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1. Introduced house sparrows ( Passer domesticus ) have greater variation in DNA methylation than native house sparrows

   https://doi.org/10.1093/jhered/esad067  

   Lauer, M Ellesse; Kodak, Haley; Albayrak, Tamer; Lima, Marcos R; Ray, Daniella; Simpson-Wade, Emma; Tevs, David R; Sheldon, Elizabeth L; Martin, Lynn B; Schrey, Aaron W (October 2023, Journal of Heredity)

   vonHoldt, Bridgett (Ed.)

   Abstract As a highly successful introduced species, house sparrows (Passer domesticus) respond rapidly to their new habitats, generating phenotypic patterns across their introduced range that resemble variation in native regions. Epigenetic mechanisms likely facilitate the success of introduced house sparrows by aiding particular individuals to adjust their phenotypes plastically to novel conditions. Our objective here was to investigate patterns of DNA methylation among populations of house sparrows at a broad geographic scale that included different introduction histories: invading, established, and native. We defined the invading category as the locations with introductions less than 70 years ago and the established category as the locations with greater than 70 years since introduction. We screened DNA methylation among individuals (n = 45) by epiRADseq, expecting that variation in DNA methylation among individuals from invading populations would be higher when compared with individuals from established and native populations. Invading house sparrows had the highest variance in DNA methylation of all three groups, but established house sparrows also had higher variance than native ones. The highest number of differently methylated regions were detected between invading and native populations of house sparrow. Additionally, DNA methylation was negatively correlated to time-since introduction, which further suggests that DNA methylation had a role in the successful colonization’s of house sparrows. 

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2. High epigenetic potential protects songbirds against pathogenic Salmonella enterica infection

   https://doi.org/10.1242/jeb.245475  

   Sheldon, Elizabeth; Zimmer, Cedric; Hanson, Haley; Koussayer, Bilal; Schrey, Aaron; Reese, Darrys; Wigley, Paul; Wedley, Amy L; Martin, Lynn B (July 2023, Journal of Experimental Biology)

   ABSTRACT Animals encounter many novel and unpredictable challenges when moving into new areas, including pathogen exposure. Because effective immune defenses against such threats can be costly, plastic immune responses could be particularly advantageous, as such defenses can be engaged only when context warrants activation. DNA methylation is a key regulator of plasticity via its effects on gene expression. In vertebrates, DNA methylation occurs exclusively at CpG dinucleotides and, typically, high DNA methylation decreases gene expression, particularly when it occurs in promoters. The CpG content of gene regulatory regions may therefore represent one form of epigenetic potential (EP), a genomic means to enable gene expression and hence adaptive phenotypic plasticity. Non-native populations of house sparrows (Passer domesticus) – one of the world's most cosmopolitan species – have high EP in the promoter of a key microbial surveillance gene, Toll-like receptor 4 (TLR4), compared with native populations. We previously hypothesized that high EP may enable sparrows to balance the costs and benefits of inflammatory immune responses well, a trait critical to success in novel environments. In the present study, we found support for this hypothesis: house sparrows with high EP in the TLR4 promoter were better able to resist a pathogenic Salmonella enterica infection than sparrows with low EP. These results support the idea that high EP contributes to invasion and perhaps adaptation in novel environments, but the mechanistic details whereby these organismal effects arise remain obscure. 

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3. Screening H3 Histone Acetylation in a Wild Bird, the House Sparrow ( Passer Domesticus )

   https://doi.org/10.1093/iob/obae004  

   Ray, D; Sheldon, E L; Zimmer, C; Martin, L B; Schrey, A W (January 2024, Integrative Organismal Biology)

   Synopsis Epigenetic mechanisms are increasingly understood to have major impacts across ecology. However, one molecular epigenetic mechanism, DNA methylation, currently dominates the literature. A second mechanism, histone modification, is likely important to ecologically relevant phenotypes and thus warrants investigation, especially because molecular interplay between methylation and histone acetylation can strongly affect gene expression. There are a limited number of histone acetylation studies on non-model organisms, yet those that exist show that it can impact gene expression and phenotypic plasticity. Wild birds provide an excellent system to investigate histone acetylation, as free-living individuals must rapidly adjust to environmental change. Here, we screen histone acetylation in the house sparrow (Passer domesticus); we studied this species because DNA methylation was important in the spread of this bird globally. This species has one of the broadest geographic distributions in the world, and part of this success is related to the way that it uses methylation to regulate its gene expression. Here, we verify that a commercially available assay that was developed for mammals can be used in house sparrows. We detected high variance in histone acetylation among individuals in both liver and spleen tissue. Further, house sparrows with higher epigenetic potential in the Toll Like Receptor-4 (TLR-4) promoter (i.e., CpG content) had higher histone acetylation in liver. Also, there was a negative correlation between histone acetylation in spleen and TLR-4 expression. In addition to validating a method for measuring histone acetylation in wild songbirds, this study also shows that histone acetylation is related to epigenetic potential and gene expression, adding a new study option for ecological epigenetics. 

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4. Microbial surveillance versus cytokine responsiveness in native and non-native house sparrows

   https://doi.org/10.1098/rsbl.2024.0431  

   McCain, Kailey M; Mansilla, Gabby; Sheldon, Elizabeth L; Zimmer, Cedric; Schrey, Aaron W; Rowe, Melissah; Dor, Roi; Kohl, Kevin D; Søraker, Jørgen S; Jensen, Henrik; et al (January 2025, Biology Letters)

   The success of introduced species often relies on flexible traits, including immune system traits. While theories predict non-natives will have weak defences due to decreased parasite pressure, effective parasite surveillance remains crucial, as infection risk is rarely zero and the evolutionary novelty of infection is elevated in non-native areas. This study examines the relationship between parasite surveillance and cytokine responsiveness in native and non-native house sparrows, hypothesizing that non-natives maintain high pathogen surveillance while avoiding costly inflammation. We made this specific prediction, as this pattern could enable invaders to effectively mitigate pathogen risk in a manner commensurate with the life-history priorities of a colonizing organism (i.e. rapid maturation and high reproductive effort). To test this hypothesis, we measuredTLR-2 andTLR-4 expression, markers of pathogen surveillance and cytokine responses (changes inIL-1βandIL-10), regulators of inflammation, to a simulated bacterial infection. In non-native sparrows, we found that asTLR-4 expression increased,IL-1βandIL-10responses decreased, a relationship not observed in native sparrows. Additionally, higher body condition predicted largerIL-1βandIL-10responses in all birds. These findings suggest that highTLR-4surveillance may mitigate strong inflammatory responses in non-native sparrows, with pathological and resource-based costs driving immune variation among and within populations. 

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5. Temperature predictability and introduction history affect the expression of genes regulating DNA methylation in a globally distributed songbird

   https://doi.org/10.1002/jav.03468  

   Martin, Lynn B; McCain, Kailey M; Sheldon, Elizabeth L; Zimmer, Cedric; Rowe, Melissah; Dor, Roi; Kohl, Kevin D; Søraker, Jorgen S; Jensen, Henrik; Mathot, Kimberley J; et al (November 2025, Journal of Avian Biology)

   Phenotypic plasticity is a major mechanism whereby organisms adjust their traits within‐generations to changes in environmental conditions. In the context of range expansions, plasticity is thought to be especially important, as plastic changes in traits can lead to rapid adaptation. 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 range expansions. At present, we know little about how methylation is regulated in wildlife, especially expression of the enzymes responsible for altering methyl marks on the genome. In this study, we compared expression of three epigenetic regulator genes (DNA methyltransferase 1, DNMT1; DNA methyltransferase 3, DNMT3; and one ten‐eleven translocation methylcytosine dioxygenase, TET2) in three tissues (gut, liver, and spleen) of house sparrowsPasser domesticusfrom nine countries. Some countries are in the native range of the species (Israel, the Netherlands, Norway, Spain, and Vietnam) whereas others are sites the species has colonized in the last 150 years (i.e. Australia, Canada, New Zealand, and Senegal). In this exploratory study, we asked whether non‐native birds and/or birds from sites with comparatively unpredictable climates would express different levels of these genes. We found that all three genes were expressed more in sparrows from the native range and from areas with more stable temperatures. Expression of all three genes was also strongly correlated among‐locations and within‐individuals, but mean expression was quite different among tissues. Many factors (e.g. urbanization of the capture site, sex of the bird) did not significantly affect gene expression, but others surprisingly did (e.g. latitude). Our results suggest that these enzymes could be important in range expansions or geographic distribution generally, but more detailed investigations will be insightful. 

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