Everywhere I’m Local
The cost of travel: how dispersal ability limits local adaptation in host–parasite interactions (2020) Johnson et al., Journal of Evolutionary Biology. https://doi:10.1111/jeb.13754
There are countless parasites in nature, and many of them tend to have relatively short life-cycles. For example, ticks live for about two years, while may of their hosts (us included) live for much longer. Because there is such a disparity in lifespan, parasites are predicted to have a greater evolutionary potential than their hosts. In other words, parasites should evolve faster than their hosts, which theoretically means that parasites should be more fit on local hosts than they would be on non-local hosts, as they would have had more time to adapt (i.e., local adaption, see Did You Know?).
Despite these predictions, the evidence from experimental studies of parasite local adaptation is mixed at best. Some studies show the adaptation to local hosts we’d expect, but some studies don’t. One reason for the lack of consistent evidence is that parasite dispersal between habitats can limit the ability of parasites to adapt. To help explain that I’ll use a comparison to cooking. If you are cooking a dish and you want to make it spicier you add in more spice. But imagine that when you add in that spice, you are also adding a lot of cream. The dish could be spicy, because you are adding spice, but the cream is diluting the spice and masking any potential heat. That is what parasite dispersal does to local adaptation: parasites within a given habitat (the dish) may have the ability to adapt to their hosts (become spicier), but because parasites from other habitats (the cream) are coming into their habitat and diluting those adaptations it masks any overall adaptation to the host (never gets spicy). Today’s authors therefore wanted to test how parasite dispersal affected local adaptation to hosts.
Did You Know: Local Adaptation
Local adaptation is exactly what it sounds like, in that a given organism is adapted to the habitat that it lives in. In parasite studies, this is usually tested using reciprocal transplant experiments where hosts and/or parasites are taken from their local environment and put into a different one. The better a parasite/host performs in their local habitat compared to the non-local habitat, the better locally adapted we say they are.
What They Did
To test how parasite dispersal affects adaptation to local hosts, the authors used two trematode species. The first trematode (Ribeiroia ondatrae, hereafter referred to as “bird trematode”) uses predatory birds as its final host, and because of this it tends to be dispersed over large distances by the birds. The second trematode (Paralechriorchis syntomentera, hereafter referred to as “snake trematode”) uses snakes as its final host, which means that it is dispersed over considerably smaller distances than the bird trematode. Both parasites often use the pacific treefrog (Pseudacris regilla) as an intermediate host, before jumping to the snake or bird when the frog gets eaten.
For their experiment, the authors first collected pacific treefrog hosts, bird trematodes, and snake trematodes from 15 populations in the western US spanning a distance of ~900km. They then made multiple crosses of using both local and non-local hosts and parasites. This allowed them to compare the effects of local parasites on local hosts, non-local parasites on local hosts, local parasites on non-local hosts, and non-local parasites on non-local hosts.
The authors used “infection success” as there measure of local adaptation. Infection success is simply a measure of how many parasites successfully parasitized their host. Each host received 30 trematode cercariae (infective stages of the parasite), so if local parasites had an infection success of 0.50 on local hosts that means that 15 of the parasites successfully parasitized the frog.
What They Found
Bird trematodes were equally successful on local and non-local hosts, even when those non-local hosts came from hundreds of kilometers away. Snake trematodes, however, were indeed locally adapted to their hosts, showing decreasing infection success with greater distance from their local hosts. An interesting results was the the bird trematodes were not only not locally adapted, but they were also more successful overall than the snake trematodes.
While not a issue, an important caveat for this paper is that it is one of only a few studies that have experimentally tested the effects dispersal on parasite infection success and local adaptation. That does not mean that these results won’t hold up as more tests are conducted, but it simply means that further research is necessary to discover how ubiquitous the pattern uncovered here is.
In science, it’s hard to test every idea out there. Plenty of theory involves predictions and models without doing experiments, and that is a completely valid and important thing to do, but what I really appreciate about this paper was the authors taking theory on parasite local adaptation and applying it to a relevant model system to experimentally test exactly what theory predicted. Not only are these results super interesting (I mean LOOK at that figure!), but they can also be applied to predict novel interactions between hosts and parasites.
Adam Hasik is an evolutionary ecologist interested in the ecological and evolutionary dynamics of host-parasite interactions and is currently working like a madman to finish his PhD. You can read more about his research and his work for Ecology for the Masses here, see his personal website here, or follow him on Twitter here.