Introduction
There is a growing body of evidence that epigenetics is important to many aspects of avian biology. There are three molecular epigenetic mechanisms: DNA methylation, histone modification, and chromatin structure. All are critical for the function and survival of multicellular species, and therefore all are important to birds. Of the three, DNA methylation is the most well-studied to date (Schrey et al. 2013; Kilvitis et al. 2014).
In birds, DNA methylation can regulate gene expression (Kilvitis et al. 2019) and vary both among tissues (Siller and Rubenstein 2019) and developmental stages (Sun et al. 2020). DNA methylation is important for an individual bird’s response to stress (Taff et al. 2019, 2024; Siller Wilks et al. 2024) and changes in the environment (Sheldon et al. 2018a; Chen et al. 2022; McNew et al. 2024). DNA methylation differs in birds among developmental temperatures (Sheldon et al. 2020), but not in response to all developmental stressors (Sepers et al. 2023). DNA methylation in birds also varies in response to infection (Lundregan et al. 2022), arsenic contamination (Laine et al. 2021), lead pollution (Makinen et al. 2022), and urbanization (Watson et al. 2020). Further, DNA methylation differs with brood size (Sheldon et al. 2018b), among postnatal environments (Sepers et al. 2024) and with early life condition (Rubenstein et al. 2016).
A defining characteristic of DNA methylation is that it can change rapidly and dynamically over time and be correlated to changes in RNA expression (Lindner et al. 2021b). This temporal change can be driven by reproductive behavior (Liebl et al. 2021), the initiation of reproduction (Lindner et al. 2021a), seasonal factors (Viitaniemi et al. 2019), and the myriad environmental changes faced by organisms introduced to areas outside their native ranges (Lauer et al. 2024). Introduced species provide a unique opportunity to ask how rapid changes in DNA methylation occur in populations with different histories. One suggestion is that birds in introduced populations are successful colonizers because they can use methylation to adjust gene expression rapidly in response to changes in the environment (Chen et al. 2022). Indeed, phenotypic plasticity is one of the best predictors of the ability of a lineage to thrive outside its native range.
The house sparrow (Passer domesticus ) is one of the world’s most successful introduced species (Liebl et al. 2015). This success is likely the result of its ability to rapidly respond to new environments (Anderson 2006; Lima et al. 2012; Martin et al. 2014), including through DNA methylation. DNA methylation is important to the success of introduced populations. DNA methylation varies among house sparrows from different introduced locations (Schrey et al. 2011, 2012; Sheldon et al. 2018a), and it may compensate for decreases in genetic diversity associated with introduction (Liebl et al. 2013). In house sparrows, DNA methylation varies with time since introduction among putativeToll-like Receptor (TLR ) promoters (Hanson et al. 2022), and DNA methylation of a specific CpG site (genomic motif where a cytosine is immediately followed by u a guanine) in the putativeTLR-4 promoter is associated with the expression of this gene (Kilvitis et al. 2019). Further, DNA methylation is more variable among individuals from introduced locations compared to native locations (Lauer et al. 2024), a pattern that is consistent with epigenetic buffering (O’dea et al. 2016), a mechanism in which individuals responding to a stressor leverage rapid epigenic-based modifications to facilitate resiliency and suppress transposons (Deniz, et al. 2019).
Our objectives were to investigate the change in DNA methylation within individuals, over time, in response to a simulated infection. We compared patterns of DNA methylation among wild-caught individuals from both the introduced and native range of house sparrows before and after exposure to a highly immunostimulatory element of E.coli (i.e., lipopolysaccharide). We characterized the number of CpG sites with significant change in DNA methylation before and after simulated infection, the direction of the change, and the magnitude and variance of the change. We hypothesized that individuals from introduced populations would change DNA methylation at more CpG sites, with greater magnitude, and greater variance, indicative of an “introduced-bird” phenotype of higher reliance on epigenetic mechanisms and supporting epigenetic buffering.