Introduction
Epigenetic mechanisms can alter gene expression without variation in
underlying DNA sequence (Bossdorf et al. 2008). Of the three molecular
epigenetic mechanisms: DNA methylation, histone modification, and
chromatin structure, only DNA methylation has received much attention in
non-model organisms, especially vertebrates (Schrey et al. 2014; Sheldon
et al. 2018).
Histones are highly conserved proteins that package DNA into the
nucleosome (reviewed by Bartova et al. 2008). The ammino acid residues
comprising histone tails can be covalently modified by up to 15
different subunits (i.e., methylation, acetylation, phosphorylation,
ubiquitylation). Modifications to histone residues alter the reactivity
of that histone-DNA complex and hence the conformation of DNA within the
nucleus. Histone acetylation is generally an activating modification,
promoting transcription by making the DNA more euchromatin like. By
decreasing a histone’s affinity for DNA, transcription factor binding
sites become more accessible, thus bolstering transcription (Bartova et
al. 2008).
There are a limited number of histone acetylation studies on non-model
organisms, and those that exist indicate that histone acetylation can
have a major impact on gene expression, phenotypic plasticity, and
plasticity of life history traits (Choppin et al. 2021, Reynolds et al.
2016). For instance, altered histone acetylation facilitated the shift
in physiology of worker ants (Temnothorax rugatulus ) following
the loss of a queen, increasing their fecundity, expression of immune
genes, and longevity of the hive (Choppin et al. 2021). Changes in
histone acetylation were also associated with pupal diapause (i.e., a
period of dormancy that is induced by harsh environmental conditions) in
the flesh fly (Sarcophaga bullata ; Reynolds et al. 2016).
Introduced species provide an excellent system to investigate the role
of histone acetylation in variation in gene expression. Introduced
individuals must rapidly adjust to new environments, a particularly
fraught challenge because introduced populations tend to have low
genetic variation and differentiation compared to native populations
(Schrey et al. 2011; Liebl et al. 2015; Hanson et al. 2022). Ample
evidence exists to support that DNA methylation, is important in this
process (Liebl et al. 2013; Mounger et al. 2021). Indeed, the house
sparrow (Passer domesticus ) has been successfully introduced
throughout the world (Liebl et al. 2015), and DNA methylation is
important at the individual level to variation in gene expression
(Kilvitis et al. 2019) and at the population level in terms of success
in colonizing new areas (Liebl et al. 2013; Sheldon et al. 2018;
Swaegers et al. 2023). Partly, these population-level effects arise
because individuals possess genomes with different dispositions to be
epigenetically modified, a trait termed epigenetic potential (Kilvitis
et al. 2017; Sheldon et al. 2023). One form of epigenetic potential has
been estimated as the number of CpG sites in promoters. In past work, we
have found that epigenetic potential varies among individuals (Hanson et
al. 2020), is related to gene expression levels (Hanson et al. 2021),
and is greater in house sparrows collected in more recently introduced
areas than native ones (Hanson et al. 2022). We propose that histone
modification is also important to introduced house sparrows, given that
it has an impact on gene expression and phenotypic plasticity (Choppin
et al. 2021, Reynolds et al. 2016).
Here, we screened histone modification in wild-collected house sparrows.
Our goal was to facilitate investigations of histone acetylation and
encourage the use of additional epigenetic markers beyond DNA
methylation in ecological epigenetics. To do that we determined whether
1) histone acetylation can be measured in wild collected house sparrows
using a commercially available assay developed for mammals, and 2)
histone acetylation in house sparrows varies in an
ecologically-intelligible manner. We measured histone acetylation in
house sparrows that had been previously collected for a study on the
response of individuals to Salmonella infection (Sheldon et al.in press ). These experimental samples provide us with the
ecologically-relevant framework, in a controlled setting, to investigate
relationships among epigenetic potential, acetylation and gene
expression.