Phenotypic plasticity is a major mechanism whereby organisms adjust their traits to changes in environmental conditions. In the context of range expansions, plasticity is especially important, as plastic changes in traits can lead to rapid adaptation. For these reasons, there has been growing interest in the role of molecular epigenetic processes in range expansions. One epigenetic process in particular, DNA methylation, enables organisms to adjust gene expression contingent on the environment, which suggests it may play a role in some invasions. Nevertheless, we know little about how methylation is regulated in wildlife, especially expression of the enzymes responsible for altering methyl marks on the genome. The most important forms of these enzymes in vertebrates are DNA methyltransferase 1, which largely maintains existing methyl marks, DNA methyltransferase 3, which creates most de novo methyl marks, and TET2, which is a major demethylator of CpG motifs, genomic regions where most methyl marks occur. In this study, we compared expression of these genes in three tissues (i.e., gut, liver, and spleen) of house sparrows (Passer domesticus) from 9 locations. Some sparrow populations derived from the native range (i.e., Israel, Netherlands, Norway, Spain, and Vietnam) whereas others were introduced <150 years ago (i.e., Australia, Canada, New Zealand, Senegal). Our hypothesis was that non-native birds and/or birds from sites with comparatively unpredictable climates would express more of all three genes. We found, however, that DNMT expression differences, while extensive, were reversed of predictions: all three genes were expressed more in sparrows from the native range and from areas with more predictable temperatures. Surprisingly, gene expression was also strongly correlated among populations and within-individuals. Our results reveal no simple role for these enzymes in range expansions, but the appreciable among and within-population variation in these enzymes warrants more detailed investigations.

DNA methylation, which can change within-individuals over time and regulate gene expression, is important to many aspects of avian biology. It is particularly important in avian responses to various stressors associated with introductions, such as infection and environmental changes. However, it remains unclear whether native and invasive bird species differ in their epigenetic responses to stress, and how DNA methylation may contribute to the success of invasive species because of the limited availability of longitudinal epigenetic studies. To address this knowledge gap, we used epiRADseq to investigate changes in DNA methylation within-individual house sparrows (Passer domesticus) over an eight hour period in response to simulated infection. We compare wild-caught house sparrows from introduced populations with those from native populations, assessing the number of genomic locations that exhibit changes, the magnitude of those changes, and the variance among individuals. Our results show that individuals from introduced populations experience more widespread changes in DNA methylation, with greater magnitude and higher variance, compared to their counterparts from native populations. Together, these findings suggest that DNA methylation plays a significant role in an individual’s response to infection. They also indicate that individuals from introduced populations may exhibit distinct epigenetic responses compared to their native counterparts, consistent with the concept of epigenetic buffering.

Biological invasions disrupt ecosystems, economies, and disease transmission pathways, yet the mechanisms underlying invasion success remain debated. The Enemy Release Hypothesis (ERH) posits that non-native host populations thrive due to reduced pathogen pressure. While widely tested in plants and in vector-borne diseases, ERH remains understudied for enteric pathogens, which have generalist host ranges and are transmitted via faecal-oral and environmental routes. We examined the prevalence of two enteric bacteria, avian pathogenic Escherichia coli (APEC) and Salmonella enterica, in native and non-native house sparrows (Passer domesticus) from eight global populations (n = 200). We tested whether population status, sex, and urbanization influenced infection risk and examined effects on condition. Contrary to ERH, pathogen prevalence was not lower in non-native populations. Instead, we detected sex-specific amplification of infections: females from non-native populations had significantly higher odds of APEC infection than native females. Urbanization also disproportionately increased infection risk in females, highlighting sex-specific vulnerability. Infection had no measurable impact on condition, suggesting house sparrows may tolerate enteric infections, although differential mortality cannot be ruled out. These findings challenge the generality of ERH and suggest that successful invaders like house sparrows may persist by tolerating, rather than avoiding, pathogen exposure.