One gene, many fish

The Great Lakes of East Africa swarm with fish--particulary with one kind of fish known as cichlids. In Lake Victoria alone you can find over 500 species. These species come in different colors and make their living in many different ways--sucking out eyeballs of other cichlids, scraping algae off of rocks, and so on. What's strange about all this is that the Great Lakes of East Africa are some of the youngest lakes on Earth. By some estimates, Lake Victoria was a dry lake bed 15,000 years ago. All that diversity has evolved in a very short period of time.

East African cichlids are therefore not just pretty fish. They are natural experiments in evolution--in particular, in the capacity that animals and other organisms have to explode into new forms. These adaptive radiations have happened many times over the history of life, at lots of different scales. The so-called Cambrian explosion 530 million years ago, for example, saw the rapid rise of many different kinds of animals, including our own earliest vertebrate ancestors.

In the PNAS Early Edition, Japanese biologists pinpoint one ingredient in the cichlid explosion: turning a gene into more than one protein. The rule of one protein for one gene is at the heart of the old Central Dogma of molecular biology, but in recent years it's become abundantly clear that the genome operates in a much more sophisticated way than that. When the DNA of a gene gets converted into a template of RNA, different segments of the gene may get spliced together to create different sequences. You can get hundreds, even thousands of different proteins from the same gene through alternative splicing.

The Japanese biologists studied hag, a gene that's responsible for the patterns of pigments in cichlids. They found three different versions of hag RNA in African river cichlids, which have relatively low diversity. By contrast, the cichlids of the Great Lakes had two or three times as many versions. The researchers point out that it takes very little evolutionary change to create new alternative splicing. They propose that alternative splicing offered a quick way to create dramatically new color patterns in cichlids. For cichlids, pigment is a key to sexual success, and so adding new colors to a population of fish could quicky divide it up into smaller populations that only mated with one another. From there, it's a quick path to new species.

Alternative splicing had its mass media debut three years ago when the Human Genome Project found surprisingly few genes. How could we be so much more complex than a fruit fly if we had only twice as many genes? Alternative splicing seemed like the obvious answer--we must make a lot more alternative proteins from our genes. But the papers that followed, like this one, failed to find a correlation between complexity and alternative splicing. That doesn't mean alternative splicing hasn't been an important player in evolution--scientists just have to be careful that they look for the right aspects of evolution to see it at work.


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