Flies alter lice evolution

At a glance, bird-dwelling feather lice species seem nearly indistinguishable, with the parasites infecting similar species, feeding upon the same feathers and dead skin, and sharing a similar lifecycle. But they can exhibit more subtle behaviors and physical features. Body lice, for example, stay close to their host's skin, digging in with their small legs. Wing lice, on the other hand, have longer legs and tend to grasp wing feathers, hiding between feather barbs to avoid being picked off during preening.

Furthermore, the two lice species have distinct patterns of speciation. While body lice tend to form a new species whenever their pigeon hosts do, wing lice seem to speciate on a schedule of their own. If you overlay the phylogenetic trees of lice and their host pigeon, "body lice evolution closely follows that of the bird but wing lice evolution is decoupled," said Harbison, who taught the writer undergraduate ornithology a few years ago. "Can we figure out any ecological factors that could be driving this pretty drastic difference in coevolutionary patterns?"

The obvious answer, said Harbison, was that wing lice tend to infect multiple host species. It could be that the lice are better able to reach novel hosts, or they are more successful once they arrive. Previous research nixed the latter hypothesis -- body and wing lice were equally able to establish populations on different bird species when placed there experimentally. But some evidence suggested that wing lice are better than body lice at grabbing a hold of pigeon flies, parasites that feed on bird blood, providing a possible vehicle on which the wing lice spread to new bird species.

To test this hypothesis, Harbison and his colleagues housed wild-caught rock pigeons with mourning doves in sheds with or without pigeon flies. While the pigeons had a full suite of ectoparasites on their bodies, the doves had been cleaned of parasites to serve as potential new hosts for the lice.

In the fly-filled shed, wing lice successfully infected seven (21 percent) of the once parasite-free doves, but only one was infected by body lice. In the shed without flies, no wing lice and only one body lice infection occurred on the doves, suggesting the flies were integral in the spread of wing lice between species.

"On face value, the paper is a very simple study: It's just birds in a shed with flies," said evolutionary biologist Noah Whiteman of the University of Arizona who was not involved in the research. "But what it does is follow macroevolutionary pattern right down to ecological process. [That] is a tall order."

C.W. Harbison and D.H. Clayton, "Community interactions govern host-switching with implications for host-parasite coevolutionary history," Proceedings of the National Academy of Sciences, doi: 10.1073/pnas.1102129108, 2011.