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40 genes aid in monarch migration
Activity differs between southbound, homebody butterflies
By Tina Hesman Saey
Come fall, monarch butterflies feel the
need for a change in latitude. A new study
shows that changes in the activity of a
suite of genes in the butterflies’ brains
help the insects find their way to overwintering grounds in Mexico.
Steven Reppert, a neurobiologist from
the University of Massachusetts Medical School in Worcester, leads a team
of scientists on an ongoing mission to
uncover the monarch’s migratory signals. The team describes a new genetic
analysis of stationary summer monarchs and fall migratory monarchs in the
March 31 BMC Biology.
At least 40 genes are involved in keeping the monarchs Mexico-bound once
they head out, Reppert and his colleagues
report. The team analyzed more than
9,000 of the monarch butterfly’s genes,
about half of the genes in its genome.
Each fall, hundreds of millions of monarchs in the eastern United States and
Canada begin flying south for the winter
and forego reproduction. The butter-
butterflies still had accurate compasses,
indicating that reproduction and navigation are controlled by separate systems.
Next, the scientists analyzed gene
activity in summer and fall butterflies
and found 40 genes showing differences.
Of those, 14 were more active in fall butterflies and 26 were more active in
summer butterflies. Only two of
the genes had any
obvious, known
connection to
migration — vrille,
which is part of the butterfly’s circadian clock, and
the gene for tyramine beta
hydroxylase, which is involved
in motor behavior. The other genes are
involved in metabolic processes, brain
development and other processes, and
the functions of 15 genes are unknown.
“Nothing stood out and said, ‘I’m a migration gene,’ ” Reppert says.
Orley “Chip” Taylor, an insect ecologist
at the University of Kansas in Lawrence
and director of Monarch Watch, says it
is disappointing that none of the genes
Reppert and his colleagues found is definitively linked to navigation. But he says,
“I think they’re doing exactly what needs
to be done to unravel all of this.”
flies navigate with internal clocks and
use the sun as a compass to find their
way to oyamel fir forests in central Mexico. No one knows what environmental signals flip the switch that causes
butterflies to start
migrating.
Some of these
40 genes may be
involved in the flip, but
the new study didn’t address
that question.
Reppert and his colleagues
collected monarch butterflies in the
summer and fall. As expected, stationary summer butterflies have high levels
of a reproductive chemical called juvenile hormone, while migratory fall butterflies are deficient in the hormone.
Summer butterflies stay near the place
they hatch and reproduce, and they don’t
orient themselves according to the sun.
The researchers gave some of the fall
butterflies a chemical that mimics juvenile hormone and then placed them in a
flight simulator to see whether the hormone could block navigation. But the fall
Louse-y genome surprise
Although generally despised, the blood-sucking human body louse, Pediculus humanus,
has gained newfound popularity among scientists for a surprising genetic feature.
Instead of carrying mitochondrial DNA in a single chromosome loop, the vermin
(shown) splits this DNA among many chromosomes, making the louse an anomaly in
the animal world, scientists report online March 31 in Genome Research. Animal cells
stash DNA in the nucleus, where the bulk of genetic material is packed, and in the
mitochondria, specialized energy factories in the cell. The mitochondrial DNA of more
than 1,200 animals has been sequenced, says study coauthor Renfu Shao of the
University of Queensland in Brisbane, Australia. “With very few exceptions, all these
animals have a single circular mitochondrial chromosome,” Shao says. In contrast,
the mitochondrial genes of the blood-sucking louse are split among 18 circular mini-chromosomes, the new research shows. Study coauthor Ewen Kirkness of the J. Craig
Venter Institute in Rockville, Md., speculates that the multiple chromosomes may
bump up genetic variation. “We see bits of genes being swapped between different
circles,” Kirkness says. This gene mingling may have allowed lice to quickly adapt
when something new showed up on the menu: human blood. — Laura Sanders
