Population genetics and the genetics of migration in monarch butterflies

Population genetics of monarchs around the world

Monarchs are well known for their annual migration from eastern North America to overwintering sites in Mexico, but they also occur in western North America, where they overwinter along the Californian coast. Furthermore, monarchs form non-migratory populations around the world, ranging from locations in New Zealand to Costa Rica and Spain. Although these different populations have been known for a long time, it remained unknown how monarchs spread around the world and whether monarchs in different populations are genetically different. We addressed these questions by developing microsatellite markers and using monarch genome resequencing approaches. Surprisingly, this work showed that monarchs originate in North America, and that they subsequently colonized the world through independent dispersal events across the Atlantic Ocean, across Central and South America, and across the Atlantic Ocean. During these dispersal events, monarchs formed genetically distinct populations and lost their migration. Another surprising finding was that monarchs in eastern and western North America, despite their drastically different migration destinations and distances, did not show genetic differentiation across their neutral microsatellite markers.

Questions:

• How much gene flow occurs between different populations of monarchs?
• Has genetic differentiation between monarch populations contributed to local adaptation?
• Are there specific genomic regions that show strong genetic differentiation between migratory monarchs with different migration distances and destinations?

Monarchs in eastern North America migrate to Mexico (black arrows) while those in western North America overwinter along the California coast (red arrows). Monarchs originate in North America and colonized the world through three independent dispersal events (orange arrows).

The genetics of seasonal migration

The occurrence of natural variation in migration distance, and the three independent losses of migration during their worldwide dispersal, make monarchs a wonderful system to study the genetic basis of migration. Our work has shown that non-migratory monarchs have smaller wings than migratory monarchs, and collaborative work has shown genetic differentiation between migratory and non-migratory monarchs at a number of genetic loci involved in muscle functioning and metabolism. Current work is indicating that muscle-related genes are also differentiated between eastern and western North American butterflies, and that eastern monarchs have greater flight endurance than western monarchs. We are expanding this work to include genetic crosses between monarchs that vary in migration phenotypes to further our understanding of monarch migration.

We are addressing the following questions:

• How do migratory and non-migratory monarchs differ in morphology, development, flight speed and flight orientation?
• What genes are involved in the initiation and execution of monarch migration?
• Can we knock out candidate genes to unequivocally demonstrate the role of particular genes in migration phenotypes?