Natalie Simmonds - Fish Out of Water? Parasites and Evolution in New Environments

The humble stickleback is an excellent example of evolutionary change and adaptation. In this article, Natalie Simmonds of the Department of Biology asks what is it that causes their evolutionary change?

About My Research

Species adapt by natural selection, with the most successful individuals passing their genes on to the next generation. As a species moves to a new environment this can prompt an evolutionary change to adapt to their surroundings. An excellent example of such a species is the three-spined stickleback, previously a marine fish that has colonised freshwater rivers and lakes across the northern hemisphere. Commonly the stickleback adapts by reducing the amount of bony armour over generations of freshwater living. But what is it about the new environment that makes the freshwater form so successful?

There are several possible factors that could have caused the evolutionary change in sticklebacks, but as of yet no ‘agent of selection’ has been firmly established. One possibility is that an increase in pressure from new, freshwater parasites, leads to the low plated form being more successful. My research focuses on the impact of parasites on the host stickleback, to see if infection could have contributed to the long term evolution of sticklebacks.

Research Approach

A three-spined stickleback with four of the most common parasites. From top; Gyrodactylus, Diplostomum spathaceum, Diplostomum gasterostei, Schistocephalus solidus
Sticklebacks are common in the wild making them an excellent species to look at real world evolutionary change. I have taken samples from a wild population with a mixture of plate morphs and brought them back to the lab. Using microscopy, parasites can be identified and quantified in the different body tissues.

A gene called eda controls the plate morph of sticklebacks. Each fish has two copies of the eda gene, either ‘C’ or ‘L’. Fish with ‘CC’ have most plates, and ‘LL’ have fewer plates, with ‘CL’ fish being intermediate. From a DNA sample the genes any individual has can be determined by molecular techniques.

Within each plate morph (CC, CL, LL) the exact number of plates is also variable. For finer detail of bone structure the tissue must be stained allowing the position and number of plates to be determined.

Using this combination of techniques, we can document the underlying genes, the plate morph produced and the level and type of parasite infection for each individual fish.

Research Findings

The population of sticklebacks under investigation had a diverse range of parasites, with 8 species identified both externally and internally. Results show that most parasites were evenly distributed between the plate morphs, indicating that not all parasites influence natural selection. However, some infections were more common in the low plated morph. It is possible that these low plated fish are better able to withstand the effects of these parasite infections, allowing them to survive, which could explain why selection favours this form over the marine form.

Completely plated fish (CC) were relatively rare in the population (6%). This could indicate that completely plated fish have a low survival rate, which could explain why selection has favoured low plated fish in freshwaters. However, by taking a natural sample we cannot conclusively determine what the reason for low survival rates is. The next stage in this research is to conduct a lab-based experiment and challenge fish of different plate morphs with parasite infections in a controlled study. By doing this I will be able to see if parasite infections have a greater effect on some plate morphs than others. This will then provide strong evidence for whether infections have influenced evolution in the wild.

About Natalie Simmonds

Natalie SimmondsNatalie Simmonds is a research student working towards completion of her doctoral degree in the Department of Biology. Natalie holds a studentship from the Fisheries Society of the British Isles.

Natalie is supervised by Dr Iain Barber.

Department of Biology
University of Leicester
Adrian Building
University Road

Natalie will present her work at the Festival of Postgraduate Research 27 June 2013 - see Natalie's Festival poster.

The Festival is open to all members of the University community and the public - book your place here.

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