Why fish?

First the fly, then the worm. This time, our series of articles exploring model organisms turns its attention to the zebrafish.

Model organisms are beloved by researchers investigating developmental biology - the transformation of an organism from a fertilised egg into an adult. The most famous of these models is of course the fruit fly, which has nearly a century's rich history of research behind it, but remarkable insights have also come from studies of frogs, chicks, worms and mice. In recent years, a newcomer has joined this cadre of biologists' favourites: the zebrafish.

The abiding images of zebrafish research are the often stunning photos of the developing embryo - its transparency allowing researchers to watch cells move and genes at work. With sophisticated molecular, genetic and imaging techniques available, thousands of mutant lines, and the genome being sequenced by the Wellcome Trust Sanger Institute, the zebrafish is now a well-established model system. Its rise to prominence has been meteoric - only a decade ago, zebrafish research was small fry.

Hometown Oregon

Although zebrafish research began in the 1960s, it is only in the last decade or so that it has shot to prominence. Like other model organisms, the zebrafish research field had a founding father and a 'hometown'. Drosophila can look to Thomas Hunt Morgan and the 'flyroom' at Columbia University, and the nematode worm to Sydney Brenner and Cambridge; likewise the zebrafish pays homage to George Streisinger, a tropical fish enthusiast at the University of Oregon.

"Streisinger was interested in trying to develop a vertebrate model for genetic studies - particularly of the development and function of the nervous system - a model that would be simpler than the mouse," says Professor Steve Wilson, who studies zebrafish development at University College London. "It was not that zebrafish are particularly different to many other species of fish, but this is the one that was explored and favoured by Streisinger. In the face of some scepticism, he was able to show that you could do various genetic techniques in zebrafish, tricks that would be useful for genetic studies of the future."

Other researchers in Oregon also began to study the zebrafish. Charles (Chuck) Kimmel, for example, looked at the anatomy of the nervous system in the developing fish, and he and others found a remarkably simple and predictable arrangement of many neurons in the brain. This showed that for studies of the development of the nervous system, the fish was going to be very useful.

In the early 1980s, a number of papers from the Oregon groups brought zebrafish to worldwide attention. Other researchers began to explore zebrafish as a model system, attracted not only by its possibilities for developmental studies but also by how easy it is to keep. "Zebrafish is a small, robust, tough and placid species [unlike, for example, sticklebacks; see box below], which is important when you keep thousands of fish, as you don't want to have to worry about them on an individual level," says Professor Wilson. "They're simple to keep as adults: as long as they're fed, and kept happy, that's it. And the happier the fish are, the happier we are - because then they breed well." Indeed, it is really only the eggs that interest most developmental biologists: the adult fish are kept as a source of embryos for study - the fish conveniently laying eggs when the lights are switched on.

The 'big screen'

The big boost to the zebrafish field came when two big genetic screens, searching for mutants, were carried out in the early 1990s. The identification of mutants is one of the most important strategies for the study of development - and indeed for many areas of biology. A mutation provides a 'way in' to understanding the role of the gene in normal development.

A similar tactic in the early 1980s - systematically looking for mutants that affected Drosophila development - had proved stunningly useful and insightful. This labour of love was done by Christianne Nüsslein-Volhard and Eric Wieschaus in Germany (and for which they were awarded the Nobel Prize), so it comes as no surprise that one of the zebrafish screens was led by Dr Nüsslein-Volhard, and the other by Wolfgang Driever, another scientist with a background of working with flies.

Although conceptually the same as the Drosophila screen, searching for mutants in the fish was a far more difficult and logistically challenging proposition. "The screens went through thousands of families of fish looking for mutations that affected early development," says Professor Wilson. "There was no guarantee that it would be successful; no idea what would come out of the screen." The persistence paid off: about 4000 mutants were identified, and the results were published in a single, mammoth issue of the journal Development in December 1996. "As soon as this happened, it provided a resource for the zebrafish field to expand exponentially," says Professor Wilson. "For the last seven or eight years, most zebrafish labs around the world have been working with the mutants that came out of those screens."

Perhaps an even more important impact of the big screen on the zebrafish field has been on the scientists themselves. "The people involved in those screens - the postdocs and students - were truly exceptional," says Professor Wilson. "They were fantastic scientists who devoted a few years of their life to doing a very difficult screen and they did it very well. Many have since set up research programmes on their own and, together with postdocs coming out of Oregon and elsewhere, have helped to seed a group of young, active and very successful labs around the world."

Doing a screen and finding a mutant is just one part of the story: you can see the effect of the mutation on the zebrafish embryo, but you do not know what gene is affected. Zebrafish researchers therefore got together to develop genome resources - "an impressive community effort" according to Professor Wilson. The development of these resources culminated with the Wellcome Trust Sanger Institute beginning to sequence the zebrafish genome in February 2001. "The initial projects [finding the mutated genes] were often very difficult," says Professor Wilson. "Now the genome sequence is almost complete, and the cloning of mutations will become relatively straightforward."

Zebrafish in action

"In one sense, the field has been quite narrow," says Professor Wilson. "It has largely been developmental biologists who have been interested in the fish, and the screens have been geared to finding mutations that affect early development." This approach has proved extremely productive, however, with zebrafish researchers providing insights into key pathways that control the development of the embryo and its nervous system. Many such studies complement - rather than compete with - those in other model organisms, with the fish often being used to tackle specific questions that are difficult to address in frogs, chicks or mice. "This is a trend that I think will increase," says Professor Wilson. "In parallel, the range of studies using zebrafish is expanding and so, in addition to exploring vertebrate embryology, it is anticipated that fish will be adopted by more and more researchers in other fields." Indeed, significant progress has already been made to establish zebrafish models for studies of cancer and nervous system physiology. Many biotech and pharmaceutical companies have also been attracted to the ease with which fish can be used to study the function of genes.

A technique that zebrafish researchers have embraced enthusiastically uses the gene for 'green fluorescent protein' to label specific cells in the embryo. As the protein is harmless and does indeed glow bright green (the gene having come from jellyfish originally), the fate of the cells can be followed as the embryo develops. "One of the most exciting things about fish is that you can study dynamic processes," says Professor Wilson. "The embryo is transparent, so you can watch things happen in the living embryo. I think this will allow the next generation of experiments to be undertaken. If you can watch what a cell is doing in the presence or absence of a gene product, you can say a lot more about what a gene is required for."

The availability of the zebrafish genome sequence not only helps speed up studies of development, but also allows researchers to think about the evolution of vertebrate gene function. The genome of the pufferfish is already complete, and, in Japan, work is progressing on that of Medaka (see box below). "I think new studies will be examining gene function in different fish," says Professor Wilson. "Many fish are closely related but have evolved different morphologies - not just between species, but between subspecies in nearby lakes. The fish genomes will be useful for looking at how these different morphologies have developed genetically, and we'll be able to find the genes subject to evolutionary change."

"In fact, there are a myriad of fantastic fish out there. It would be fascinating to find out how they have acquired these astounding shapes."

Zebrafish is not alone

Several other fish are studied by biologists. Professor Steve Wilson comments on how some of them are used.

Medaka
SW: "Medaka is a small bony fish, similar to zebrafish, which is particularly popular as a model system in Japan for studies of development. Its genome is being sequenced and resources are being developed to bring it up to speed with zebrafish. For people interested in evolutionary aspects of gene function this will be very useful."

Pufferfish (Fugu and Tetraodon)
SW: "Pufferfish are useful primarily for their DNA. Very few people actually keep living pufferfish, as it’s not a practical model system. But its genome is very small: the non-essential DNA has been discarded. So you can look at the Fugu genome to examine genes, and the pieces of DNA that control the genes, and compare them to the sequence of your own vertebrate species."

Sticklebacks
SW: "Sticklebacks are studied from the point of view of ecology, behaviour and, more recently, because of their variable body shapes. However, not all is easy with sticklebacks. Whereas zebrafish don’t exhibit many dramatic behaviours, sticklebacks can be very aggressive, territorial, individual fish. I imagine it would be a bit of a nightmare to keep large families of sticklebacks in the same tank, they’d be fighting all the time. On the positive side, perhaps they will end up becoming a model for human social interactions!"

Why fish?

Hometown Oregon

The 'big screen'

Zebrafish in action

Related links

See also

Further reading