Evolution and mechanisms of disease tolerance

Healthy (left) and MG infected (right) house finch.

Sampling house finches in Davis, CA for our population comparison study. Photo courtesy of James Adelman.

I am exploring the evolution and underlying mechanisms of disease tolerance in house finches (Haemorhous mexicanus) infected with an emerging bacterial pathogen, Mycoplasma gallisepticum (MG). MG causes severe conjunctivitis in house finches and the introduction of this pathogen to naïve populations results in severe population declines. Since the initial spillover of MG from domestic poultry in the early 1990's in Virginia (USA), MG has steadily spread to most house finch populations across the United States. With my postdoc mentor, James Adelman (University of Memphis), and our collaborator Dana Hawley (Virginia Tech), I am investigating how house finch populations are adapting to MG through a large-scale population comparison. By comparing both pathogen load and pathology (severity of conjunctivitis) during infections, we are able to determine if populations of house finches are evolving resistance, tolerance, or both to MG. While resistance mechanisms directly decrease pathogen load and are relatively well characterized in animals, tolerance mechanisms decrease the per-pathogen fitness cost of infections without directly decreasing pathogen load and have received far less attention in animal studies.

This project has three main components:

1) Determining if tolerance is evolving to MG in house finches.

We are comparing populations with differing histories of MG endemism (Fig. 1) and predict that tolerance to MG will be higher in populations with a longer history of MG endemism.

2) Identifying the underlying mechanisms of both resistance and tolerance.

We are using RNAseq data to compare gene expression during MG infection between individuals from populations that vary in tolerance and resistance to identify pathways that may control response to MG.

3) Understanding how tolerance affects disease transmission.

Tolerance generally decreases sickness behaviors and increases the infectious period and is thus predicted to increase disease transmission. However, tolerance may also decrease transmission-relevant pathology (e.g., conjunctivitis in house fiches infected with MG) and therefore may ultimately decrease transmission. We explore these ideas in Henschen and Adelman (2019) and will address them within the house finch-MG system in the future.

Fig. 1. Years of MG endemism for each of our sample populations. Our populations span the range of MG endemism in house finches and include Blacksburg, VA and Auburn, AL (~25 years), Ames, IA (~20 years), Bothell, WA and Davis, CA (10-15 years), and Tempe, AZ and Honolulu, HI (0-10 years).

A house finch perches among the grapevines in Davis, CA. Photo courtesy of James Adelman.

Male ornamentation and physiological quality in male common yellowthroats

Fig. 2. The relationship between mask size and resistance to oxidative stress. Males with larger masks are more resistant to oxidative stress (time to 50% hemolysis of red blood cells after exposure to pro-oxidant).

Fig. 3. The relationship between mask size and increase in corticosterone (CORT). Males with larger masks have a greater increase in CORT after 30 minutes of capture and handling stress than males with smaller masks.

A male common yellowthroat (Geothlypis trichas).

Male ornaments are thought to honestly signal quality to potential mates. However, exactly what aspects of physiological and genetic quality are signaled by these often flamboyant traits is less clear. I am exploring links between male ornaments and quality in the common yellowthroat, Geothlypis trichas. Male yellowthroats have both a melanin-based mask and a carotenoid-based bib, which are under selection by both female choice and male competition. In Wisconsin, female yellowthroats prefer mates with a larger black mask and males with larger masks are dominant over males with smaller masks.

Through work with my dissertation advisors, Peter Dunn and Linda Whittingham, I have shown that males with larger masks are more resistant to oxidative stress (Fig. 2.) (Henschen et al., 2016) and have a more robust acute stress response (Fig. 3.) (Henschen et al., 2018) than males with smaller masks, but mask size is not related to blood parasite load (Henschen et al., 2017). This work suggests that large masks are, indeed, a marker of a high quality mate.

We are also using RNAseq to compare gene expression between developing ornamental (mask and bib) and non-ornamental (body) feathers to determine which physiological processes are important for ornament expression.

Preparing mist nets in the Cedarburg Bog State Natural Area, Saukville, Wisconsin (USA). Photo courtesy of UW-Milwaukee Alumni Magazine.

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