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About Blog Author: Dr Peri Bolton (@periperipatus) is a Postdoctoral Scholar at East Carolina University, and a Fellow at the Smithsonian National Museum of Natural History. She uses genetic techniques to study questions about bird behavior and conservation. This project is in collaboration with head curator of Sylvan Heights Bird Park (@sylvanheights), Dustin Foote and Dr Christopher Balakrishnan (@EvolutionPirate).
Diseases can be threatening for conservation of wild populations, as well as captive breeding programs. The captive population of the Endangered white-winged duck (Asarcornis scutulata) is extremely susceptible to avian tuberculosis (Mycobacteriosis) (Figure 1).
Arboreal ducks more broadly appear to be susceptible to Mycobacteriosis (Cromie et al. 1991), but this pathogen accounts for at least 85% of mortality in UK and USA captive populations of white-winged ducks (Cromie et al 1992; Saggese et al 2007). Sylvan Heights Bird Park owns the largest captive breeding population of white-winged ducks in north America, and they have been working with East Carolina University for years to help improve conservation of these ducks. Genome sequencing of captive populations suggest that inbred individuals are dying younger from Mycobacteriosis (Figure 2). This suggests a genetic component to reduced immunity in these captive birds.
One of the key components in the immune system are the products of the Major Histocompatibility Complex (MHC) genes. These genes have multiple alleles and paralogs, and form an important component of antigen recognition in cell-mediated immunity. Indeed, mice with MHC mutations have different antigen reactivity to a related Mycobacterium (Huygen et al 1993). In collaboration with Dustin, I have already generated preliminary estimates of MHC copy number using whole genome sequencing (~10-15X). The sequencing results suggest the WWD has two MHC Class-I and Class-II paralogs each, less than in the closely related tufted duck (Aythya fuligula), and a substantial loss of heterozygosity in Class-I alleles in captive populations (Foote et al. 2021). Therefore, the genetic diversity of the MHC could be an important determinant in susceptibility to mycobacterial infection in ducks.
I aim to characterise the immune response to Mycobacterial infection in ducks using RNA sequencing. The first way I will do this is identifying differentially expressed genes in between white-winged ducks with Mycobacteriosis, compared with blood from healthy individuals. To get a sense of any dysfunction in immune response, I will then compare these samples against blood samples representing sick and healthy redhead ducks (Aythya americana). We already have one of these paired samples from redhead ducks at Sylvan Heights Bird Park, and the remainder will be collected this year. Then, I will test the hypothesis that reduced MHC diversity influences susceptibility to Mycobacterial infection. To do this, I will use these transcriptomic data to characterize MHC copy number and allelic diversity within and between each species.
We also have ten lung tissue samples from white-winged ducks that were necropsied. Half of these individuals were infected with mycobacteria at death, and half died from unrelated causes. In most birds Mycobacteriosis manifests as gastrointestinal lesions, but lesions are common in the lungs of white-winged ducks. I will compare gene expression between these samples to understand local immune responses, and against the blood samples to explore tissue specific responses.
This study will be the first to use transcriptomic data to holistically explore immune responses to mycobacterial infection in any bird. We also hope to clarify the role of MHC in disease susceptibility within and between duck species. Our study results will be used by Sylvan Heights Bird Park, and other conservation organizations, to improve health outcomes for white-winged ducks through captive breeding management.
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