Underground orchid is evolutionary enigma

By Alexandra Back 15 February 2011
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An orchid that lives and blooms completely undergound shows evolution in action.

A RARE AND UNUSUAL ORCHID that lives and blooms in complete darkness underground is providing new clues about plant evolution.

The critically endangered orchid (Rhizanthella gardneri) from Western Australia has no greenery or chlorophyll with which to produce its own nutrients; instead it acts as a parasite, feeding on a species of fungus found around the roots of an outback shrub.

Professor Ian Small from the University of Western Australia (UWA), the co-author of a new study on the species, says that through evolution it has lost features that aren’t vital for its parasitic lifestyle. “It has gone nearly as far as it can go in terms of gene loss,” he told Australian Geographic.

No photosynthesis but chloroplast genes remain

The orchid has just a third of the chloroplast genes – which help turn sunlight into energy – normally found in plants, says lead author Dr Etienne Delannoy also at UWA. With only 37 genes, “it the smallest of all known plant chloroplast genomes,” he says.

But what is interesting is that the species has kept any chloroplasts at all, given that it has not photosynthesised for tens of millions of years and lives its entire life underground.

It is a timely discovery for the orchid, of which researchers estimate there are fewer than 50 individuals in the wild. This number is inexact, because the orchid is a particularly challenging specimen to find.

“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,” says Professor Mark Brundrett from the joint government and community funded Wheatbelt Orchid Rescue Project, which was involved in the research.

Unraveling the function of remaining chloroplast genes

“This work demonstrates the leaps and bounds that plant science is making now, with the ability to sequence whole genomes incredibly quickly,” comments Professor Tony Larkum from the School of Biological Sciences at the University of Sydney.

But deciphering just what those genes do is proving difficult. The non-photosynthesising role of chloroplasts in plants is hard to study because many genes are responsible for multiple tasks. “It is difficult to look for effects over and above the major function of photosynthesis,” he says.

Researchers do however know that the remaining four chloroplast genes are responsible for the production of proteins essential for the orchid’s survival.

The research may also lead to better understanding about other parasitic species like the malaria parasite, says Ian, which has followed a similar evolutionary path by streamlining its genome.

The findings are published this month in the journal Molecular Biology and Evolution.