The monarch butterfly's annual migration from North America to Mexico, a spectacular event at risk of disappearing forever, is the result of a single, millions-year old gene, biologists said Wednesday.

Analysis of the insect's long-guarded genetic secrets has changed what we knew about its history, migration and distinctive bright orange-and-black wing colouring, they wrote in the journal Nature.

"Prior to our work, monarch migration was thought by some to be a very recent phenomenon, but we have shown that it evolved millions of years ago," study co-author Marcus Kronforst told AFP of the findings that he said "overturn past thinking about monarch butterflies".

"I believe this raises the stakes considerably when we consider that we may be witnessing the end of an amazing biological phenomenon that these little insects have been carrying out every year for the past two million-plus years."

There has been a sharp decline in the number of monarchs who undertake the annual trek from as far north as Canada to Mexico for the winter before their descendants, over several generations, make the journey back north for summer.

In 1996, about a billion of the fluttering insects known to scientists as Danaus plexippus, completed the gruelling north-south trip, but only 35 million in the past year, according to the study authors.

The drop has been blamed on deforestation, drought and a sharp decline ascribed to herbicide use in the milkweed plants on which they feed and lay eggs.

"You used to see huge numbers of monarchs, clouds of them passing by," said Kronforst. "Now it looks quite possible that in the not too distant future, this annual migration won't happen."

At the same time, many groups that used to migrate are morphing into stationary populations.

- 101 genomes -

Kronforst and an international team of experts sequenced 101 genomes of butterflies from around the world, including non-migratory and white Danaus varieties.

Because most members of the monarch butterfly family outside North America are tropical and non-migratory, the common ancestor was long thought to have had those same characteristics and to have acquired migration much later.

Under that theory, monarchs first crossed the Pacific and Atlantic oceans in the 1800s.

However, the mapping of their gene tree has shown that the insects originated from a migratory ancestor in North America about two million years ago.

From there, they moved into Central and South America some 20,000 years ago and probably crossed the Atlantic and Pacific already 2,000 to 3,000 years ago, said Kronforst.

Today, they exist as far afield as Europe, Africa and Australia.

To their surprise, the team discovered that migratory ability, a complex animal behaviour, was linked to a single gene implicated in wing muscle formation and function.

It was likely responsible for the long-haul fliers' more efficient oxygen consumption and flight metabolism.

"The butterflies that moved out of North America lost migration," Kronforst added, and the migratory gene "changed".

"I believe our results have implications for helping conserve the monarch migration simply by accentuating the biological significance of the phenomenon," the researcher said.

The team also found that a different single gene was responsible for the monarch's famous colouring.