Bdelloid rotifers

New Delhi, Jan. 29: Scientists have discovered how tiny, primitive leech-like animals found in mud and water have thrived for over 30 million years without sex, solving a long-standing mystery of biology.

Two researchers at Cornell University in the US have shown how the invertebrate animals called bdelloid rotifers have survived so long despite being asexual creatures that reproduce exclusively through cloning. Their research, published today in the US journal Science, also bolsters evidence for a hypothesis to explain why sex is near-ubiquitous among virtually all known organisms.

The history of biology suggests that asexual animals become extinct relatively quickly. Less than 1 per cent of the world’s animals are asexual, but all except the rotifers are young on the tree of evolution.

Scientists have hypothesised in the past that asexual organisms are wiped out by parasites and microbes, but sex allows organisms to shuffle genes and over time evolve resistance to microbes. However, bdelloid rotifers have not just survived without sex for 30 to 50 million years, they have even proliferated into 460 different species.

Now, Cornell scientists Paul Sherman and Chris Wilson have shown that bdelloid rotifers can survive complete drying, fly in the wind, and settle on the ground far away, ready to be awakened on contact with water.

“These animals have survived by staying ahead of their enemies demographically, colonising parasite-free habitat patches, reproducing and departing before they’re attacked and destroyed,” said Sherman, professor of neurobiology and behaviour.

“In essence, they play an unending evolutionary game of hide-and-seek,” Sherman told The Telegraph. The study shows bdelloid rotifers are more resistant to extreme drying and long flights in the wind than their fungal parasites.

“This is exciting research — the survival of asexual bdelloid rotifers has been an evolutionary scandal and this study reveals a mechanism to explain it,” said T. Ramakrishna Rao, a zoologist who has himself conducted research on rotifers at the School of Environmental Sciences, University of Delhi, but who was not connected with the Cornell study.

Scientists believe that the study also throws light on why sex is near-ubiquitous. “It adds to a growing body of evidence that sex functions to protect organisms against parasites and pathogens,” a team member Chris Wilson said.

Sexual reproduction, while common in nature, is a big paradox for biologists. “It is biologically an expensive affair — it costs energy whether for courtship dances or for nest-building,” Rao said.

“Sexual reproduction may also disrupt the flow of favourable genes, and only half of the progeny are females — yet why sexual reproduction has endured in so many creatures has been a big puzzle,” Rao said.

In the Cornell study, Wilson infected bdelloid rotifers with fungi and observed that they all died within weeks. When the rotifers were dried, he found that the fungi were far more vulnerable to dehydration than the rotifers.

In a second experiment, Wilson placed dried, fungus-infected rotifers in a wind chamber and found that the tiny invertebrates flew in the wind to establish new fungus-free colonies.

The bdelloid rotifers have evolved a physiological mechanism to tolerate complete dehydration, Sherman said. Their small and aerodynamic features allow them to be dispersed over thousands of kilometres to establish colonies, the scientists said.