Thursday, 14 October 2010

It's easy to fool a fly


Orchids do it most famously, but so do lots of plants. They trick insects into taking pollen from one flower to another by producing chemicals that mimic things insects like.

It may be the smell of a female insect or something they like to eat. In some animals I gather these can be the same thing...

The Solomon's Lily (not illustrated above) is the latest system to be studied - you can find a media report at EurekAlert! or the full scientific paper at Current Biology.

EurekAlert! describes the work from scientist at Max Planck Institute for Chemical Ecology in Germany as having "solved a case of fraud that has been pending for 40 million years". A nice line.

Arum palaestinum, native to the Middle East, attracts vinegar flies (species of Drosophila) by releasing chemicals similar to those produced by rotting fruit. Fungal yeasts are the culprits in rotting fruit, fermenting the alcohol as part of their digesting process.

Our lily produces sixe chemicals which in the magnificent evolutionary achievement that is the fly brain, create the "impression of fermentation". Two of the chemicals are very rare in plants but are found in wine and vinegar. It turns out these two chemicals attract quite a few different vinegar fly species and the plant "exploiting" an instinct that dates back a million years or so, to the early evolution of Drosera.

Vinegar flies eat mostly rotting fruit which they find by tracking down the smell of active yeast.

The Solomon Lily has violet-black coloured flowers that smell like a "fruity wine". The vingear flies attracted by this intoxicating odor get trapped inside the flower overnight. Lodgings perhaps, but no board. There is nothing for the fly to eat inside the flower. Its 'reward' is to carry pollen from this flower to the next tricky bloom, or perhaps less helpfully to the plant, a piece of rotting fruit.

The scientist studied the fly-lily interaction in detail. On average 140 flies visited each flower, represening 8 species of Drosophila including (as EurekaAlert! put it) "the well-known laboratory work horse and kitchen nuisance Drosophila melanogaster". The reactions in the fly antennae were measured and analysed, revealing the most attractive chemicals to be ones normally found in vinegar and wine, and not usually in flower perfumes.

The conclusion by Johnannes Stökl was that "The flies are unable to distinguish the lily from rotting fruit – they are deceived by the lily because it imitates the yeasty odor although it does not offer yeast as food".

They took the study even further, delving deep inside the head of the flies to find where the yeast odour activated the brain. Eleven odour receptors were activated and because different species were deceived in the same way they concluded the "yeast dector" trait was an ancient one in evolutionary terms - that is, most or even all, vinegar flies would have it.

Just as every story on carnivorous plants has to end with "And could you use this to kills flies/mosquitoes in your home?" (answer: not many), the media story ends with:

"Is keeping a Solomon's lily now the ultimate solution to rid your kitchen of flies? "Well, since they only flower once per year, and then only for a few hours, a cup of vinegar is still a better option. However, during the hours it flowers, I can assure you there will be no flies left in your kitchen!" says Marcus Stensmyr."

Image: this is a different species of arum lily but also one with dark purple-black flowers. The bloom in this case produces a melodorous odour and attracts flies that like rotting meat. It's Dracunculus vulgaris, an arum also native to the Mediterranean region but photographed here in Chelsea Physic Garden in London (we also grow it in Sydney's Royal Botanic Gardens).

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