Ular dan Kadal air Beracun berpacu Dalam Evolusi --T-REC-komunitas reptil-semarang--KSE-komunitas satwa eksotik

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Ular dan  Kadal air Beracun berpacu Dalam Evolusi 

Kadal air berkulit kasar dengan dosis besar  racun mematikan dikulitnya  yang sama ditemukan di blowfish . Ular garter yang makan  kadal air juga telah berevolusi resistensi terhadap racun tersebut  , memacu toksisitas yang lebih besar pada  kadal air oleh seleksi alam . Tapi kini peneliti melaporkan bahwa di beberapa daerah , ular entah bagaimana telah  berevolusi pada tingkat resistensi yang jauh melampaui apa yang kadal air mampu melawan ....read more

Snakes Vault Past Toxic Newts In
Evolutionary Arms Race

Date:

March 13, 2008

Source:

Stanford University

Summary:

Rough-skinned newts harbor in their skin massive doses of the same deadly
toxin found in blowfish. Garter snakes that dine on the newts have evolved
resistance to the toxin, spurring greater toxicity in the newts by natural
selection. But now researchers report that in some areas, the snakes have
somehow evolved levels of resistance far beyond what the newts are capable of
countering.

.......................

Snakes don't eat fugu, the seafood delicacy prepared from blowfish meat and
famed for its poisonous potential. However, should a common garter snake wander
into a sushi restaurant, it could fearlessly order a fugu dinner.

The snakes have evolved resistance to the blowfish poison, tetrodotoxin
(TTX), by preying on rough-skinned newts, which also secrete the toxin. Some
newts are so poisonous that they harbor enough TTX to kill a roomful of adult
humans.

Why would a small animal produce such an excessive amount of poison? The
answer lies in the evolutionary back-and-forth between newts and garter snakes.
Throughout much of their shared territory, newts and snakes have been locked in
a kind of arms race: TTX-resistant snakes cause natural selection to favor
ever-more poisonous newts, and the new-and-improved newts drive selection for
higher resistance in snakes.

In a new study Charles Hanifin, a postdoctoral scholar at Stanford's
Hopkins Marine Station, and his co-authors say that snakes in some areas may
have prevailed in the evolutionary arms race between predator and prey.
Surprisingly, snakes in several geographic locations have developed such
extreme resistance to TTX that newt production of the toxin cannot keep up.

Most toxic amphibians in the world

Some populations of newts produce enough TTX to kill thousands of mice or
10 to 20 humans. Ounce for ounce, Hanifin said, they are even more toxic than
South America's famed poison dart frogs.

"Some populations of these newts may very well represent the most
toxic amphibians on the planet," Hanifin said.

The poisonous newts have even killed off humans. The Journal of the
American Medical Association reports the case of a 29-year-old man who died
after swallowing an 8-inch-long newt on a dare. The journal also describes the
case of a 26-year-old man in Oregon who managed to survive his encounter with
the newts. After swallowing five of the animals to win a bet, he felt dizzy,
began vomiting and was too weak to walk, though he later recovered under a
doctor's care.

These incidents aside, the newts rarely harm humans. It is safe to handle
the newts with bare hands, since the toxin is not absorbed through the skin. A
newt must be ingested to be toxic, and Hanifin said the animal emits an acrid
smell that probably discourages most pets and children from tasting it.

Escaping the arms race

At first glance, the newt and garter snake populations seem to be evenly
matched. The most toxic newts are found in the same areas as highly resistant
snakes, and areas without toxic newts house only non-resistant snakes.

Data on the garter snakes came from Hanifin's collaborators, Edmund Brodie
Jr. of Utah State University and Edmund Brodie III of the University of
Virginia, who measured snake resistance to TTX by injecting the animals with
the toxin and measuring how fast they subsequently slithered. Although TTX does
not kill resistant snakes, it often slows them down for a while. Less-resistant
snakes move slower after TTX injection, and some are even temporarily paralyzed.

To get a closer look at the snake-newt interaction, Hanifin and colleagues
tested 383 newts from 28 locations where the Brodies had previously examined
garter snake TTX resistance. Collection spots stretched down the West Coast
from British Columbia to Central California.

Hanifin found that snakes were pulling ahead of the newts in several
places. In one third of the locations, the most toxic newt could still be eaten
by the least resistant snake. This means that all snakes in the population do
just as well regardless of their TTX resistance level, and there is no
evolutionary pressure for the snakes to develop stronger resistance.

"In these areas, I think the snakes have won," Hanifin said.

How have snakes managed to become super-resistant to TTX, while newt
production of the toxin lagged behind? It seems there are only a handful of
snake genes involved in resistance, meaning TTX resistance in snakes can evolve
quickly and in great leaps, Hanifin said. Newt genetics appear to work
differently, with increasing toxicity arriving only through smaller incremental
changes.

Newts are also limited by their own biology. They are only resistant to
TTX, not immune to it. A few milligrams of TTX injected into a newt's gut are
lethal, so the animal sequesters the toxin in its skin. While the most toxic
newts had 14 to 15 milligrams of TTX, some garter snakes are resistant to up to
100 milligrams of TTX. To hold that much toxin, the tiny newts would be one
part toxin to nine parts skin-a near physical impossibility, according to
Hanifin.

Though snakes may have won this round, Hanifin said their good fortune may
not last forever. There is some evidence that TTX resistance comes at a price:
Really resistant snakes have slower crawl speeds than snakes with little or no resistance.
If there is no advantage to a snake for being super-resistant, and
super-resistance has an evolutionary cost, the snakes could eventually end up
with a lowered resistance, to the point where the newts' toxin levels would
again be effective. Though Hanifin said the idea was plausible, it would take
years of experiments to confirm.

Collecting and testing newts

Together with his father, a research dermatologist, Hanifin devised a
method of measuring the newt's toxin levels using the same kind of surgical
punch used to take skin for biopsies. Hanifin removed a half-centimeter circle
of skin from the backs of anesthetized newts and then ground up the skin
samples to analyze the amount of toxin present.

Getting accurate measurements was tough, and Hanifin spent two months in
Japan learning techniques from blowfish researchers. After the procedures were
ironed out, Hanifin and his colleagues spent five years collecting enough newts
to test.

Hanifin said the newts make convenient field animals. "They're pretty
mellow; they don't get real worked up about being handled," he said.
"If you're collecting them in a pond, they just kind of float around
you."

The newts' toxicity means they can afford to be lax about evading
rubber-booted researchers, and Hanifin caught most of the animals by hand. He
said he did not envy the snake collectors, who chased the rapid-slithering
animals through grass and underbrush.

Hanifin got help collecting from people in his lab and from Oregon State
University, the University of Oregon, researchers in Washington and California,
and the California Department of Fish and Wildlife.

"Literally dozens of people contributed," Hanifin said. "It
was a really collaborative effort."

Future directions of Hanifin's research include learning more about human
disease by exploring the genetics of resistant garter snakes. TTX blocks
electrical signaling in nerve cells by stopping up a sodium channel, and
TTX-resistant snakes have a modified channel that the toxin does not recognize.
In humans, defects in similar sodium channels can lead to serious illness,
including some types of epilepsy, and insight into sodium channel biology could
help treat these diseases.

Journal reference: Hanifin CT, Brodie ED Jr, Brodie ED III (2008)
Phenotypic mismatches reveal escape from arms-race coevolution. PLoS Biol 6(3):
e60. doi:10.1371/journal.pbio. 0060060







Story Source:

The above story is based on materials provided
by Stanford University. Note: Materials may be edited
for content and length.

http://www.sciencedaily.com/releases/2008/03/080311075326.htm

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