Butterfly genomics: Monarchs migrate and fly differently, but meet up
and mate
A genome-wide comparison of eastern and western monarchs
Date:
July 29, 2020
Source:
Emory Health Sciences
Summary:
A new study confirms that while the eastern and western butterflies
fly differently, they are genetically the same.
FULL STORY ==========================================================================
Each year, millions of monarch butterflies migrate across eastern North
America to fly from as far north as the U.S.-Canadian border to overwinter
in central Mexico -- covering as much as 3,000 miles. Meanwhile, on the
other side of the Rocky Mountains, western monarchs generally fly 300
miles down to the Pacific Coast to spend the winter in California. It
was long believed that the eastern and western monarchs were genetically distinct populations.
==========================================================================
A new study, however, confirms that while the eastern and western
butterflies fly differently, they are genetically the same. The journal Molecular Ecology published the findings, led by evolutionary biologists
at Emory University.
"It was surprising," says Jaap de Roode, Emory professor of biology and
senior author of the study. His lab is one of a handful in the world
that studies monarch butterflies.
"You would expect that organisms with different behaviors and ecologies
would show some genetic differences," de Roode says. "But we found
that you cannot distinguish genetically between the western and eastern butterflies." The current paper builds on previous work by the de Roode
lab that found similarities between 11 genetic markers of the eastern
and western monarchs, as well as other more limited genetic studies and observational and tracking data.
"This is the first genome-wide comparison of eastern and western monarchs
to try to understand their behavioral differences better," says Venkat
Talla, first author of the current study and an Emory post-doctoral
fellow in the lab.
========================================================================== Talla analyzed more than 20 million DNA mutations in 43 monarch genomes
and found no evidence for genomic differentiation between eastern and
western monarchs. Instead, he found identical levels of genetic diversity.
"Our work shows that the eastern and western monarchs are mating
together and exchanging genetic material to a much greater extent than
was previously realized," Talla says. "And it adds to the evidence
that it is likely differences in their environments that shapes the
differences in their patterns of migration." Co-author Amanda Pierce,
who led the earlier study on 11 genetic markers, launched the project
while she was a graduate student in the De Roode Lab.
"Monarch butterflies are so fragile and so lightweight, and yet
they are able to travel thousands of miles," Pierce says. "They are
beautiful creatures and a great model system to understand unique,
innate behaviors. We know that migration is ingrained in their genetic
wiring in some way." After monarchs leave their overwintering sites,
they fly north and lay eggs.
The caterpillars turn into butterflies and then fly further, mating
and laying another generation of eggs. The process repeats for
several generations until finally, as the days grow shorter and the temperatures cooler, monarchs emerge from their chrysalises and start
to fly south. This migratory generation does not expend any energy on
breeding or laying eggs, saving it all for the long journey.
==========================================================================
"For every butterfly that makes it to California or to Mexico, that's
its first journey there," Pierce marvels.
Previous work had identified a propensity for the eastern and western
monarchs to have slight differences in their wing shapes. For the
current paper, the researchers wanted to identify any variations in
their flight styles.
They collected eastern monarchs from a migratory stopover site in Saint
Marks, Florida, and western monarchs from one of their overwintering sites
near Oceano, California. Pierce ran flight trials with the butterflies
by tethering them to a mill that restricted their flight patterns to
circles with a circumference of about 25 feet. The trials were performed
in a laboratory under controlled light and temperature conditions that
mimicked overwintering sites.
Artificial flowers were arranged around the circumference of the flight
mills.
"The idea was to try to give them some semblance of a 'natural'
environment to help motivate them and to orient them," Pierce explains.
Butterflies were released unharmed from the flight mills after performing
short trials.
The results showed that the eastern monarchs would choose to fly for
longer distances while the western monarchs flew shorter distances
but with stronger bursts of speed. "The more powerful flight trait
of the western monarch is like a sprinter, essentially," Pierce says,
"while the eastern monarchs show a flight trait more like marathoners."
Pierce has since graduated from Emory and now works as a geneticist for
the Environmental Protection Agency in Washington, D.C.
Talla, who specializes in bioinformatics, grew up in India where the rich diversity of wildlife inspired him to become an evolutionary biologist. He moved to Sweden to get his PhD, where he studied the genomics of the
European wood white butterfly. Although all wood whites appear identical visually, they are actually three different species.
"One of the big questions I'm interested in answering is how does an
individual species wind up becoming multiple species?" Talla says. "I want
to understand all the processes involved in that evolution." He jumped
at the chance to join the De Roode Lab. "Monarchs have always been at
the top of my list of butterflies I wanted to study because of their
incredible migrations," Talla says. "They are a fascinating species."
Last November, he joined de Roode on a lab field trip to the eastern
monarch overwintering site, inside and adjacent to the Monarch Butterfly Biosphere Reserve in central Mexico. Tens to hundreds of millions of
monarchs blanket the trees and landscape through the winter. "It's a mind-blowing sight," Talla says. "It makes you wonder how they all know
how to get there." Previous tracking and observational studies had shown
that at least some western monarchs fly south to Mexico instead of west
to California. The full- genome analysis suggests that more than just a
few of the western monarchs may be making the trip to Mexico where they
mix with the eastern monarchs. And when the butterflies depart Mexico,
some may fly west instead of east.
"Evidence from multiple directions is coming together to support the
same view," de Roode says.
The findings may help in the conservation of monarchs. Due to
a combination of habitat loss, climate change and lack of nectaring
flowers, numbers of both eastern and western monarchs have declined
in recent decades, with the western ones showing the most precipitous
drop. The U.S. Fish and Wildlife Service is currently considering whether
the butterflies need special protections.
"If environmental factors are all that drives the differences between
the eastern and western monarchs, it's possible that we could help the
western population by transplanting some of the eastern ones to the west,"
de Roode says.
The De Roode lab now plans to investigate what exactly in the environments
of the butterflies triggers different expressions of their genes.
The work was funded by Emory University, the National Science Foundation
and the National Institutes of Health.
========================================================================== Story Source: Materials provided by Emory_Health_Sciences. Note: Content
may be edited for style and length.
========================================================================== Journal Reference:
1. Venkat Talla, Amanda A. Pierce, Kandis L. Adams, Tom J. B. de Man,
Sumitha Nallu, Francis X. Villablanca, Marcus R. Kronforst, Jacobus
C. de Roode. Genomic evidence for gene flow between monarchs with
divergent migratory phenotypes and flight performance. Molecular
Ecology, 2020; DOI: 10.1111/mec.15508 ==========================================================================
Link to news story:
https://www.sciencedaily.com/releases/2020/07/200729124418.htm
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