A new study has revealed that less resistance can sometimes be better than more especially when it comes to battling an epidemic of a deadly parasite.
A freshwater zooplankton species known as Daphnia dentifera
endures periodic epidemics of a virulent yeast parasite that can infect more than 60 percent of the Daphnia population. During these epidemics, the Daphnia
population evolves quickly, balancing infection resistance and reproduction.
A new study led by Georgia Institute of Technology researchers reveals that the number of vertebrate predators in the water and the amount of food available for Daphnia
to eat influence the size of the epidemics and how these "water fleas" evolve during epidemics to survive.
The study shows that lakes with high nutrient concentrations and lower predation levels exhibit large epidemics and Daphnia
that become more resistant to infection by the yeast Metschnikowia bicuspidata
. However, in lakes with fewer resources and high predation, epidemics remain small and Daphnia
evolve increased susceptibility to the parasite.
"It's counterintuitive to think that hosts would ever evolve greater susceptibility to virulent parasites during an epidemic, but we found that ecological factors determine whether it is better for them to evolve enhanced resistance or susceptibility to infection," said the study's lead author Meghan Duffy, an assistant professor in the School of Biology at Georgia Tech. "There is a trade-off between resistance and reproduction because any resources an animal devotes to defense are not available for reproduction. When ecological factors favor small epidemics, it is better for hosts to invest in reproduction rather than defense."