Busy bees and helpful fungi are the secret to orchids' success

8th Feb 2011

The way orchids share pollinators and guard their own unique nutrient-supplying fungus has made them one of the world's most successful plant groups.

That's the conclusion of a ten-year study on how the relationship between the flowers and their pollinators leads to so many different types of orchid.

The orchid family includes over 22,000 species worldwide. Many have complex, beautiful flowers. Some look so much like insects, they fool wasps into mating with them: as the wasp embraces the flower, the orchid deposits a small pollen package on its abdomen to be taken to the next flower.

The study highlights that the continued decline in bees, wasps, butterflies and other pollinators could put some orchids at a greater risk of being driven to extinction.

Scientists have long known about these specialised relationships with insects which pollinate the flowers in return for food such as nectar or oils. But until now, they weren't sure how this created such a wealth of orchid diversity.

After studying orchids in a small region of South Africa, the researchers found that many different types of orchids can live next to one another, because they deposit their pollen on different parts of the same bee. This means the pollen doesn't get mixed together so the same bee can serve two different orchid species. The plants effectively share the bees, allowing many different orchids to survive happily in one place.

For example, when the orchids Pterygodium pentherianum and Pterydoium schelpei live side-by-side, pentherianum puts its pollen on the bee's front legs, but schelpei sticks it to its abdomen.

The reason orchid flowers have complex shapes is because they attract particular pollinators. They deliver their pollen to a precise location on insects' bodies,' explains Professor Tim Barraclough from Imperial College London, a specialist in the evolution of species diversity and co-author of the study.

But when it comes to fungi, the orchids aren't so keen on sharing. The team found that orchids tend to use a different fungus to their neighbours, 'probably to avoid competing for nutrients provided by individual fungi,' adds Barraclough.

Most flowering plants have fungi living on their roots, which break down soil, allowing plants access to nutrients they couldn't get had the fungi not been there.

The international team of researchers from Imperial College London, the Royal Botanic Gardens, Kew, Stellenbosch University and the universities of Washington and Bayreuth studied 52 species of orchid from the Namaqualand and the Drakensberg regions of South Africa. 'Both areas are among the world's top hotspots for botanical diversity,' says Barraclough.

The orchids they studied all secrete oil inside their flowers, which female bees collect to feed their larvae. To find out which bees visited which orchid, they collected the pollen from the insects and analysed its DNA.

They found that when an orchid moves to a new area, it adapts to a new bee species, while some competing orchids adjust by placing pollen on different parts of the same bee.

'It's given us a fundamental insight into how so many new species can originate, and once they originate how they are able to coexist without exchanging genes,' says Barraclough. 'Our work shows that shifts in pollination traits are important for generating new species and allowing coexistence, but fungi are important for coexistence, but not speciation.'

Barraclough says the next obvious thing to find out is if these findings apply to other orchids from other countries.

The study is published in The American Naturalist.

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