Experimental evolution of jellyfish-algal
The Paradox of infectiously transmitted
My empirical work on jellyfish-algal symbiosis was developed
with a postdoctoral fellow formerly at the University of Texas; T.P.Wilcox.
We study the upside-down jellyfish, a Caribbean species, at sites throughout
the Florida Keys. Jellyfish (and their relatives: corals, cubazoans
and hydras) have an ancient mutualism with marine algae in which there
is exchange of algal photosynthetic nutrients for shelter and nitrogen
from the host. This mutualism is considered paradoxical in some species,
because there is infectious (horizontal) transmission of algae between
hosts. Optimal virulence theory predicts that parasitism (cheating)
can evolve in symbionts that spread infectiously between hosts. This
is because cheater strains can arise, which benefit by taking extra
resources from their host then simply move on to new hosts. Cooperation,
however, is expected when symbionts are directly inherited from host
parents to their offspring; vertical transmission. Cheating is not
expected here since a cheater that takes extra resources from its host
would ultimately lower its own reproduction. In infectiously transmitted
symbionts, explaining the maintenance of cooperation remains elusive.
This work will further the understanding the maintenance of this poorly
understood yet widespread type of symbiosis.
Experimental Evolution on Jellyfish
Here, we use experimental evolution to demonstrate that cheating algal
symbionts exist in nature. We experimentally manipulated transmission
mode in jellyfish symbionts to test virulence theory on symbiont evolution.
Wild algal symbionts were infected into clonal hosts and experimentally
evolved. In one treatment, repeated vertical transmission of algae selected
for symbiont cooperation (because symbiont fitness was tied to host
reproduction). In the other treatment repeated horizontal transmission
promoted symbiont cheating. Fitness estimates revealed evolution of
cheating symbionts in the horizontal treatment. Cheater algae emerged
with fast within-host growth and high dispersal rates from hosts (compared
to vertical treatment) causing declined host growth. Cheaters could
not dominate because a trade-off existed between harm to hosts and symbiont
transmission. Such trade-offs are predicted by virulence theory and
may be critical in stabilizing ‘infectious’ symbioses.
This work was recently highlighted in Science Magazine PDF