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Boundaries between species
By Bijal P. Trivedi

Mating two different species often produces a hybrid creature that either dies during early development or is sterile if it survives. Now researchers are beginning to piece together a genetic explanation for this species incompatibility.

Take the example of a horse and a donkey. They can mate, but they produce sterile offspring. Many inter-species matings fail to produce live offspring because complications arise during early development.

But sometimes gender determines whether an inter-species mating will work—for instance, when the female of one species is mated with the male of the other. When a female mouse of the species Peromyscus maniculatus is mated to a Peromyscus polionotus male the result is a mouse that is smaller than either parent.

When a P. maniculatus female and a P. polionotus male mate, the fully grown offspring are smaller (left) than either parent (right).

When the mating is reversed, using a male P. maniculatus and a female P. polionotus, the placenta becomes too large and the fetus dies.

When a P. polionotus female and a P. maniculatus male mate, the placenta grows quickly, becoming abnormally large (right), and aborts early in development. Normal placenta (left).

The polarized outcomes of these two matings could be due to a phenomenon called imprinting, according to Paul Vrana, a researcher at Princeton University and an author of the study which appears in the May issue of Nature Genetics.

Whether a gene is turned off or on can depend on which parent it comes from. A paternally imprinted gene means the father contributes the active copy and the mother's copy is turned off. Vrana believes that the oversized placenta occurred because imprinting was disrupted. Somehow a growth gene that should have been turned off in the mother became active. The offspring had two copies of a growth gene that were "on" rather than just one from the father.

In the first mating, two copies of a growth-retarding gene seemed to be active, rather than just one from the mother, which is why the offspring is smaller than the parents, says Vrana.

Vrana and his colleagues believe that each species maintain a unique selection of imprinted genes. Imprinting may be one mechanism that maintains boundaries between species.

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Vrana, P.B. et al. Genetic and epigenetic incompatibilities underlie hybrid dysgenesis in Peromyscus. Nat Genet 25, 120-124 (May 2000).

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