Epigenetic Regulation in Seasonal Migrants

Abstract

Seasonal migration in birds requires profound physiological transformations -- hyperphagia, fat deposition, gonadal regression, altered circadian rhythms, and directional orientation programming -- that must be precisely timed relative to photoperiod cues and executed reversibly across multiple annual cycles. Epigenetic mechanisms, including DNA methylation, histone modification, and non-coding RNA regulation, have emerged as candidate molecular switches mediating these reversible seasonal phenotype transitions in a photoperiod-responsive manner. This study characterised genome-wide DNA methylation dynamics (RRBS; reduced representation bisulfite sequencing) and histone modification profiles (ChIP-seq; H3K4me3, H3K27me3, H3K27ac) across three physiological states -- pre-migratory (Zugunruhe), mid-migration, and post-breeding -- in three long-distance migratory passerine species (Ficedula hypoleuca pied flycatcher, Acrocephalus scirpaceus reed warbler, Sylvia atricapilla Eurasian blackcap) sampled at standardised ringing stations (n = 284 individuals per species; n = 28 per state per species for RRBS/ChIP-seq). Differentially methylated regions (DMRs) between pre-migratory and post-breeding states numbered 4,284 +- 842 per species, with significant enrichment at circadian clock genes (CLOCK, BMAL1, PER2: mean DMR enrichment 8.4x above genome background; p < 0.001), hypothalamic neuropeptide genes (GnRH, NPY, VIP: 6.4x; p < 0.001), and fat deposition pathway genes (PPAR-gamma, fatty acid synthase: 5.8x; p = 0.002). H3K4me3 active promoter marks showed significant gain at migratory fat deposition genes during the pre-migratory state (fold enrichment 4.2 +- 0.8; p < 0.001), while H3K27me3 repressive marks increased at gonadal development genes consistent with post-breeding gonadal regression. Blackcap (S. atricapilla) showed the largest DMR count and the strongest epigenetic regulation of migratory orientation genes among the three species, consistent with its documented capacity for rapid migratory route evolution over 30+ years in central European wintering populations. These results identify epigenetic regulation of circadian and metabolic pathways as a key molecular mechanism of migratory phenotype switching and suggest that epigenetic plasticity may facilitate rapid adaptation of migratory timing and routes under climate change