Evolution in action. Thale cress seeds develop only with the right combination of the histidine gene.

Olivier Loudet/INRA

Why Can't Mules Breed?

Nature has a way of keeping species separate: Most hybrid organisms, such as mules or ligers, are sterile. The mechanisms behind this were unclear, but now scientists think they may have caught the genetic culprit in action.

Hoping to learn more about what keeps species apart--and how new species form--biologist Olivier Loudet of the French National Institute for Agricultural Research in Paris turned to the thale cress (Arabidopsis thaliana), a weed from the mustard family. Scientists sequenced the complete Arabidopsis genome in 2000, making genetic analysis relatively easy; and, with many populations growing throughout the world, thale cress has a wide range of genetic variety.

Loudet and colleagues sampled two populations of thale cress, from Poland and the Cape Verde Islands in the middle Atlantic Ocean. Immediately, they noticed a subtle genetic difference: One of two copies of the gene for the essential amino acid histidine is partially deleted in chromosome 5 of the island thale, and it is not expressed at all in chromosome 1 of the Polish thale. When these two genetic variants combined during breeding, 11% of the embryos died, the team reports in tomorrow's issue of Science. Other genetic combinations resulted in a measly thale cress with shorter-than-normal roots. The team chalks both problems up to a reduced supply of histidine.

Crossing 30 other variants of thale cress resulted in inviable offspring about one-fourth of the time. This means that evolution of a single gene can rapidly lead to differences within a species, says Loudet.

Evolutionary geneticist Leonie Moyle of Indiana University, Bloomington, says the results are "quite exciting" because this is the first clear example of genetically incompatible lineages within the same species.

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