Tasmanian devils & cancer provide new insights into the evolution of sex


EXCERPT: For 50 years we have known that it makes sense to have sex sometimes. Sometimes [...] it means only having sex once every few generations — like the cockroach, who can be chaste for generations on end. An occasional dalliance, evolutionarily speaking, provides a useful shuffling of genes between individuals. But why is sex the go-to reproductive strategy for so many creatures? ... One possible answer has been put forward ... sex may have evolved to combat transmissible cancer.

[...] the plants and few animals who reproduce without sex can get the job done a lot faster. When you don't rely on sex, every individual has the capacity to produce young, said study co-author Tomas Madsen of Deakin University. "You don't have to carry the cost of having males around, because we do bugger all," Professor Madsen said. "We contribute with our sperm but females that can produce females, and those females can produce females in turn, will have much higher reproductive output than a female investing half in males." And then there is all the energy that goes into trying to find and compete for a mate.

But sex does have upsides. Sex mixes up genes, keeping one step ahead of parasites. It also introduces exciting new combinations of genes and reduces the build-up of harmful mutations, Professor Madsen said. The thing is, you can get these benefits from only having sex occur every few generations. Some animals, such as the Komodo dragon, can reproduce both sexually and asexually. "So why don't you stop having sex for 500 generations and then, when you need to, switch [to having sex again]?" Professor Madsen said. The research team thinks their hypothesis answers this long-standing question in biology.

Cancer has been around for more than a billion years. As a result, it has had a huge impact on evolutionary history, said Dr Hamede, who studies Tasmanian devils. "In wildlife, cancer and its effects, at individual, population and ecosystem levels are completely overlooked, as most animals die from cancer unseen and undetected," he said. "It's easy to forget that cancer is a disease that has been present for millions of years and affects almost all multicellular organisms."

The cancers we are most familiar with originate in and die within the same individual. But a transmissible tumour can live forever. "Transmissible cancers are an immortal cell line, they can be regarded as immortal living entities," Dr Hamede said. "The oldest cell line known in biology is transmissible cancer in dogs, which originated in wild canids [dogs] around 10,000 years ago.

If transmissible cancers were common, the result would be devastating and lethal for species around the globe. This, according to Dr Hamede, created pressure for the evolution of strategies to combat it. When an organism reproduces without having sex, its offspring are genetically identical clones. The New Mexico whiptail is an example of an animal that reproduces asexually. This small lizard species only consists of females.

Cancer can spread between clones, because a clone's immune system will not identify the cancer cells as foreign and fight them, Dr Madsen said. "If you have sex in each generation, you change the genome," he said. "When a cancer cell comes from individual A to individual B, individual B will recognise that [cancer cell] is not the same as me. But if they are clonal, they can't tell them apart … So the cancer cells can sneak in without being detected by the immune system."

Because cancer occurs in every generation, if the genetics of a population are not shaken up with sex every time reproduction occurs, that population will be vulnerable to transmissible cancer. It's a novel idea, said James DeGregori, an expert on the evolution of cancer of the University of Colorado. "My first reaction was scepticism. [But] once I read through the argument, I thought this is a very reasonable hypothesis," he said.

But Tassie devils have sex, so why do they get transmissible cancer? Although Tasmanian devils are not clones, inbreeding has taken its toll on their immune system. The Tasmanian devils running around today are the descendants of very few ancestors. This history means that they are very genetically similar to each other. Being inbred makes them susceptible to transmissible cancer, Professor Madsen said. "A healthy devil bites into the tumour of the sick devil and cancer cells come into the mouths of the healthy devil," he said. (MORE - details)

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