The mass migration of the monarch butterfly (Danaus plexippus), from Mexico to North America is a unique biological phenomenon that has fascinated many for hundreds of years. Writing about the monarch’s long journey in our Friday Feature Five mind-blowing migration events seen in nature only sparked more questions in my mind. Advances in the field of genetics mean that researchers can now sequence whole genomes. With these advances, can we uncover the secrets behind the monarch butterflies’ unique abilities?

Monarch butterflies have a strong association with the milkweed plant (Asclepias syriaca). In summer the monarch butterfly lays its eggs on the milkweed plant. Monarch caterpillars consume the milkweed plant and ingest toxins that are stored inside them throughout their lifespan and these toxins protection from predators. Monarchs also possess warning coloration of black and orange or black and white. In autumn the adult butterflies migrate to Mexico and ‘roost’ in trees in groups of thousands large. However, there are many monarch populations outside of North America and Mexico that are on different continents that do not perform long migrations.





This poses the question as to how did the monarch butterfly species become so widespread?

Map of the sampling locations used by Zhan et al. (2014)

Scientists have determined the evolutionary history of the monarch butterfly by analysing the genomes of 101 monarch butterflies from all existing populations! These results showed the ancestral monarch population originated from North America and three separate dispersal events occurred. 20,000 years ago, after the last glacial maximum (Ice Age) some monarchs migrated to Central and South America due to increased milkweed abundances. Then 2,000-3,000 years ago some monarchs migrated from North America across the Atlantic and Pacific. This demonstrates that monarchs were ancestrally migratory and dispersed to create their broad distribution.





How were some monarchs able to carry out these incredibly long migrations across oceans?

Photograph by Joel Sartore

By analysing the monarch genome, it is possible to determine genes that are under natural selection. Genes under positive natural selection are usually advantageous to the organism and therefore prevalent in the population. Regions of the genome in migratory monarchs under the strongest positive selection were a single group of genes responsible for the formation of Collagen IV. Collagen IV is vital in the flight muscle function in monarchs, and other species of Lepidoptera (butterfly). This means in migratory populations of monarchs there is a single amino acid substitution (mutation) in one protein under strong positive selection. Potentially this single mutation led to more efficient oxygen consumption and flight metabolic rates specifically in the flight muscles these monarchs possess. Is this adaptation enough for such a delicate-looking insect to migrate thousands of kilometers?





What is the likelihood of a single gene being important for a complex trait like migration?

Firstly, other genetic factors have been implicated in monarch migration such as those responsible for the monarch’s ‘internal body clock’ (circadian rhythm and clock genes). Navigation in monarchs has been linked to genetic control of magno-receptors in their antennae and retina development, implicated in sun navigation. It is unlikely that the ability of a monarch butterfly to perform such impressive long-distance migration, is because of one mutation, but a combination of several. Random chance is involved too, in regard to when advantageous mutations arise in the population and whether the conditions will promote positive natural selection. Regardless, it is exciting to see evidence of natural selection occurring in wild populations today! Unfortunately, the monarch migration phenomenon is under serious threat, with mass deforestation in Mexico reducing the available ‘roosting’ sites for the butterflies.

Hopefully this article will increase the awareness of the threat to the monarch migration phenomenon and inspire people to understand and protect these unique behaviours seen in nature. If you enjoyed this article, give us a like or share! Never miss an article by subscribing to Scientific Souls or following us on social media channels.





#science #biology #migration #monarch #butterfly #genetics #history #nature #research #education #facts #universe #knowledge #science #facts #environment #writing





References



