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Eastern Diamondback Rattlesnake |
If you're one of
the unfortunate few to be bitten by a venomous snake, having access to
effective antivenom to combat the swelling, pain and tissue damage to these
bites is critical.
But new research by a
team of biologists at Florida State University has revealed that creating
antivenom is a bit tricky.
That's because the
type of venom a snake produces can change according to where it lives.
Mark Marges, a Florida
State doctoral student in Professor Darin Rokyta's laboratory, led a research
study that examined the venom of 65 eastern diamondback rattlesnakes and 49
eastern coral snakes from all over the state of Florida to determine whether
snake venoms varied by geography.
The venom from an
eastern diamondback rattlesnake in the Florida panhandle is very different than
the venom from a rattlesnake 500 miles south in the Everglades, and this has
huge implications for snakebite treatment.
"So if you use
just southern venoms when making the antivenom, it would be ineffective against
some of the more common toxins found in northern diamondback
rattlesnakes," said Florida State University doctoral student Mark
Margres.
In the rattlesnakes,
they found significant variation linked to geography. But, in the coral snakes,
they found the venom to be identical no matter where the snakes were found.
"This can tell us
a bit of the history and evolutionary patterns of the snakes," said Kenny
Wray, a post-doctoral research associate in Rokyta's lab. "This suggests
that the coral snakes may be recent invaders to the region and haven't had time
to evolve different venoms in different areas."
This information also
will help with the development of coral snake antivenom, because scientists now
know there is uniformity in coral snake venom. According to a 2012 estimate by
the Center for Disease Control, 7,000 to 8,000 people in the United States are
bitten by venomous snakes every year.
Not only are there
medical implications, this information is also important for conservation
purposes.
The eastern
diamondback rattlesnake is being considered for federal protection under the
Endangered Species Act. But, if the snakes are removed from one geographic
area, they will be irrevocably deleted from the ecosystem altogether.
"If we lose some
of these populations, we lose a whole venom type," Rokyta said. "That
really changes conservation."
Venom from an eastern diamondback rattlesnake
in the Everglades is distinct from the cocktail of toxins delivered by the same
species in the Florida panhandle area, some 500 miles away. But no matter where
you go in the Southeastern United States, the venom of the eastern coral snake
is always the same.
Each venomous snake
species produces a unique venom, a mixture of around 50-200 toxic proteins and
protein fragments that co-evolve with the typical prey of the snake, such as the
smaller reptiles eaten by the eastern coral snake or the rodents preferred by
rattlesnakes. In this cycle of evolutionary attack and counterattack, any
genetic variants that enhance venom resistance tend to spread through the prey
population, prompting tweaks to the snake venom recipe that restore its
effectiveness.
The result should be
distinctive local co-adaptations between predator and prey, as well as
considerable regional diversity in the types and amounts of the different venom
proteins. But when Darin Rokyta (Florida State University) and his colleagues
collected and profiled venom from eastern coral snakes at many sites within
Florida, they found no variation at all. The mix of proteins in coral snake
venom from one part of the state was indistinguishable from that collected
anywhere else. In contrast, eastern diamondbacks, which live in the same parts
of the country as the coral snakes, produce venom with different ratios of
toxic proteins in nearly every sub-population across their range. For example,
two venom components, including one known to cause paralysis in prey, are found
at high levels in the northernmost populations, and were completely absent in
the snakes from Caladesi Island, near Tampa.
"We were
shocked," Rokyta said. "This is the first time anyone has looked at
venom variation at this scale, and everybody has assumed that the
co-evolutionary arms race would cause local populations to diverge
quickly."
Rokyta says there
could be several explanations for the lack of variation in eastern coral snake
venom. For example, a small population of the species might have recently
expanded and taken over the entire range, displacing other populations and
reducing genetic diversity. Or it could reflect a difference in co-evolutionary
dynamics between the species and its typically reptilian prey, compared to the
small mammals preferred by rattlesnakes. The team is now using genetic clues to
the population histories of each species to investigate possible explanations.
The results of the
study will be helpful to researchers developing eastern coral snake antivenom.
Making an antivenom requires samples of venom, but if the mix varies
substantially from place to place, this will affect the drug's effectiveness
and reliability. For this species, sampling from many populations should not be
necessary. "This tells us it doesn't matter where we catch these
relatively elusive snakes; we can stick to using those locations where they're
easy to find," Rokyta said.
The variation between
eastern diamondback populations could provide crucial information to
authorities managing the conservation of this species, which is in decline and
under consideration for listing as threatened under the Endangered Species Act.
Eastern diamondback rattlesnake declines are thought to have been caused by
habitat loss compounded by hunting and persecution by humans. The data from
this study can be used for population management, to ensure the full range of
venom subtypes are conserved for the long-term viability of the species.
"The received
wisdom was that venoms are rapidly-evolving, but now we know that's not
necessarily the case." said Mark Johnston, Editor-in-Chief of GENETICS.
"Clearly, venom evolution in these two snake species has been shaped by
different forces. The next challenge is to understand why."
Citation
Margres MJ, McGivern JJ, Seavy
M, Wray KP, Facente J, Rokyta DR. 2014. Contrasting
Modes and Tempos of Venom Expression Evolution in Two Snake Species. Genetics, 2014; DOI: 10.1534/genetics.114.172437