Sex is weird. But for evolutionary biologists, it's mystifying.

Key points: Sex as the primary way organisms reproduce may have evolved to combat transmissible cancer

Sex as the primary way organisms reproduce may have evolved to combat transmissible cancer Studying cancer in wildlife can help us understand evolution

Studying cancer in wildlife can help us understand evolution Transmissible cancer is rare, but the disease has plagued Tasmanian devils

For 50 years we have known that it makes sense to have sex sometimes.

Sometimes, in this case, doesn't mean a cheeky once-a-month; 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? Why are most animals obligated to get it on to produce offspring?

One possible answer has been put forward by a team of ecologists, evolutionary biologists and cancer researchers: sex may have evolved to combat transmissible cancer.



"Cancer has shaped the ecology and evolution of wildlife for thousands and millions of years," said Rodrigo Hamede of the University of Tasmania, co-author of a recent paper on the subject in PLOS Biology.

Rodrigo Hamede holds a Tasmanian devil with a facial tumour.

Cancer itself is very common because wherever you have an organism that has more than two cells rubbing together, there is the potential for cancer.

But transmissible cancer is very rare. It only affects a handful of animals we know of, such as oysters, dogs and Tasmanian devils.

The new hypothesis suggests transmissible cancer may be so rare because having sex at every generation puts the brakes on the disease.

In evolutionary terms, however, having sex all the time is a bit of a drag.

Why does sex suck?

Sex is by far the most common way to reproduce. But the plants and few animals who reproduce without it 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.

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"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.

Komodo dragons are one of the rare species which can reproduce both sexually and without having sex. ( Flickr: Mark Dumont )

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.



What does cancer have to do with evolution?

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.

Cancer has been around for over a billion years, meaning it has affected evolutionary history.

"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.

"That means this tumour has managed to maintain itself, jumping from host to host for thousands of years."

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.

How could sex combat transmissible cancer?

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."

The New Mexico whiptail lizard is a female-only species, where daughters are genetically identical their mothers. ( Bryant Olsen: Flickr )

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.

"If the immune system had been very different between the two devils, those cancer cells would have been killed straightaway."

Studying transmissible cancers in animals such as devils, the researchers suggest, offers an insight into what life on earth might be like if sex was not the norm and genetically very similar individuals were more common.

Tasmanian devils pass transmissible cancer cells to each other through biting. A reason they can get transmissible cancer is because they are very genetically similar. ( Supplied: Rodrigo Hamede )

"Just by studying transmissible cancers we can start making assumptions and then refine our hypothesis about the evolution of sex," Dr Hamede said.

And the study of cancer in wildlife may also help develop future treatment options for humans, he added.

"Our research is the tip of the iceberg. The more learn about cancer and how has it shaped wildlife, the more we learn about cancer biology in general. And that also has implications for treatments for novel therapies for improving the way we treat people living with cancer."