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A contagious cancer has been killing off Tasmanian devils in large numbers. But some have avoided the disease. The key to their salvation, a new study shows, has been their genes.
Tasmanian devils are small, aggressive marsupials on the Australian island of Tasmania. In 1996, researchers noticed some devils had tumors on their faces. Those cancers can stop an animal from eating and breathing, causing death. Unlike most human cancers, this one can spread from animal to animal. The infectious disease was first traced to a female devil. It spread its cancer to other devils through biting. (These animals frequently bite each other during mating.) The disease has now wiped out about 80 percent of the fierce animals. In some places, it has killed up to 95 percent of them.
Usually cells from different animals can’t grow in each other’s’ bodies. If you kiss your mom, her cells don’t start growing on your lips. The immune system, which helps fight off germs, also kills cells from other individuals. That is why people who get organ transplants have to take drugs to stop their bodies from rejecting those “foreign” organs. But devil facial tumors can hide from the immune system.
This was one reason scientists worried about the devils. “What we reluctantly felt was that this was the end for the Tasmanian devil,” says Jim Kaufman. There were worries it could go extinct “because they really didn’t have a defense.” Kaufman is a scientist who studies how the immune system evolved. He works at the University of Cambridge in England.
But researchers have just turned up good news. Many devils have versions of genes that may protect them from the cancer. Researchers reported this August 30 in Nature Communications. Concludes Kaufman, this “is really the most hopeful thing I’ve heard in a long time.”
In the genes
Menna Jones is a conservation biologist at the University of Tasmania in Hobart. As the cancer spread across Tasmania, she and her coworkers collected DNA from devils in three places. They did this both before and after the disease arrived at the locations.
Jones then teamed up with Andrew Storfer. He is an evolutionary geneticist from Washington State University in Pullman. His team examined the devil’s genetic instruction book, or genome. These researchers wanted to see if there was something special about devils who remained healthy after the infectious disease arrived. That might explain why they survived while the rest did not.
Some scientists had suspected the survivors just hadn’t caught the cancer yet, Kaufman notes. It was thought they had been too young to breed and get bitten. The new study, though, indicates that differences in their DNA may have protected some devils from the cancer.
Storfer and his team found more than 90,000 DNA spots where a small number of devils have a different base — an information-carrying component of DNA — than in most devils. These genetic spelling differences are known as single nucleotide polymorphisms (PAH-lee-MORF-isms), or SNPs, pronounced “snips.” The team looked for SNPs that had been rare before the tumor swept through a population but then became common after the disease arrived. Such a pattern could indicate that natural selection was working. This evolutionary process meant devils with the right SNP variants would avoid the deadly infection.
Two parts of the genome in all three of the devil populations contained SNPs that fit such a profile. Because the two regions changed in all three populations, the change probably didn’t happen by chance, the researchers say. These variants aren’t new. They were already there. But very few devils had them before the tumor came. What changed? Only devils with those SNPs survived long enough to breed and have babies. And their offspring all inherited the protective SNPs.
Those two genome regions contain a total of seven genes. Genes are the specific instructions in the big genome instruction book. They tell cells how to build particular proteins. Proteins do most of the work of building cells, digesting food and all the other things a body must do. Some genes that fight the facial tumor have, in other mammals, been shown to combat cancer or control the immune system. The researchers, however, aren’t sure which of the genes were most protective for the devils, let alone how they might work.
One day, scientists might use the new SNP data to better predict which devils will be at risk of the deadly face cancer. Breeding programs set up to save the devils might also use these data. Such programs might mate animals that carry the survival variants to ones that don’t. That would up the chance that the next generation would carry the guardian SNPs.
But there is a new complication. Last year, a second devil facial cancer emerged. Also infectious, it looks very much like the first tumor. In this case, it started in a male. Katherine Belov is a comparative genomicist at the University of Sydney in Australia. Researchers don’t know whether the SNPs that allow devils to resist the first infectious cancer will also work against the second, he notes. That’s why conservation biologists should not breed the resistance genes into all Tasmanian devils, she warns.
Limiting which devils breed with which others could reduce the diversity of genes in this species. Genetic diversity is high when there are many versions of genes present throughout members of a species. And devils need all the diversity they can get to cope with other diseases and other unknown challenges in the future, Belov says. Restricting breeding to animals that have these anti-tumor SNPs could limit the species’ ability to overcome other problems in the future.
Cancers that spread
Several cancer can be spread by infectious microbes, such as viruses. Feline leukemia is a well-known example. But what afflicts the devils is different. It’s not a germ that infects them but a sickening cell — a cancer cell — from a member of their own species.
Scientists used to think that such contagious cancer cells were rare. For a long time, only one type was known — in dogs. For 11,000 years, some of them had been spreading such an infectious dog cancer through mating. Then the first contagious devil cancer emerged 20 years ago. Scientists reported the second one last year.
But it was far from the only such new infectious cancer. Also in 2015, researchers found such a cancer — in this case, a leukemia — infecting soft shell clams. This past June, researchers reported finding more contagious shellfish cancers. They infected mussels, cockles and golden shell clams. Golden shell clams (Polititapes aureus) caught their cancer from pullet carpet shell clams (Venerupis corrugate), which no longer carry the disease. That was the first report of such contagious cancer cells infecting a new species.
With two such infectious cancers in Tasmanian devils, some researchers now wonder if their species is especially prone to contagious cancers. No one can yet answer that.
But in more good news for the devils, scientists have reared many healthy animals in captivity. Last month, 33 of them were released into the wild. Those devils had been given vaccines to help them ward off the facial tumor. Scientists hope they will breed with wild devils remaining to increase their genetic diversity.
The researchers have outfitted the newly released devils with collars with tracking “tags.” This will help them map the animals as they move around. The collars also are reflective. That may help to save these animals from the fate of devils previously released: Many were hit and killed by cars.
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breed (noun) Animals within the same species that are so genetically similar that they produce reliable and characteristic traits. German shepherds and dachshunds, for instance, are examples of dog breeds. (verb) To produce offspring through reproduction.
cancer Any of more than 100 different diseases, each characterized by the rapid, uncontrolled growth of abnormal cells. The development and growth of cancers, also known as malignancies, can lead to tumors, pain and death.
cell The smallest structural and functional unit of an organism. Typically too small to see with the naked eye, it consists of watery fluid surrounded by a membrane or wall. Animals are made of anywhere from thousands to trillions of cells, depending on their size. Some organisms, such as yeasts, molds, bacteria and some algae, are composed of only one cell.
cockles A type of clam that with a somewhat ribbed, triangular shell. These mollusks have a strong, muscular foot that allows them to effectively hop along the ocean floor when they need to move. They tend to burrow just below the mud or sand in tidal beach areas and to depths of 15 meters (50 feet).
colleague Someone who works with another; a co-worker or team member.
component An item that is part of something else, such as pieces that go on an electronic circuit board.
conservation The act of preserving or protecting something. The focus of this work can range from art objects to endangered species and other aspects of the natural environment.
conservation biologist A scientist who investigates strategies for helping preserve ecosystems and especially species that are in danger of extinction.
contagious Likely to infect or spread to others through direct or indirect contact; infectious.
defense (in biology) A natural protective action taken or chemical response that occurs when a species confront predators or agents that might harm it. (adj. defensive)
diversity (in biology) A range of different life forms.
evolutionary An adjective that refers to changes that occur within a species over time as it adapts to its environment. Such evolutionary changes usually reflect genetic variation and natural selection, which leave a new type of organism better suited for its environment than its ancestors. The newer type is not necessarily more “advanced,” just better adapted to the conditions in which it developed.
gene (adj. genetic) A segment of DNA that codes, or holds instructions, for producing a protein. Offspring inherit genes from their parents. Genes influence how an organism looks and behaves.
genetic Having to do with chromosomes, DNA and the genes contained within DNA. The field of science dealing with these biological instructions is known as genetics. People who work in this field are geneticists.
genetic diversity The range of genes types — and traits — within a population.
genome The complete set of genes or genetic material in a cell or an organism. The study of this genetic inheritance housed within cells is known as genomics.
germ Any one-celled microorganism, such as a bacterium, fungal species or virus particle. Some germs cause disease. Others can promote the health of higher-order organisms, including birds and mammals. The health effects of most germs, however, remain unknown.
immune system The collection of cells and their responses that help the body fight off infections and deal with foreign substances that may provoke allergies.
infectious An adjective that describes a type of germ that can be transmitted to people, animals or other living things.
leukemia A type of cancer in which the bone marrow makes high numbers of immature or abnormal white blood cells. This can lead to anemia, a shortage of red blood cells.
mammal A warm-blooded animal distinguished by the possession of hair or fur, the secretion of milk by females for feeding the young, and (typically) the bearing of live young.
marsupials Mammals that carry their young for a period after birth in external pouches where the developing babies will have access to their mother’s nipples — and milk. Most of these species evolved in Australian and have especially long hind-legs. Examples of marsupials include kargaroos, opossums and koalas.
natural selection This is guiding concept underlying evolution, or natural adaptation. It holds that natural mutations within a population of organisms will create some new forms that are better adapted to their environment. That adaptation makes them more likely to survive and reproduce. Over time, these survivors may come to dominate the original population. If their adaptive changes are significant enough, those survivors may also constitute a new species.
organ (in biology) Various parts of an organism that perform one or more particular functions. For instance, an ovary is an organ that makes eggs, the brain is an organ that interprets nerve signals and a plant’s roots are organs that take in nutrients and moisture.
population (in biology) A group of individuals from the same species that lives in the same area.
proteins Compounds made from one or more long chains of amino acids. Proteins are an essential part of all living organisms. They form the basis of living cells, muscle and tissues; they also do the work inside of cells. The hemoglobin in blood and the antibodies that attempt to fight infections are among the better-known, stand-alone proteins. Medicines frequently work by latching onto proteins.
resistance (as in drug resistance) The reduction in the effectiveness of a drug to cure a disease, usually a microbial infection. (as in disease resistance) The ability of an organism to fight off disease. (as in exercise) A type of rather sedentary exercise that relies on the contraction of muscles to build strength in localized tissues.
single nucleotide polymorphism (or SNP) A SNP (pronounced “snip”) is DNA in which one of its original nucleotides has been naturally substituted for another. This variation may alter the function of DNA. SNPs are inherited. Each person carries millions of SNPs, making them unique from other people.
species A group of similar organisms capable of producing offspring that can survive and reproduce.
tag (in biology) To attach some rugged band or package of instruments onto an animal. Sometimes the tag is used to give each individual a unique identification number. Once attached to the leg, ear or other part of the body of a critter, it can effectively become the animal’s “name.” In some instances, a tag can collect information from the environment around the animal as well. This helps scientists understand both the environment and the animal’s role within it.
tumor A mass of cells characterized by atypical and often uncontrolled growth. Benign tumors will not spread; they just grow and cause problems if they press against or tighten around healthy tissue. Malignant tumors will ultimately shed cells that can seed the body with new tumors. Malignant tumors are also known as cancers.
vaccine A biological mixture that resembles a disease-causing agent. It is given to help the body create immunity to a particular disease. The injections used to administer most vaccines are known as vaccinations.
variant A version of something that may come in different forms. (in biology) Members of a species that possess some feature (size, coloration or lifespan, for example) that make them distinct. (in genetics) A gene having a slight mutation that may have left its host species somewhat better adapted for its environment.
Journal: B. Epstein et al. Rapid evolutionary response to a transmissible cancer in Tasmanian devils. Nature Communications. Vol. 7, August 30, 2016, p. 12684. doi: 10.1038/ncomms12684.
Journal: R.J. Pye et al. A second transmissible cancer in Tasmanian devils. Proceedings of the National Academy of Sciences. Vol. 113, January 12, 2016, p. 374. doi: 10.1073/pnas.1519691113.