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What a diminutive, freshwater crustacean can reveal about the consequences of infection across generations

When researchers from the University of Michigan investigated the implications of a parasite on a diminutive freshwater crustacean, they encountered an unforeseen result.

The organism infected by the parasite thrived despite its ailment; however, a significant number of its offspring, which were not infected, faced early mortality. This discovery provides insight into how infections may impact not only the initial hosts but also their progeny.

“Researchers have invested considerable effort studying virulence, which is typically defined as the effect a parasite has on the fitness of its host. However, this often only takes into account the impact on the particular infected host,” stated Meghan Duffy, a U-M professor of ecology and evolutionary biology and the principal author of the study, funded by the U.S. National Science Foundation and the Gordon & Betty Moore Foundation.

“We understand that for various situations, such as when a mother is malnourished and lacks sufficient resources, it can impact her progeny. Likewise, exposure to a predator can influence her offspring. We know that these effects can transcend generations; it’s just that we haven’t fully considered this for virulence, as far as we can determine.”

Duffy investigates symbiotic interactions in freshwater plankton, often concentrating on daphnia. These crustaceans are ubiquitous in freshwater environments, resembling tiny shrimps. A mere scoop of water from a North American lake could harbor hundreds of these small, translucent creatures. They also serve as a barometer for water quality: daphnia are responsive to various pollutants, making an abundance of them in a lake a good sign, according to Duffy.

“They form a vital connection in the aquatic food web. They consume algae, preventing lakes from appearing murky. Subsequently, fish feed on them,” she explained. “There are billions of them in a lake, allowing them to consume vast quantities of algae daily. Young bluegill find them especially delectable.”

The research, published in Ecology and led by postdoctoral researcher Kristina McIntire, started almost by chance. The team was engaged in a distinct research project that required them to raise young from mother daphnia exposed to various parasites. McIntire encountered difficulties because most offspring of female daphnia subjected to a parasite, specifically the fungal-like microsporidia, were dying off.

This observation was unexpected because the parasite is not regarded as particularly harmful, Duffy maintains. Daphnia typically suffer minimal consequences from infection, exhibiting only slight impacts on their reproduction—at least until the subsequent generation of daphnia emerges.

To investigate this phenomenon, the researchers placed individual mother daphnia in beakers filled with filtered lake water. Daphnia are advantageous study subjects for various reasons, one being their clonal reproduction: a mother creates a clone of herself for her offspring. This characteristic ensures that her offspring carry identical copies of her DNA. Moreover, since each mother was isolated in a beaker, Duffy and her team could monitor the quantity of young produced by each mother.

The team introduced spores from the microsporidian to some of the mothers. Following this, they exposed a portion of the offspring from those mothers to the microsporidian, while others remained unexposed. Another beneficial attribute of daphnia is their transparency, making them visible to the naked eye. Researchers—under slight magnification—can determine whether a daphnia is infected with the microsporidian. When infected, their gut, which runs in a tube only one cell thick down its center and usually appears green, turns opaque and appears almost crystallized.

The findings revealed that numerous young produced by infected mothers perished in their juvenile stages, failing to reach maturity. Conversely, the offspring generated by mothers who had not encountered the parasite survived robustly, producing many young of their own.

Duffy hopes other researchers will explore whether the virulence of different parasites also manifests in subsequent generations, contemplating if similar effects arise from a variety of parasite types and diseases.

“We have learned from other studies that these generational impacts certainly do not occur with all parasites, but we suspect it may be more widespread than just this singular parasite,” Duffy remarked. “We need additional investigations since if you only gauge virulence in the traditional way within a single generation, you might overlook the most significant consequences the parasite can have.”

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