Certain groups of stickleback fish have evolved over time to resist tapeworms found in freshwater lakes, but this resistance comes at a cost.
Biological Sciences – Assistant. Professor Natalie Steinel and a team of researchers examined stickleback fish in British Columbia and found that the fish living in Roberts Lake, about 160 miles northwest of Vancouver, had developed scar tissue, known as fibrosis, in response to tapeworm growth within it.
Their findings, which were recently published in Sciences The journal shows that this immune response has negative effects on the fertility of sticklebacks, and this may be why other stickleback populations, such as those in Lake Gosling just miles from Lake Roberts, tolerate tapeworms rather than develop tissues scar around. Tapeworms infesting fish in Gosling Lake can grow to one-third of the fish’s body weight.
Steinel recently gave us a closer look at this research.
Q: Your research team started this study in 2013. What was the initial goal?
a: The goal was to understand the diversity of infections seen in wild populations. We study wild populations of stickleback fish that all suffered from the same tapeworm, but respond differently to that infection. In one lake in British Columbia we were seeing sticklebacks full of very large parasites, and then in another lake about 15 kilometers away, sticklebacks had almost no infections. We were really interested to see if there were environmental factors at play or if something was going on genetically and immunologically causing the different outcomes we were seeing.
Q: Why is the research centered on sticklebacks?
a: We know a lot about the evolutionary history of sticklebacks, so we have a lot of basic knowledge to build on. They have been a model for evolutionary biology and ecology for a very long time, but they are a relatively new model for the work of immunology. And there are a lot of interesting differences between these fish, which are found throughout the northern hemisphere. There are huge opportunities to take advantage of all this diversity and study interesting phenotypes [observable traits].
Q: Your team looked at how stickleback scar tissue plays a role in tapeworm infection. Why this focus?
a: I was doing a side project on stickleback and noticed this strange fibrosis [scarring of the tissue] in their body cavity. I could see adhesions and extracellular matrix, which is almost like glue, all the organs fused together. I brought this to Daniel Polnik [the principal investigator of the research project and professor at the University of Connecticut]He stated that he had seen this before in preserved fish and thought that the scars may have occurred due to the preservation process. But when we began to pay attention to it, we realized that it is a feature of fish. We started scoring fish in the lab by how severe the fibrosis was. We’ve also taken this scoring system into the field, and so we’ve been collecting data for several years, tracking fibrosis in wild populations. It highlights the importance of being vigilant while doing your work and noticing things that seem strange to you because over time you may see a trend that leads to the formation of new scientific questions.
Q: What are the most important results of this research?
a: We discovered that fibrosis appears to be an evolving feature of stickleback fish, which forms scar tissue to stop tapeworm growth. These fish came from marine ancestors 12,000 years ago, and they did not have fibrosis. This trait evolved over time when fish were introduced to tapeworms in freshwater lakes. But not all stickleback populations produce fibrosis. Instead, they tolerate tapeworms. This may be because fibrosis comes with negative effects. It sticks the internal visceral organs together, impeding the movement of the fish. And from what we’ve shown from our work, it appears to suppress fertility. The highlight of this research for me is this immune trade-off. Is it better to fight pathogens, or is it better to preserve your fertility?
Q: What does this research tell us about the human immune system?
a: Fish immunity is somewhat similar to human immunity; They have similar components and pathways. We research fish, but our findings are relevant to any host with an infection. There will be costs, benefits, and trade-offs to fighting those infections, so these general principles will apply to human disease.