements. In other animals, this sort of gene shuffling is enough to create separate species that cannot interbreed. But it seems the shrews can still make babies even if their genes are radically reordered.

Dance of the chromosomes

DNA is organised into chromosomes, and each cell of an animal usually carries two copies of each, one inherited from the mother and one from the father.

Sperm and egg cells, however, contain just one copy of each chromosome. This ensures that when a sperm fertilises an egg the resulting cell ends up with two of each. But making sperm and eggs is tricky, as each must get one copy of each chromosome, rather than two of one and none of another.

To ensure they separate correctly, the chromosomes line up in their pairs in the centre of the cell. Then they are pulled apart by protein “ropes”. If all goes well, each sperm or egg will have one of each chromosome.

But now imagine that one member of a pair has swapped a chunk of its DNA for a piece from a chromosome that belongs to a different pair. Now the dividing cell has a problem, because it must make sure the two DNA-swapping chromosomes end up in the same place. If it doesn’t, one of the daughter cells will miss out on some genes.

And now suppose that lots of the chromosomes have swapped bits of themselves, passing them from one to another. Worse, some have fused together into super-size chromosomes, while others have broken in two – and then fused with others. Given a jumbled mess like that, most embryos would fail to develop – their genetic machinery just wouldn’t work properly. But not when they’re in a common shrew.

Carry on karyotyping

Different races of the shrew have shuffled their chromosomes in different ways. But because each race shuffles in the same way, shrews from the same race can breed normally. The surprise is that shrews from different races can breed too.

Jeremy Searle of Cornell University in Ithaca, New York, and colleagues studied two neighbouring races in Siberia. Despite the shrews’ different chromosome arrangements, they manage to interbreed. Their hybrid offspring are less fertile than their parents, however.

That’s because the offspring do not have matching pairs of chromosomes, so it is hard for them to make eggs or sperm that have the full complement of genes. But although their fertility is lower, it isn’t zero: they can still have litters of their own. Somehow they are able to assemble up to nine chromosomes in the centre of a dividing cell, and pull them apart one by one in the right way to make viable sperm and eggs. “I think it’s fairly astonishing that they manage it,” Searle says.

A second pair of races living near Moscow, Russia, were “as extremely different as you could get”, Searle says, but they had viable offspring all the same – although it’s not clear if the offspring could reproduce.

By contrast, house mice are also divided into chromosomal races, many of which cannot interbreed at all. “We don’t know why the shrews are better than the mice,” Searle says.

Eventually the different races may become separate species. Searle says there are shrews that look just like the common shrew, but do not breed with them – and the only difference is how they arrange their chromosomes.

Journal references: Journal of Evolutionary Biology, DOIs: 10.1111/j.1420-9101.2010.02191.x and 10.1111/j.1420-9101.2011.02266.x

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