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Scientists have discovered why orchids are one of the most successful groups of flowering plants – it is all down to their relationships with the bees that pollinate them and the fungi that nourish them.

The orchid family is one of the largest groups of flowering plants, with over 22,000 species worldwide. Today’s research suggests that there is such a huge range of species because orchids are highly adaptable and individual species can interact with bees, and other pollinators, in different ways.

For example, when orchids Pterygodium pentherianum and Pterygodium schelpei live side by side, Pterygodium pentherianum puts its pollen on the bee’s front legs, whereas Pterygodium schelpei puts it on the bee’s abdomen. This means that one bee can carry pollen from two distinct species without mixing it.

The study also shows how orchids are able to live harmoniously together, with different species working in partnership with different microscopic fungi in the soil, ensuring they do not compete with each other.

Prior to today’s study, it was known that orchids have strong interactions with bees, which pollinate the flowers in return for food such as nectar or oils, and also with fungi, which supply minerals to the roots in return for sugars. These relationships are amongst the best examples of nature’s system of ‘mutual benefit’ and are believed to have been important for enabling orchids to evolve into so many different species. However, the mechanisms by which these relationships affect the number of plant species, and these species’ ability to coexist, had remained obscure.

The group studied 52 orchid species in a small region of South Africa, which all secrete oil inside their flowers that female bees collect to feed to their larvae. In order to investigate which pollinating bees were visiting the different species, they collected orchid pollen from the bees for DNA sequencing and analysis. They found strong evidence that when an orchid moved to a new geographical area it adapted to a different pollinating bee species, and interestingly, some competing orchid species were able to adapt by placing pollen on different body parts of the same bee.

“What is remarkable in these orchids is that diversity is generated not only through switches between bees, but also by switches between different body parts of the same bee, so two closely related orchids might place pollen on different segments of one bee’s front leg,” added Professor Barraclough. “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.”

The researchers also studied the microscopic fungi living on the roots of the orchid, to see how this relationship was affecting plant diversity. Most flowering plants host microscopic fungi in their roots that help the plant take up nutrients from the soil. Until now it has been difficult to investigate this interaction, as most of the fungi belong to species that are difficult to culture. The researchers overcame this challenge by combining a molecular technique known as DNA barcoding with field experiments. In contrast to the bees, where co-occurring orchid species normally share the same insect pollinator, the plants needed to use different fungal partners in order to coexist in the same region.

“By tapping into different kinds of fungi, different plant species access different pools of nutrients and so the problem of living together without competing for the same resources is solved,” said Professor Barraclough. However, the same fungal partners are found in different geographical areas and so orchid species that originate in different areas, by adapting to different pollinators, tend still to use the same fungi.

The team’s fieldwork shows that shifts in pollination traits were important for bringing about new species and allowing coexistence in a diverse group of orchids, whereas shifts in fungal partner were important for coexistence but not for speciation. Many other groups of flowering plants enter into similar relationships with pollinators and fungi, and both the origins and the future survival of that diversity could depend critically on understanding these relationships.

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The Effects of Above- and Belowground Mutualism on Orchid Speciation and Coexistence

Rhizanthella gardneri is a cute, quirky and critically endangered orchid that lives all its life underground. It even blooms underground, making it virtually unique amongst plants.

Last year, using radioactive tracers, scientists at The University of Western Australia showed that the orchid gets all its nutrients by parasitising fungi associated with the roots of broom bush, a woody shrub of the WA outback.

Now, with less than 50 individuals left in the wild, scientists have made a timely and remarkable discovery about its genome.

Despite the fact that this fully subterranean orchid cannot photosynthesise and has no green parts at all, it still retains chloroplasts – the site of photosynthesis in plants.

“We found that compared with normal plants, 70 per cent of the genes in the chloroplast have been lost,” said Dr Etienne Delannoy, of the ARC Centre for Excellence in Plant Energy Biology, the lead researcher of a study published in Molecular Biology and Evolution. “With only 37 genes, this makes it the smallest of all known plant chloroplast genomes.”

“The chloroplast genome was known to code for functions other than photosynthesis, but in normal plants, these functions are hard to study,” said ARC Centre Director Professor Ian Small.

“In Rhizanthella, everything that isn’t essential for its parasitic lifestyle has gone. We discovered that it has retained a chloroplast genome to make only four crucial proteins.

Our results are relevant to understanding gene loss in other parasites, for example, the Plasmodium parasite that causes malaria.”

Associate Professor Mark Brundrett from the Wheatbelt Orchid Rescue Project describes Rhizanthella as one of the most beautiful, strange and iconic orchids in the world.

“Combining on-the-ground conservation efforts with cutting edge laboratory technologies has led to a great discovery with impacts for both science and conservation. The genome sequence is a very valuable resource, as it makes it possible to estimate the genetic diversity of this Declared Rare plant”.

Professor Brundrett has been working with the Department of Environment and Conservation and volunteers from the West Australian Native Orchid Study and Conservation Group to locate these unique orchids.

“We needed all the help we could get since it often took hours of searching under shrubs on hands and knees to find just one underground orchid!”
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