Pulling around a tapeworm that is a third of your body weight can be a real drag. In an article published in Science Thursday, a team of researchers showed that stickleback fish have developed resistance to tapeworms, but resistance has its own costs.
When stickleback fish left marine waters to colonize northern freshwater lakes nearly 12,000 years ago, they encountered freshwater tapeworms. Parasites invaded their stomachs and grew, reaching enormous sizes of up to a third of the host fish’s body weight. This is equivalent to a human carrying a 50-pound tapeworm. However, some stickleback groups soon developed defensiveness. When confronted with a tapeworm, their immune systems form scar tissue around the parasite, stopping its growth. But other groups of thornbushes bore the worms instead, and they scarred little or no scarring at all.
Populations of sticklebacks that scare off tapeworms, and those that don’t, can live close to each other in lakes just miles away. Until now, no one has understood why some groups of echinoderms have evolved one way or the other.
“We see this in Alaska, in British Columbia. Colleagues in Scandinavia have seen it,” says University of Connecticut biologist Dan Polnik.
“The wonderful thing about coevolution between tapeworms and fish is that it is a remarkably dynamic process, and there are different outcomes of this evolutionary battle everywhere we look,” says Jesse Webber, a biologist at the University of Wisconsin-Madison.
Bolnick, Weber, and Steinel worked together to answer the question of stickleback resistance. Along the way, they’ve shown resistance to parasites isn’t always a good thing.
The trio studied sticklebacks from Lake Roberts and Gosling on Vancouver Island in British Columbia. Both lakes contain tapeworms and both contain echinoderms. The two groups of stickleback are very similar. The main difference is that Roberts fish scar strongly to prevent tapeworms from growing while gosling fish do not. The only other obvious difference is that Roberts females reproduce much less successfully than Gosling females, apparently because the scar tissue in their stomachs makes it more difficult.
The researchers wanted to find out what genes are responsible for scarring and whether the scarring was the reason why Roberts females did not reproduce either. But if they simply compared the genomes of Roberts and Gosling fish directly, they might be confused by other genetic differences between the populations that have nothing to do with the scars. They had to confuse the two groups, so the only consistent difference between the two was the trait of the scars.
To rearrange the genetic surface, the researchers crossed the fish from Roberts and Gosling. These Roberts-Gosling hybrids were all alike, each with half of their genes from each group. These hybrids were then mated to create a second generation. The second generation had many different combinations of genes with individual fish having different traits from each other, their hybrid parents, and from the Roberts-Gosling grandparents generation.
This genetically mixed second generation was the one to which the researchers were exposed to tapeworms.
After exposing them for a set number of days, the team looked at the relative amounts of scars and tapeworms in each fish. They analyzed the genomes of fish with heavy worms, and compared them to the DNA of fish with heavy worms. They narrowed the differences down to a handful of genes and looked carefully to see which genes were most active.
They found that one of the most active genes was closely linked to scarring in mice.
You might be surprised to see a rat scar in the same way as fish. But the immune system controls scarring, and it’s similar in all vertebrates, from fish to mice to humans.
The researchers then looked at this gene in the two original groups. The researchers found that the gene had recently evolved in the genome of the stickleback, the fish that tolerates tapeworms without scarring. There appears to be ongoing evolutionary pressure to tolerate tapeworms rather than their scars.
Steinel of UMass Lowell said: “In this article, we address the issue of immune/pathogen co-evolution using fish, but these principles are broadly applicable to other animal systems, including human infection. To successfully manage infectious diseases, we must understand the balance between The costs and benefits of the immune response”.
“This is one of very few research papers that have been done both in the wild and in the lab to show a significant fitness cost” to parasite resistance, Polnik said. But this is logical. Female sticklebacks with a lot of stigmata are 80% less likely to reproduce successfully. Tapeworms do not appear to affect reproduction, although they slow the movement of fish and make them more likely to be eaten by birds.
“When we jump in and look at these systems, we can learn a lot not only about the process of evolution, but also about new mechanisms that are of value applied to people and livestock — mechanisms like how your immune system recognizes your immune system, how you fight off a parasite, how you turn off an unwanted immune response. in them,” Weber said.
This work was funded by an Early Professional Scientist Fellowship at the Howard Hughes Medical Institute, as well as grants from the National Institutes of Health.