YSM Issue 90.2

In the mid-20th century, there was a huge breakthrough in molecular
biology, when DNA was discovered to be nature’s hereditary
material and researchers developed the ability to detect differences
in protein and ultimately DNA sequences. These new
molecular approaches provided important tools for studying evolution,
but they led to a splintering in many biology departments—
with molecular biologists and biochemists falling on one side and
evolutionary biologists and ecologists falling on another. Dubbed
the “molecular wars,” this conflict has had effects that are still present
today.
Collaboration between evolutionary biologists and biochemists
is not very common, perhaps because of these historical differences.
However, many scientists in both fields are working to bridge
the gap. “People are starting to realize that by taking a multidisciplinary
approach, we can get a deeper overall understanding,” Siddiq
said.
A key, overlapping topic for these two fields is the search for genetic
changes in a species that help it adapt to its particular environment.
Genes dictate how the organism functions, just like a recipe
tells chefs how to make food. If the customer isn’t happy with a dish,
the chef might swap the recipe, forever changing how that dish is
made in the future. The same is true of natural selection on genes,
except nature selects for genetic changes that make organisms better
capable of surviving and reproducing in their environments (and
natural selection takes a lot longer). Often, distinct patterns of DNA
sequence—called signatures of selection—are produced when natural
selection is acting or has acted on a gene, and this can point the
way towards genetic changes that might have been important in the
evolutionary history of a species. However, many other factors such
as random mutations can also lead to rapid gene changes during evolution,
and some scientists remain cautious about interpreting what
signatures of selection mean for biological functions.
Siddiq and his advisor Joe Thornton, a professor of Ecology &
Evolution, set out to test the direct link between genetic changes
in ADH and whether those changes actually played a role in the
adaptation of Drosophila melanogaster to alcohol-rich environments.
They chose this system because there was a clear hypothesis
of cause and effect, and the genetic tools available in the fruit fly—a
widely-studied model in evolutionary biology—made it easily testable,
according to the researchers.
D. melanogaster evolved to survive in ethanol-rich environments
after it split from its sister fly species, Drosophila simulans, between
two and four million years ago. So, the scientists set out to determine
the ADH gene sequence from the last common ancestor between
Drosophila melanogaster and Drosophila simulans. Using
statistical methods, they traced back the gene sequence of ancient
ADH by comparing the genetic sequence of modern Drosophila
melanogaster to that of its modern relatives.
With the ancient ADH sequence in hand, Siddiq then biochemically
synthesized, expressed, purified, and characterized the ancient
ADH protein. To do this, the gene sequence for ancient ADH
was cloned into a plasmid—a small circular piece of DNA that is
easily inserted into E. coli bacterial cells. The bacteria then multiplied,
replicating their plasmids with each cell division and producing
large amounts of the protein. The expressed ADH protein
was isolated and tested for its ability to process ethanol. The result
was stunning: modern ADH and ancient ADH have the same ethanol-processing
ability.
genetics
FEATURE
IMAGE COURTESY OF WIKIMEDIA
►High ethanol tolerance allows D. Melanogaster to featon rotting
fruit, which produces alcolhol as it ferments.
To further verify this result and to test how the ancient protein
functions in the context of the modern organism, Siddiq partnered
with David Loehlin from the University of Wisconsin and Kristi
Montooth from the University of Nebraska to recreate living Drosophila
melanogaster fruit flies that express the ancient ADH gene.
The team produced DNA coding for the ancient ADH sequence
and injected this DNA into fruitfly eggs. They bred thousands of
these “ancestralized” flies and tested their ability to process ethanol
by putting them in chambers filled with different ethanol concentrations.
This result mirrored the previous one: the ancestralized
flies were just as good at metabolizing ethanol as their modern
counterparts. The scientists concluded that the species’ enhanced
ethanol-processing ability is not a result of adaptation in the ADH
protein, despite the signature of selection on ADH gene sequence
during evolution. This suggests that the previous hypothesis—
which assumed that changes in the Drosophila gene sequence were
an environmental adaptation—may be wrong, and other genes
need to be studied to understand how alcohol tolerance evolved.
But why might a generation of evolutionary biologists have been
wrong all this time about the role of ADH? The main reason could
be that alcohol tolerance is complex, and dozens of different genes
not directly involved in alcohol metabolism might have played an
important role.
Unlike college students, most biologists don’t care much about
how “buzzed” fruit flies can get. However, fruit flies are a central
model species that molecular biologists and geneticists use to
test their theories. This research could overturn a long-standing
hypothesis in evolutionary biology and show us a new way with
which hypotheses about the past can be tested. Moreover, the study
helps to verify the utility of newly-developed tools in molecular biology
that have a wide-ranging impact for future research.
Ultimately, this research only came through an incredible collaboration
between a biochemist in Chicago, a geneticist in Wisconsin,
and a physiologist in Nebraska. These three individuals had
met throughout their academic careers, at conferences and nextdoor
labs, and developed a connection that led them to an interdisciplinary
approach for testing their scientific hypotheses—an
approach that would have never come from a single department.
Perhaps this bodes well for young biologists: like the humble fruit
fly, they can go out with their colleagues and get some drinks—all
the while developing novel hypotheses in evolutionary biology.
www.yalescientific.org
March 2017
Yale Scientific Magazine
33