sexing sticklebacks

To prove his point Richard Griffiths shows me the transactions of the Third International Conference on Stickleback Behaviour and Evolution held in Vancouver this June. "Sticklebacks are an important international model used by researchers all over the world. Sticklebacks are a fantastically successful species occurring everywhere. Some have heavy armour, and some have none. Some have no spines at all in areas short on calcium. In fjords in Norway different sorts of sticklebacks are found from one fjord to the next, showing different forms and adapted to many niches like Darwin's finches. They can live in seawater or fresh. They are an extremely robust species.

The problem for researchers is that until now, no-one has been able to sex them until they are fully grown, and that has made some experiments more difficult and other completely impossible. When the fish are mature, the male develops a characteristic blue iris and red throat. Males also dance and make nests.

Richard Griffiths expalins "For researchers it is extremely important to know what sex individual fish are. Male fish tend to be more aggressive and territorial and that can translate ibnto them not growing well and also being more disease prone. Some behaviour patterns are typically male, and others female. Anyone looking at these behaviours without knowing gender would be missing a whole dimension. Imagine trying to study human behaviour without knowing what sex the individuals are. If you can say that playing football or fishing are Y linked markers you start to have the basis for understanding human behaviour.

The stickleback test that Dr Richard Griffiths is devising uses DNA techniques pioneered to sex birds. The first step in devising the test involves a general test that compares the genetic sequence of a number of individuals of known gender. This is like comparing the libraries of different individuals. What is needed is some piece of genetic material that all males have and females don't. Once this marker is identified, then a more specific test is devised honing in on that genetic marker sequence. The result is a test rather like a pregnancy test. Two bands would indicate a male, one band would be female. The single band is shared between both sexes and is just there to show that the test has worked successfully. It involves clipping a spine from a fish, causing minimal damage, and not effecting its behaviour.

In fact, as with science generally, it is not quite so simple because fish are pretty mixed up sex wise. Some fish including sticklebacks and salmon have XY chromosomes, as with humans. In other words males have both X and Y chromosomes, and are terms heterogametous. Other fish like eels are the opposite, and the so-called WZ system applies where males are ZZ and females WZ. Richard Griffith's test should allow people to work out which system applies easily. Still other species of fish, such as Silverside fish, have environmental sex determination, so they choose their sexual orientation according to factors such as temperature, or their dominance in their group. There is therefore no chromosomal difference and these species defy any lab test.

The arrival of the new stickleback sex test has been greeted with enthusiasm by researchers who see it as a way of opening up new areas of research. Professor Felicity Huntingford, of Glasgow University said, "Sticklebacks have no economic value, of course. But they are a valuable experimental species. They are an internationally recognised scientific tool to strengthen our understanding of fish biology and evolution, much like the white rat and the fruit fly. They are used to explore fundamental evolutionary questions. For instance male sticklebacks exhibit exotic colours when breeding, but what advantage does the female get in choosing a highly coloured male. In exploring this question it would be very valuable to be able to know sex ratios and survival in broods.

"The test Richard Griffiths has devised allows us to do much more powerful experiments. If you can look at sex rations of eggs or small fry it allows studies which were previously impossible. One could, for instance see how male fish devote resources to male and female fry. Sticklebacks and their resource allocation is also a far from ludicrous model for looking at migrating salmon as well. Some do move from fresh water into salt water to breed. Sticklebacks often fail the taxi driver test. They cannot believe that you seriously study sticklebacks. Still they are a crucial research tool and this sex test will increase their experimental usefulness considerably.

Glasgow, University of

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