Merlin Lab
Department of Biology
Texas A&M University
Neurogenomics of monarch migration
Fall migration pathway and spring journey north of monarch butterflies (above).
Every fall, in response to seasonal changes in daylength and temperature, monarchs initiate a long journey south from Canada & the northern United States to overwintering grounds in Mexico. A single generation of monarchs makes the entire southward migration, and overwinters in Mexico. During this migration and overwintering period the migrant butterflies are in reproductive dormancy (i.e., diapause). In the spring, they break diapause following increase in tempearture, mate, and travel back northwards to lay eggs on milkweed plants, the unique host plant of monarch larvae, in southern Texas. 3-4 subsequent generations of monarchs progressively complete the return migration to breeding grounds in the north.
To guide this migration, monarchs orient using both a time-compensated sun compass and a magnetic compass.
Their time-compensated sun compass uses the sun's position in the sky and circadian clocks in their antennae to compensate for time-dependent sun position changes as a means by which to maintain direction. A magnetic compass relies on information from Earth's magnetic field. See magnetoreception. Similar to the switch in behavior and physiology observed in fall migrants, the reversal of flight orientation in spring remigrants has been shown to be environmentally induced as well, but, in this case, by prolonged exposure to low temperatures experienced at the overwintering sites.
Environmental induction of the two-way migration of eastern North American monarchs suggests that epigenetic mechanisms triggered by environmental changes regulate migratory behavior in this species. To uncover some yet unknown molecular mechanisms driving orientation behavior in migratory species, we are currently leveraging the monarch behavioral plasticity within individuals that fly south in the fall and north in the spring following prolonged exposure to coldness and cutting-edge epigenetic tools to identify 'migratory' genes and their mode of regulation.
(Left) Monarchs display a south-southwest compass orientation in the fall when migrating to their overwintering grounds (above). In the spring, compass orientation reverses and is northward. This reversal in flight orientation can be mimicked in the lab by exposing monarchs to the cold temperatures they would experience during the overwintering period for 26 days (the entire overwintering period is ~3 months). (Right) The switch in flight orientation upon exposure to environmental coldness suggests an epigenetic reprogramming of flight orientation. The genes, cis-regulatory elements (CREs) and putative transcription factors (TFs) that control their expression, which may be involved in this molecular switch, could be identified through integrated approaches combining RNA-seq, ATAC-seq, and CUT&RUN in brains of fall migrants and cold-treated fall migrants.