Why Are Hybrids Sterile ?

Because of gene evolution

Normally, hybrids between two different species, even if offering beneficial traits, are sterile. And in many cases, hybrids are not viable at all.

A mule (photo), the result of the mating of a horse and a donkey, is sterile.

Researchers at Cornell University have made the first discovery of a gene pair that provokes problems at hybridization.

Two genes from two fruit fly species (Drosophila melanogaster and D. simulans) interfere with each other, preventing the production of male offspring.

The finding explains what causes lethality or sterility in hybrids and also offers clues to how species evolve from common ancestors.

A rare mutation in a D. melanogaster gene called "Hmr" (Hybrid male rescue) and a similar mutation in a D. simulans gene called "Lhr" (Lethal hybrid rescue) make these genes nonfunctional.

When either of these genes is eliminated, the hybrid males survive.

"We have found the first example of two genes that interact to cause lethality in a species hybrid," said the paper's senior author, Daniel Barbash, assistant professor in Cornell's Department of Molecular Biology and Genetics.

This confirms the Dobzhansky-Muller model, a theory from the 1930s that suggests hybrid incompatibilities (such as death or sterility) are caused by genes that have evolved from a common ancestor but diverged in each of the species.

In the common ancestor, these genes may have worked perfectly well together.

But, as each gene evolved in its own species, it began to code for proteins that no longer work in the other species.

When genes from each species were compared with each other, the Hmr gene in D. melanogaster and the Lhr gene in D. simulans each evolved much faster than most genes and diverged due to natural selection, a genetic change due to a pressure that benefits the survival of a species.

The Dobzhansky-Muller model also proposes that these evolved genes depend on each other to cause hybrid incompatibilities.

However, when Barbash and his colleagues cloned each gene and inserted an Lhr gene from D. simulans into D. melanogaster, the two genes did not interfere with each other in the engineered D. melanogaster strain even though the Lhr and Hmr genes interfere with each other in hybrids.

"This tells us there must be other things involved in the hybrid" that impacts the incompatible pairing of these genes, said Barbash.

The scientists hope to determine whether the hybrids die because of additional genes like Hmr and Lhr, or because of more subtle differences between the chromosomes of the species.

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