By Brooke Coupal
Certain populations of threespine stickleback fish have evolved over time to resist tapeworms found in freshwater lakes, but this resistance comes at a cost.
Biological Sciences Asst. Prof. Natalie Steinel and a team of researchers examined stickleback fish in British Columbia and found that fish living in Roberts Lake, about 160 miles northwest of Vancouver, had developed scar tissue, known as fibrosis, in response to tapeworms growing inside of them.
Their findings, recently published in Science magazine, show that this immune response has negative effects on the fertility of stickleback fish, which may be why other stickleback fish populations, like those found in Gosling Lake just miles away from Roberts Lake, tolerate tapeworms as opposed to developing scar tissue around them. The tapeworms that invade the fish found in Gosling Lake can grow up to one-third of the fish’s body weight.
Steinel recently gave us a closer look into 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 we were seeing in wild populations. We study wild populations of stickleback fish that all experienced the same tapeworm, but they respond differently to that infection. In one lake in British Columbia, we would see stickleback fish riddled with very large parasites, and then in another lake about 15 kilometers away, the stickleback fish had almost no infection. We were really interested in finding out if there were ecological factors at play or if something was happening genetically and immunologically that was causing the different outcomes we were seeing.
Q: Why did the research center around threespine stickleback fish?
A: We know so much about the evolutionary history of stickleback fish, so we have a lot of foundational knowledge to build off of. They have been an evolutionary biology and ecology model for a very long time, but they’re a relatively new model for immunology work. And there’s so much interesting variation amongst these fish, which are found throughout the northern hemisphere. There are enormous opportunities to take advantage of all this diversity and study interesting phenotypes [observable traits].
Q: Your team looked at how scar tissue in stickleback fish played a role in tapeworm infection. Why that focus?
A: I was doing a side project on stickleback fish and noticed this weird fibrosis [scarring of the tissue] in their body cavity. I was seeing adhesions and extracellular matrix, which is almost like glue, fusing all the organs together. I brought this up to Daniel Bolnick [the principal investigator of the research project and professor at the University of Connecticut], and he mentioned that he had seen that before in preserved fish and thought the scarring may have happened due to the preservation process. But as we started paying more attention to it, we realized it was a trait of the fish. We began scoring fish in the lab by how severe their fibrosis was. We also have taken that scoring system out to the field, and so we’ve collected several years of data, tracking fibrosis in wild populations. It highlights the importance of being observant while you’re doing your work and noting 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 were the main findings of this research?
A: We discovered that fibrosis seems to be an evolved trait for stickleback fish, who form the scar tissue to stop the growth of tapeworms. These fish came from marine ancestors 12,000 years ago, and they did not have fibrosis. This trait evolved over time when the fish were introduced to tapeworms in freshwater lakes. But not all stickleback fish populations produce fibrosis; instead, they tolerate the tapeworms. This may be because fibrosis comes with negative impacts. It glues the internal visceral organs together, which inhibits fish movement. And from what we’ve shown from our work, it looks like it suppresses fertility. The biggest highlight of this research for me is this immunological trade-off. Is it better to fight off your pathogen, or is it better to maintain your fertility?
Q: What does this research tell us about the human immune system?
A: Fish immunity is fairly similar to human immunity; they have similar components and pathways. We’re looking at fish, but our findings are relevant to any host that experiences an infection. There are going to be costs, benefits and trade-offs to fighting that infection, so those general principles will apply to human disease.