| Pheromone Notes #30 News release issued
by University Of Michigan:
Birds Do It. Bugs Do It. But Why Don't
We?
ANN ARBOR, Mich. -- Many creatures including
our fellow primates the New World Monkeys
rely on highly specific scent molecules
called pheromones to find a suitable mate.
Even our humble mammal cousin, the mouse,
was found to have 140 genes just for pheromone
receptors when its genome was completely
sequenced earlier this year.
But humans are clueless when it comes
to pheromone signals, according to University
of Michigan evolutionary biologist Jianzhi
"George" Zhang. He believes
color vision put our pheromones out of
business.
Our closest relatives
on the primate family tree rely on "sexual swelling" and gaudy, colorful
patches of skin to signal their reproductive fitness and fertility, Zhang said.
In fact, though humans and these apes still carry genes that should create pheromone
receptors in our noses, these genes have mutated to the point that they are merely
pseudogenes---they don't function any more. Zhang has used the genes of
people and primates to get at the answer to this intriguing puzzle. Zhang (pronounced
Zong), is an assistant professor in Ecology and Evolutionary Biology in the College
of Literature, Science and Arts. Zhang's paper on the topic appears this week
in the online Proceedings of the National Academy of Sciences. Zhang believes
that a significant gene duplication made the difference and that it happened sometime
between 23 million years ago and the split of the New World and Old World primates
about 35 million years ago. An ancestor of the Old World primates (humans,
chimps, gorillas, orangutans, gibbons, baboons and guerezas) developed a second
copy of the red/green color-vision gene, which resides on the X chromosome. Female
New World monkeys have full color vision because females have two X chromosomes
that harbor both red and green color vision genes. But males only have one X chromosome,
so New World males have only one copy of either the red or green gene, and that
leaves them color-blind. After the red/green gene duplication in the Old World
family however, even the males got color vision too. "Color vision
made pheromones unnecessary," Zhang said. As a channel for sexual signaling,
color vision works better at a distance than pheromones, Zhang believes. A pheromone
attaches to a water molecule, drifts about in the air currents and finally lands
on the proper receptor in someone else's nose. The receiver can't immediately
be sure who sent it, where it came from or when. But with sexual swelling, everyone
in the troop can see precisely when and where the signal is, even at a significant
distance. Sexual swelling occurs in about 10 percent of all primate species,
but only in the Old World species of Africa and Asia, which is where humans probably
originated, as well. To test their idea, Zhang's team zeroed in on a human
gene called TRP2, which makes an ion channel that is unique to the pheromone signaling
pathway. They found that in humans and Old World primates, this gene suffered
a mutation just over 23 million years ago that rendered it dysfunctional. But
because we could use color vision for mating, it didn't hurt us. In turn, the
pheromone receptor genes that rely on this ion channel fell into disuse, and in
a random fashion, mutated to a dysfunctional state because they haven't experienced
any pressure from natural selection. Zhang calls this process "evolutionary
deterioration."
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