What a tiny, freshwater crustacean can tell us about the effects of infection over generations
Study: Transgenerational pathogen effects: Maternal pathogen exposure reduces offspring fitness (DOI: 10.1002/ecy.70165)
When University of Michigan researchers were looking at the effects of a parasite on a tiny freshwater crustacean, they found something unexpected.
The organism infected with the parasite fared well despite their illness, but many of their offspring, who were not infected, died young. The finding sheds light on how illness may affect not just the organisms who originally contract the infection, but their children as well.
"Scientists have spent a lot of time studying virulence, which is generally defined as the impact a parasite has on the fitness of its host. But all of that usually only considers the impact on the particular host individual who is infected," said Meghan Duffy, U-M professor of ecology and evolutionary biology and senior author of the study, which was supported by the U.S. National Science Foundation and the Gordon & Betty Moore Foundation.
"We know for other things, like if a mother is starving and doesn't have enough resources, that can influence her offspring. If she's exposed to a predator, that can influence her offspring. We know these effects can carry across generations. We just somehow haven't really considered that for virulence, as far as we can tell."
Duffy studies symbiotic relationships in freshwater plankton, often focusing on daphnia. Daphnia are an incredibly common freshwater crustacean, sort of like a tiny shrimp. One scoop of water from a North American lake might have hundreds of the tiny, transparent crustaceans swimming through it. They are also an indicator of water quality: daphnia are sensitive to many pollutants, so having a lot of daphnia in a lake is a good sign, Duffy says.
"They are a really key link in the lake food web. They eat algae, so they keep lakes from looking like pea soup. Then, fish eat them," she said. "There are billions of them in a lake, so they can eat a lot of algae in a day. Then, young bluegill think they are just so tasty."
The study, published in Ecology and led by postdoctoral researcher Kristina McIntire, began almost accidentally. The researchers were attempting a totally different research project in which they needed to rear offspring from mother daphnia that had been exposed to different parasites. McIntire was frustrated because most of the offspring of female daphnia exposed to a parasite, a fungal-like parasite called microsporidia, were perishing.
The observation was surprising because the parasite is not considered to be very harmful, Duffy says. Daphnia tend to experience minimal effects from infection, including only small effects on how much it reproduces-at least until the next generation of daphnia is born.
To study this effect, the researchers placed individual mother daphnia in beakers filled with filtered lake water. Daphnia are good study subjects for several reasons, one of which is that they reproduce clonally: the mother makes a clone of herself to produce offspring. This means her offspring carries identical copies of her DNA. And because they were able to keep each mother isolated in a beaker, Duffy and her team were able to track how many babies the mother produces.
The researchers introduced spores from microsporidian to some of the mothers. Then, they exposed some of the offspring of those mothers to the microsporidian, while others were not exposed. Another helpful trait of daphnia is that they are transparent, and they are visible to the naked eye. Researchers-under a small amount of magnification-can see whether a daphnia is infected with the microsporidian. When infected, their gut, which runs in a tube only one cell thick down her middle and is usually green, becomes opaque and looks almost crystallized.
What they found is that many of the offspring produced by infected mothers died as juveniles, not reaching adulthood. In contrast, the offspring produced by the mothers that hadn't been exposed to the parasite survived well, producing many offspring themselves.
Duffy says she would like other researchers to look into whether virulence of other parasites also shows up in future generations, wondering if similar impacts spring from many different types of parasites and diseases.
"We know from other studies we've done that these generational effects certainly don't happen with all parasites, but we suspect it's probably happening more broadly than with just this one parasite," Duffy said. "We need other people to start looking into this because if you're only measuring virulence the way we normally do within a generation, you might miss the biggest impact the parasite is making."
