A potential role for epigenetic mechanisms enabling appropriate seasonal reproductive transitions of liver yolk-precursor production

Animals utilize mechanisms that enable breeding at times that ensure
offspring production and growth during periods of abundant resources. DNA
methylation is one mechanism by which the expression of genes necessary
for reproduction may be regulated so that they are expressed only at
appropriate times of the year. To date, much work on seasonal breeding in
vertebrates has focused on the neuroendocrine system; however, oviparous
vertebrates, including birds, must also rely on the liver for the
production of yolk precursor hormones that will provide the nutrients
necessary for developing young in ovo. We hypothesized that
changes in DNA methylation in the liver may be one mechanism ensuring
appropriately timed seasonal breeding. We observed that liver expression
of (VTG2) was upregulated in birds sampled during the early breeding
compared with those sampled during the pre-breeding period, and also
observed changes in liver expression of DNMT1 and DNMT3a between these two
periods. Contrary to our predictions, we observed an increase in
methylation from pre-breeding to breeding at one of the three CpG sites in
the promoter region of the VTG2 gene, with no differences at the other two
CpG sites. Finally, we asked if patterns of methylation of VTG2 are
similar between the liver and the blood. We observed strong correlations
between blood and liver in two sites that did not change between
pre-breeding and breeding, while there was only a trend for a significant
association between blood and liver DNA methylation at the site that
displayed a significant increase in liver CpG methylation between sampling
time periods. Together, these findings suggest that changes in DNA
methylation in a tissue important for reproduction outside of the
reproductive endocrine axis (liver) may play a critical role in
appropriate timing of seasonal clutch initiation.