Do parasites drive avian speciation?
Life as a hybrid is not always easy. They might be infertile or unattractive to potential partner. Or they might have a higher chance of dying. In other words, hybrids often have lower fitness compared to ‘pure’ species. But what factors determine this decrease in hybrid fitness? A recent study in the journal Ecology and Evolution focused on parasites.
The Yellow-rumped Warbler (Setophaga coronata) complex comprises four subspecies (coronata, auduboni, nigrifrons and goldmani). The first two interbreed along a narrow hybrid zone in British Columbia. Previous work indicated that there is selection against hybrids, but the exact mechanisms could not be unraveled. Camille-Sophie Cozzarolo (University of Lausanne) and her colleagues tested the hypothesis that hybrids are more susceptible to parasite infections. Specifically, they focused on haemosporidian parasites of the genera Haemoproteus, Leucocytozoon, and Plasmodium that are transmitted by dipteran flies.
On the one hand, hybrids might be an easy target for parasites because they lack the resistance that ‘pure’ species have evolved (this has been shown in black duck x mallard hybrids). On the other hand, the increased genetic diversity in hybrid genomes might confer an advantage when they get infected. In the present study, the authors expected that hybrids between coronata and auduboni will be infected more. In this way, parasites are (partly) driving the speciation process.
Contrary to their expectations, the researchers found no support for their parasite-driven speciation hypothesis. Hybrids did not have a higher infection prevalence. Instead elevation was the most important predictor of prevalence. This is probably because the vectors of certain parasites (particularly Leucocytozoon) thrive at higher elevations where they find suitable water bodies for reproduction.
In the case of Haemoproteus parasites, there was an effect of hybrid index (i.e. the genetic background of the birds). The probability of infection in coronata strongly increased with elevation. Possibly, coronata individuals do not cope well with high elevations, making them more vulnerable for infection.
So, it seems unlikely that haemosporidian parasites play a key role in selecting against coronata x auduboni hybrids. Although the outcome of this study is negative (it would have been very cool if parasites are driving bird speciation), it is important that this paper has been published. Only reporting positive results will lead to a publication bias and a skewed perspective on the role of parasites in bird evolution.
Cozzarolo, C.S., Jenkins, T., Toews, D.P.L., Brelsford, A. & Christe, P. (2018) Prevalence and diversity of haemosporidian parasites in the yellow-rumped warbler hybrid zone. Ecology and Evolution
This paper has been added to the Parulidae page.