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Issue 471, 22 October 2001

Sex change parasites help the invaders win battle of survival

When native species are pitted against invaders, parasites can play a decisive role in determining who survives and who does not. Researchers in Leeds have discovered parasites with radical methods of spreading themselves among their host species and which can completely alter the outcome of invasions, with devastating consequences for the native species.

Biology lecturer Dr Alison Dunn (above) and colleagues at Leeds teamed up with researchers at QueenÕs University Belfast to study invasions in communities of amphipods – shrimp-like creatures which are freshwater relatives of the common sand-hopper (below). The native amphipod has been affected by three different invaders which are spreading throughout rivers and lakes within Northern Ireland, often eliminating the native species.

Each invader came by a different route but all arrived through human intervention: one was introduced from Britain in the 1950s through fish farming, another came in ornamental pond plants and the third hitched a ride in the ballast water of American ships during the First World War.

While looking at the different parasites affecting both native amphipods and the invaders, researchers in Leeds found one which lives in the cytoplasm of the host cells. This parasite is vertically transmitted, only being passed from mother to offspring of the host in the mother's eggs and, to ensure the widest transmission possible, the parasite is able to turn male hosts into females. It ensures females give birth to larger numbers of female offspring, so continuing the cycle. Offspring of uninfected hosts are between 40% and 50% female; for infected amphipods this rises to 80%.

Dr Dunn explains: "The parasite distorts the sex ratio of the host population. Mathematical models show that it may even drive the host extinct due to lack of males, but may survive itself through recolonisation of the area by other amphipod populations. Of five different types of vertically transmitted parasites in amphipods, four have now been shown to work by feminising the host, and two of these have been new discoveries by researchers at Leeds."

While the feminising parasite infects both native and invading amphipods, the natives are far more severely affected, with 55% carrying the parasite, compared to only 15% of the invaders.

Other parasites identified by the researchers affected the amphipod's swimming ability and behaviour, making it less agile and more susceptible to predators. One grows over the host's body, forming a Ôfur coatÕ which increases drag on the swimming animal. Another affects the amphipod's main swimming muscle, inhibiting its ability to move. Another is a worm which uses two hosts – the amphipod and the duck – to complete its life cycle. This parasite forms a bright red cyst, and alters its host's behaviour, making it swim to the surface where it is visible and easily eaten by a duck, so continuing the cycle.

Dr Dunn explains: "We found a way to study the amphipods in their natural environment, using special tubes to create environmental enclosures in the river itself. We were able to pit native against invader, either healthy or infected, and measure the effect of the parasites on competition and predation as well as mating behaviour."

On site mid-stream - Dr Alison Dunn

Where healthy natives were placed in a tube with one larger, more aggressive invader the native was soon wiped out, eaten by its competitor. When the individuals were infected with parasites, this happened much more quickly. In normal, healthy conditions however, the native species will overrun another smaller invader but when infected, both native and invader could coexist.

Dr Dunn said: "Our study clearly showed that parasitism influences the biological invasions in a number of ways. It would be easy to assume that replacing one amphipod with another will not necessarily upset the ecosystem to a great extent, but even these tiny creatures are very different. For example, amphipods eat the larvae of other creatures, and natives and invaders have different predatory preferences, so changes in the amphipod population will impact on other populations too. And this scenario is repeated wherever invaders are fighting native species for survival, especially in UK waters, as our island status makes us particularly vulnerable to aquatic invasions."

Using more detailed study of the amphipods in the laboratory, and specialised mathematical modelling, the researchers are now looking at how to predict the precise characteristics of host-parasite relationships which lead to successful invasion, extinction or coexistence.

Through understanding the process and the patterns of invasion, they hope to develop the ability not only to predict, but also manipulate the outcome.

Dr Dunn and colleagues at Leeds have secured further grants to look at the intriguing feminising parasites in more detail, in order to determine their evolutionary origin, and discover more about the long-term effects on host populations .

The biological investigations project is funded by a NERC grant to Drs Alison M. Dunn and Mel Hatcher of Leeds and Drs Jaimie Dick and Calum MacNeil of Belfast. Work on feminising parasites is funded by NERC and Leverhulme Trust grants to Drs Judith Smith, Alison Dunn, and Rebecca Terry of Leeds and a NERC/CASE studentship to Mr Joe Ironside (CASE partner Dr David Rollinson at the Natural History Museum).

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