Enzyme gene expression in house sparrow populations

Phenotypic plasticity is a major mechanism whereby organisms adjust their
traits within generations to complement 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.  For these reasons, there has been growing
interest in the roles of molecular epigenetic processes in the context of
range expansions.  One epigenetic process in particular, DNA
methylation, enables organisms to adjust gene expression contingent on the
environment, which probably explains why it has played a role in some
invasions.  Nevertheless, we still know very little about how
methylation is regulated in wildlife, especially the 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 the expression of
these genes in three different tissues (i.e., gut, liver, and spleen) of
house sparrows (Passer domesticus) from 9 locations. 
Some populations derived from the native range of the species (i.e.,
Israel, the Netherlands, Norway, Spain, and Vietnam), whereas others came
from areas where birds were introduced <150 years ago (i.e.,
Australia, Canada, New Zealand, and Senegal).  We hypothesized
that non-native birds and/or birds from sites with comparatively
unpredictable climates would express the highest levels of all three
genes.  We found, however, that DNMT expression differences,
while extensive, were reversed from predictions: Sparrows from the native
range and from areas with more stable temperatures expressed more of all
three genes.  Surprisingly, too, expression of all three genes
was strongly correlated among countries and within individuals. 
Our results reveal no simple role for these enzymes in range expansions,
but they do indicate appreciable among and within-population variation,
which we hope motivates more detailed investigations of these enzymes in
other wildlife.