Researchers find natural section favors parasite fitness over host health

May 12, 2008

Why do parasites harm their hosts? Classic evolutionary theory predicts that parasites become more virulent because they must transmit themselves between hosts, yet scientists have found little data to support this idea, until now.

Led by Emory University researcher Jacobus de Roode, PhD, a team of scientists has uncovered evidence that natural selection selects for harmful parasites by maximizing parasite fitness.

De Roode and co-authors Andrew Yates, PhD, Emory University; and Sonia Altizer, PhD, University of Georgia, studied monarch butterflies Danaus plexippus infected with parasite Ophryocystis elektroscirrha and observed that higher levels of replication within the host resulted in both higher virulence and greater transmission of the parasite.

The study will be published online in the Proceedings of the National Academy of Sciences.

"A fundamental evolutionary question is why parasites that depend on their hosts for their own survival and fitness hurt or even kill them," says de Roode. "According to theory, parasites face a trade-off between the benefits of increased replication, the transmission to new hosts and the costs of host mortality, resulting in the highest fitness at intermediate parasite replication. During the past 30 years there has been very little experimental evidence that this trade-off actually exists. This is one of the first demonstrations that really shows that this trade-off model applies.

"These findings support the idea that selection for parasite transmission can favor parasite genotypes that cause substantial harm," he says.

In natural populations, D. plexippus become infected as caterpillars after they ingest spores of O. elektroscirrha that are scattered onto eggs or host plant leaves by adult butterflies. The parasites then penetrate the gut wall and replicate, forming spores around the scales of the developing butterflies.

"Greater parasite replication reduced host survival to the adult stage, with fewer monarchs emerging successfully from their pupal cases," says de Roode. "Among female monarchs that survived to the adult stage, higher parasite loads reduced mating success, in part by reducing the female lifespan.

"Harmful effects from the parasites on the host may appear maladaptive," says de Roode. "But high parasite loads were necessary to increase transmission."

Because the parasites affect the butterflies' lifespan, their ability to fly, and whether they can migrate and reproduce, de Roode says he and his colleagues are now studying how the parasites' virulence level varies among monarch populations and whether migration patterns and length affect the parasites' virulence level.

Source: Emory University

Explore further: How do our muscles work? Scientists reveal important new insights into muscle protein

add to favorites email to friend print save as pdf

Related Stories

Egg colours make cuckoos masters of disguise

Nov 19, 2014

Cuckoos are notorious cheats. Instead of building a nest, incubating their eggs and raising their chicks, they lay their eggs in the nests of other birds and leave the task of raising their offspring to the ...

Viruses impaired if their targets have diverse genes

Nov 18, 2014

When a viral infection spread through five genetically identical mice in a row, the virus replicated faster and became more virulent or severe. But when the infection spread one-by-one through five genetically ...

How mutualisms evolve in a world of selfish genes

Nov 11, 2014

Reproduction for a female fig wasp can be a nightmarish process. When she is ready to lay her eggs, she leaves the fig in which she was born and became pregnant and searches for another. After she finds it, ...

Recommended for you

Genomes of malaria-carrying mosquitoes sequenced

8 hours ago

Nora Besansky, O'Hara Professor of Biological Sciences at the University of Notre Dame and a member of the University's Eck Institute for Global Health, has led an international team of scientists in sequencing ...

Bitter food but good medicine from cucumber genetics

8 hours ago

High-tech genomics and traditional Chinese medicine come together as researchers identify the genes responsible for the intense bitter taste of wild cucumbers. Taming this bitterness made cucumber, pumpkin ...

New button mushroom varieties need better protection

13 hours ago

A working group has recently been formed to work on a better protection of button mushroom varieties. It's activities are firstly directed to generate consensus among the spawn/breeding companies to consider ...

User comments : 3

Adjust slider to filter visible comments by rank

Display comments: newest first

Ragtime
1 / 5 (2) May 12, 2008
If such mechanism wouldn't work, then the predation and the whole human civilization couldn't appear. The humans are parazites of Earth biosphere in certain extent. The parasite formation is sort of phase transition, followed by spontaneous symmetry breaking. The increasing of complexity leads to the better exploatation of sources, which increases the complexity even more.
AJW
not rated yet May 13, 2008
One:
"This is one of the first demonstrations that really shows that this trade-off model applies."
Two:
"These findings support the idea that selection for parasite transmission can favor parasite genotypes that cause substantial harm," he says.

So, which is it,
'trade-off applies'
or 'favor parasite genotypes'?

Was anything show or did the summarizer get it wrong?
PPihkala
not rated yet May 27, 2009
The title 'Researchers find natural section favors parasite fitness over host health' has a typo in it. 'section' should be 'selection'.

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.