Epigenetic Mechanisms in Developmental and Seasonal Programs

Developmental epigenetic modifications generally occur during cellular embryonic differentiation and impart permanent changes that last the individual’s lifespan. It is now recognized that epigenetic modifications also exhibit rhythmic patterns that impact the timing of seasonal transitions in physiology and behaviour. This chapter explores the role of epigenetic modifications during mammalian development and photoperiodic programming of seasonal rhythms, focussing on the molecular and cellular substrates in the hypothalamus that regulate seasonal timing of reproduction. The chapter draws evidence from the well-established literature on genomic imprinting and maternal programming during mammalian development to identify common genomic, molecular and cellular signalling mechanisms. One mechanism common across developmental and seasonal programs that is highlighted is the role of thyroid hormones. Recent data indicate that the epigenetic regulation of thyroid hormone deiodinase enzymes is a critical feature of developmental and seasonal programming.