Parasite Resistance May Not Always Be Evolutionarily Advantageous

A strong immune response to disease is beneficial, but it can also be a double-edged sword. So says new research, published in the Sept. 9 issue of Science, by a team that includes Haverford Assistant Professor of Biology Foen Peng. 

Peng and his collaborators at the University of Wisconsin–Madison, the University of Massachusetts Lowell, and the University of Connecticut studied the immune response of threespine stickleback fish to freshwater tapeworms. They found that while fish in some lakes gained resistance to the parasite, that resistance came with a substantial cost—a substantial loss of fertility—while fish in other lakes managed to live with the tapeworm and to produce a large number of offspring. It is generally assumed that natural selection favors the infection-resistant, but this paper shows that when immune response is costly, particularly to the ability to produce offspring, selection may, in fact, favor a strategy of coexistence with parasites.

“This is one of the few cases where loss of resistance has been revealed in natural populations,” said Peng. “In addition, we also found several promising candidate genes that underlie the differences in immune responses between these fish populations, and some of these genes are homologs of important human immune genes.”

Peng, who joined the Haverford faculty last fall, conducted his part of the research during his postdoctoral fellowship in Dan Bolnick’s lab at the University of Connecticut. His contributions to the paper include using bioinformatic tools  to identify the genes that could cause the differences in immune responses in the sticklebacks, and then generating transgenic fish with CRISPR genome editing technology to test the gene's effects. 

“The different fish populations demonstrated consistent responses in the lab, compared to the field, when experimentally exposed to tapeworm parasites,” he said, “so we know the finding in the lab is valid.” 

Because the genes identified in this research (e.g., PU.1 or STAT6) are important genes in the human immune system, this work has implications out of the water as well. For example, understanding how fish do or do not generate fibrosis can provide important insight for treating fibrosis disease in humans. 

Peng’s lab at Haverford is captivated by similar questions around the genetic basis of adaptation and speciation, though he and his students are now studying how plant species adapt to different animal pollinators. 

“The flowers I work with have a shorter generation time, compared to stickleback fish,” he said, “which is a big advantage for conducting genetic research in a small liberal arts college."