A new Odyssey

Geneticists discover nature's way of preventing interbreeding

Published: Monday 15 March 1999

the fruit fly Drosophila melanogaster ( D melanogaster ) is the mad scientist's ultimate dream. Toying around with some of its genes can lead to bizarre physiological changes. Mutations in these genes, called homeotic genes as homeosis means "replacement of one body part by another", can cause legs to grow where the eyes are supposed to be, or perhaps, balancers in place of wings. Scientists have known the existence of these genes in D melanogaster for years.

More recently however, such genes have been found in a number of species -- including human beings -- and, according to geneticists, they all contain a particular stretch of dioxyribose nucleic acid (dna). This stretch, also called the "homeobox", gives rise to some 60-62 amino acids when decoded. Together, these amino acids are known as "homeodomain". The fact that these genes occur in a number of species, coupled with the effects of their mutation, suggests that these homeotic genes play very important developmental and evolutionary roles. So when the researchers recently discovered a homeobox that, far from being static in terms of it sequence, evolves at a very dizzy pace they were surprised.

This is what Chau-Ti Ting and colleagues at the University of Chicago, usa, report in the American science journal Science (Vol 282, No 5393). The story involves a gene known as Odysseus (Ods) , that plays a role in making male hybrids of two closely-related fruit fly (Drosophila) species sterile. If an Ods region is transferred from D mauritiana to D simulans -- possible if the two happened to mate -- the males of D simulans become completely sterile. Hybrid sterility is crucial for evolutionary divergence, in particular for speciation. This is because when a new species arises in the beginning, its numbers are usually far less than of the parent species. And if the members of the new species can mate freely with the parental type, it will 'blend' rapidly and the new species will not be able to establish itself. So Odes can be seen as nature's extremely ingenious way of being fair to the species -- both old and new.

The Ting group managed to isolate dna from the Ods region of D melanogaster and found that it contained a homeobox. They went on to compare the Ods gene from different species of Drosophila flies, expecting some variation in regions outside the homeobox but very little or none within. To their astonishment, the results were quite the opposite: the homeodomain region of Ods varies more than other regions, not less. Indeed, the difference between the homeodomains of D simulans and D mauritiana is comparable to the difference between the homeodomains of flies and mice.

Statistical analysis shows that the observed rapid evolution is due to positive selection -- the changes seen are far more than would be expected on the basis of chance alone. Given the Ods's role in hybrid sterility, the fast rate of the homeodomain's evolution probably has a crucial message: speciation events are followed by rapid changes in those genes that are responsible for ensuring that individuals of two different species do not interbreed.

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