Parasite Mediated Sexual Trait Divergence
The comparative piece of my dissertation research was focused on understanding how parasites can influence the evolution of their hosts. Specifically, I looked at how local adaptation to parasite communities may drive sexual trait divergence across closely related subspecies.
Sexual traits often advertise information about parasites. Females may gain benefits from choosing males based on sexual traits, such as parasite resistant genes passed to offspring, or direct benefits, such as higher quality territories or better parental care. Understanding what benefits females gain within populations by using male sexual traits will help us to better understand what is driving the evolution of sexual signals and why they change across populations.
To answer these questions, I studied the local parasite community, the cost of those parasite infections and co-infections, and the information content of male sexual traits in three closely related barn swallow subspecies with divergent sexual signals. This work took place in the Czech Republic where females choose males with longer tail streamers, in Colorado where females choose males based on dark ventral plumage, and in Israel, where females choose males based on both tail streamer length and dark ventral coloration. Within each of these populations, I compared how sexual traits expression reflects information about local parasite communities and what benefits females gain by using sexual traits to select mates.
This work is possible because of several amazing collaborators, including: Dr. Tomás Albrecht in the Czech Republic, Dr Yoni Vortman in Israel, and Dr. Basma Sheeta in Egypt.
Parasite Manipulation Experiments and Mechanisms in Colorado Barn Swallows
While the overarching question of my PhD was focused on the role of parasites in speciation, and involves large comparative work across the barn swallow species complex, I was still very interested in uncovering the mechanisms behind the broad patterns that I found. While this is not always feasible to do across four countries, it is possible to get down to the details with a local population of barn swallows in Colorado.
I have found that the parasite that has the largest impact on Colorado barn swallows is the nest mite. These are small blood feeding ectoparasites that live in barn swallow nests and feed on nestlings. In order to determine the impacts that these mites have on nestlings, I did several experiments where I removed mites from nests by heating them up with a heatgun, and adding set numbers of live mites into half of my experimental nests.
I have used this experimental setup to look at how parasites impact nestling development, physiology, immune responses, survival, and sexual trait development, as well as how mites impact adult behaviors like territory settlement, incubation, and provisioning.
Melanin Sexual Traits: Linking Parasites to Gene Expression
In the North American and Israeli subspecies of Barn Swallows, we know that females use rust colored breast and throat plumage to select mates. This color is produced by melanin. Historically, melanin-based color traits were thought to be under strict genetic control, which makes it unclear how melanin sexual traits can serve as honest indicators of male quality. However, new information suggests that condition-dependence for some melanin traits may occur during early development. If so, trait variation may provide information about an individual’s developmental history to females.
The color that individual barn swallows develop as fledglings is predictive of their color as breeding adults, despite repeated molting of feathers. However, in collaboration with Dr. Joanna Hubbard, we have shown that variation in melanin-based color in barn swallows is due largely (~70%) to differences in early environment. With only about 18% of color expression explained by additive genetic variation (which is quite low for a melanin trait).
To better understand how genes and the environment are interacting to influence the expression of melanin color in barn swallows, I set up a large field experiment to cross-foster eggs and manipulate parasites in nests. Working with Dr. Scott Taylor, I am now working to analyze gene expression data from developing feather tissue collected in this experiment. The goal is to understand how the early environment interacts with genes in the melanin pathway to influence the life-long expression of a melanin-based sexual signal.
This research will have important implications for understanding the gene by environment interactions during the early developmental period for melanin-based ornamental traits, and thus, information that females gain by using these traits.
Special thank you to my amazing field crew: Sheela Turbek, Kelley McCahill, Mara Hernandez, Will Dube, Clair Mastrangelo, Kim Greene, Dan Tomlin, and Heidi Reeg.
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