Download Cancer Genes - Lincoln University

“Cancer’s Sex Appeal” by Carl Zimmer
Cancer is not just a terrible disease but a strange one. Tumor cells must switch on certain
genes in order to thrive and multiply. You might expect that natural selection would have
eliminated those genes, because they kill off their owners. Far from it. A number of cancer
genes, known as oncogenes, have actually been favored by natural selection over the past
few million years. Oncogenes, in other words, have boosted the reproductive success of their
owners, and have even been fine-tuned by evolution.
Humans are not alone in getting cancer. In fact, it seems to be a pretty inescapable risk of
being an animal. As cells divide and mutate, some mutations may make cells ignore the
needs of the body and multiply madly. That’s too bad for other animals, but there’s a silver
lining for us: by studying other animals, scientists can get some clues to how cancer evolves
in us.
The delicate swordtail (Xiphophora cortezi) is particularly prone to getting melanomas (the
bottom picture here shows a fish with a tumor in its tail). When Andre Fernandez and Molly
Morris of Ohio University went fishing for delicate swordtails in mountain streams in Mexico,
they found six fish with melanomas in a single day’s catch. These melanomas are particularly
nasty–instead of striking old fish that are going to die soon anyway, they turn up in young
breeders and kill them over a few months.
Melanomas develop from pigment-producing cells in the skin. As these tumors develop, the
cells inside them produce lots of extra proteins from a gene called Xmrk. Despite Xmrk’s
harm, it has survived in good working order for a long time. Functioning versions of Xmrk
exist not just in delicate swordtails, but in related swordtail species that descend from a
common ancestor that lived a few million years ago.
How does such a dangerous gene continue to survive for so long? Fernandez and Morris
have just published an experiment that might solve the mystery. A lot of delicate swordtails
have large dark spots on their tails, like the one shown on the top fish here. Xmrk is essential
for producing those spots. Other fish have been shown to use stripes, spots, and other visual
patterns to attract mates. So Fernandez and Morris wondered what the female delicate
swordtails thought of the Xmrk spots on males.
Turns out, they like them a lot. When offered a chance to pay a visit to one of two male fish,
female delicate swordtails from two populations in Mexico spent more time with spotted
males than spotless ones. And they also preferred to consort with males with big spots over
males with little ones.
The Xmrk gene definitely imposes an evolutionary cost on fish. But that cost may be erased
by the benefit it gives male fish through sexual selection. By the time a male delicate
swordtail dies from an Xmrk tumor, he may have mated with a number of females, which will
pass down the gene to their young.
We humans may also be shaped by the trade-off between sexual selection and the cost of
cancer. Testosterone and related hormones latch onto androgen receptors on the surface of
some cells. It’s important for the development of men’s bodies, for example, and the growth
of body hair. It also plays a role in the production of sperm. These kinds of traits can affect
the success men have in finding mates and having children. But the androgen receptor gene
also becomes active during prostate cancer. In fact, versions of the gene that increase sperm
count in men also raise the risk of cancer.
Posted on “The Loom”, September 13th, 2008
http://blogs.discovermagazine.com/loom/2008/09/10/cancers-sex-appeal/
Discussion questions
1.
Explain why you might expect that natural selection would have eliminated cancer
genes.
2.
How would you expect the effects of natural selection to differ between cancers that
strike old fish that are going to die soon anyway and those, like the example given here,
that turn up in young breeders and kill them over a few months?
3.
This article describes a trade-off between natural selection and sexual selection. What
are the differences and similarities between these two different kinds of selection?
4.
Why might females prefer spotted males over spotless ones?
5.
Why might females prefer males with big spots over males with little ones?
6.
How does the Xmrk gene impose an evolutionary cost on fish? Does this differ between
males and females?
7.
How does sexual selection erase the cost of the Xmrk gene? What benefit does it give to
the individuals that carry it? Does this differ between males and females?
8.
Which are better off, males with the Xmrk gene, or those without it? If you were a male
swordtail fish, would you want to have the Xmrk gene?
9.
What other features of males (of any species) could simultaneously lead to them to be
preferred as sexual partners, but also more likely to suffer an early death? Do not restrict
your answers to anatomical features, what about other things such as behaviour?
10. Can you think of any possible examples of trade-offs between natural selection and
sexual selection in humans?