DNA methylation predicts immune gene expression in introduced house sparrows (<i>Passer domesticus</i>)

Kilvitis, Holly; Schrey, Aaron; Ragsdale, Alexandria; Berrio, Alejandro; Phelps, Steve; Martin, Lynn

23 April 2019

Populations undergoing range expansions are often faced with novel selective pressures, and to cope with such challenges, populations must either adapt quickly to local conditions or exhibit sufficient phenotypic plasticity to overcome novel challenges. This latter option—particularly plasticity mediated by molecular epigenetic mechanisms—allows for rapid phenotypic adjustments and persistence in novel environments, and thus could be advantageous at range-edges. Our previous research on house sparrows in Kenya—a site of ongoing range expansion for this species—suggests that invasion success is facilitated by (i) differences in the expression of a microbial surveillance gene (i.e. Toll-like receptor 4—TLR4), and (ii) extensive variation in genome-wide DNA methylation among individuals. Here, our primary goal was to investigate whether these two observations are related, asking whether DNA methylation within a target sequence upstream of the TLR4 transcription start site is associated with variation in TLR4 expression. We found that DNA methylation in the aforementioned region was quite variable among individuals, and variation at one CpG site was predictive of differences in TLR4 expression. Moreover, we found genetic variation within the TLR4 target sequence, however, this variation was not predictive of TLR4 expression. To our knowledge, ours is the first study to demonstrate an association between DNA methylation and the expression of an ecologically relevant trait in the context of a range-expanding vertebrate.