The researchers crunched the numbers and found clear support of a hypothesis that is popular among scientists but, until now, had not been well studied in a naturally occurring parasite and its host. They found that female monarchs that were too heavily infected often died before they mated or, if they survived, did not mate. Females who had an intermediate parasite load were long lived and laid a large number of eggs, while females with light parasite loads also were long lived and had many offspring, but relatively few offspring were infected.
The findings of this study are significant because they provide an explanation for why so many parasites cause disease and death to their hosts, de Roode said.
The results demonstrated that the trade-off hypothesis holds for monarchs in a controlled setting, but the researchers also wanted to know how natural selection influences virulence in the wild. They isolated parasite strains from monarch butterflies in eastern and western North America that have different migratory behaviors. Eastern monarchs famously migrate from breeding grounds as far north as Canada to their wintering sites in central Mexico and then back again, traveling up to 5,000 km round-trip. Western monarchs migrate roughly a third of this distance to winter along the California coast.
We thought that if the parasites are going to be more benign in any population, its going to be in the eastern monarchs, Altizer said, because those butterflies fly the farthest distances, and parasites that kill their hosts during this long journey wont produce any offspring.
The team exposed western and eastern monarchs to parasite strains from their own and the opposite population. Monarchs from both populations were equally susceptible to infection, but parasites from
|Contact: Sam Fahmy|
University of Georgia