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