Study sheds new light on sexless creature

No sex please, we're rotifers (Credit: C Wilson)

New research backs the idea that a group of celibate animals can survive without sex because they can dry up and lie dormant to evade disease.

Published today in the journal Proceedings of the Royal Society B, the study looked at microscopic organisms called bdelloid rotifers, which reproduce by laying eggs that hatch into identical clones of themselves.  

Scientists have long been puzzled by how these creatures have survived for over 40 million years despite never having sex. In other animals, sexual reproduction is a key part of natural selection, which allows them to evolve defences against changing environmental conditions, especially new diseases.

The new study shows that bdelloid rotifers are able to avoid disease by lying dormant, drying up and drifting around on the wind.

Researchers from Imperial College London studied bdelloid rotifers living in patches of moss on tree trunks. They saw that many of the animals were suffering from fungal infections, which spread rapidly after a damp Summer.

During the exceptionally dry months of March and April 2011, the water-loving rotifers survived by withdrawing into a hardy, ball-like dormant state. When moisture returned, the rotifers emerged healthy, but the fungus was almost completely destroyed.

Lead researcher, Dr Chris Wilson, from the Department of Life Sciences at Imperial College London, said: "This unusual escape act may help to explain the other 'dry spell' that bdelloid rotifers are known for: how their whole lineage has lasted for tens of millions of years without sex.

"If you don't need to adapt constantly to some disease or other, it's not as vital to mix up your genes through sex.  So, if bdelloid rotifers escape infections by drying up or drifting to new habitats, it may help explain how they’ve managed without sex for so long.".

Dry rotifers are blown around very easily, like dust on the wind. So even in a short dry spell, they are able to escape from patches infested with fungus and start healthy populations after landing somewhere new. There was almost no disease in the animals collected in wind traps, or from moss higher up on trees. The new study is the first to demonstrate this disease-evading behaviour in the field.
 
"We've seen in laboratory work that bdelloid rotifers can survive dehydration for a period. But these results from the field now show that droughts like the one in 2011 actually keep them healthy, by killing the fungal parasites that attack them when it's wet,” added Dr Wilson.

In the future, Dr Wilson hopes the results could help scientists develop ways to control some economically destructive crop pests.  Bdelloid rotifers are harmless, but they share the soil with tiny roundworms called root-knot nematodes, which also multiply without males, and cause severe damage to plants.

"Nematode pests lack sex too, but don't have the same trick as rotifers for escaping parasites during dry spells. We hope that by doing more research we may find a way to kill them off using the same fungal diseases that affect the rotifers."