Arum is cheating flies

Black Calla imitates yeasty fragrance, luring insects into the trap

Fruit flies caught in the calyx of the black calla © Johannes Stökl
Read out

A fraud that has been going on for some 40 million years has now been explained to Max Planck scientists. Arum palaestinum, an arum plant found in the Middle East, attracts fruit flies as a pollinator by emitting exactly those fragrance molecules that are produced by yeast fungus on rotting fruits and during alcoholic fermentation.

Thanks to a novel measurement method, functional imaging, researchers from the Max Planck Institute for Chemical Ecology were able to follow the path of fragrant molecules from the plant to the insect brain. They found that the so-called black calla seduces eight different species of fruit fly and that their sensory perception and behavior were almost identical in smelling yeast fermentation and aristocratic fragrance.

In the nervous system of the animals reacted particularly clearly two developmentally original odor receptors. These receptors have probably specialized in the evolution of the perception of yeast smell. The Black Calla abuses this million-year-old instinct of flies for their own purposes, according to the researchers in the journal "Current Biology".

Fly catching plants

The genus Drosophila - referred to as fruit or vinegar flies - is rich in species and has developed many different food sources for themselves. The spectrum includes fruits to bacterial lawns that grow on certain tropical crab species. Many Drosophila species use yeast as a staple food. Their feelers and olfactory bulbs, consisting of so-called glomeruli, specialize in typical scent molecules of growing yeasts. Smallest molecule concentrations in the air are sufficient to point the fruit flies to the food source.

In turn, many flowering plants rely on insects as pollinators to ensure progeny and genetic variability. For example, they attract the animals with colored petals and scented bouquets. When the pollinator reaches the flower, he is rewarded with nectar for transferring the pollen to another plant. The arum plant Arum palaestinum, however, resorted to fraudulent means. display

The plant produces in its flowers, which are violet-black on the inside, a smell, which is compared by humans with that of fruity wine. With this fragrance, so it was to be suspected, the Black Calla attracts their pollinators, namely fruit flies. However, these are not rewarded with nectar in the flowering, but on the contrary still held overnight in the flower and only released the next day.

The black calla (Arum palaestinum), taken in the field. Johannes St kl / MPI Chemical Ecology

Drosophila melanogaster can also be fooled

The Max Planck researchers Johannes St kl and Marcus Stensmyr have not only collected and analyzed this fragrance, but at the same time determined and investigated the fruit fly species that are caught in the calyx of the plants. Together with the behavioral biologist Markus Knaden, they then checked the reactions of the animals to the various fragrance molecules. Her two colleagues Silke Sachse and Antonia Strutz in turn performed neurophysiological experiments on the flies.

An interesting result emerged: Arum palaestinum attracts on average around 140 flies per plant, which belong to eight different Drosophila species, including the well-known species Drosophila melanogaster, which is used as a model organism in many laboratories and their research Genome is completely known. In the fragrance of the plant, the researchers were able to detect 14 different chemical compounds to which the antennae of the flies responded. For these investigations, St kl electrically derived and recorded action potentials from the antennae of the animals.

Identified ester compounds

The chemical analysis of the perfumes released by the plant revealed that they are predominantly ester compounds. However, in this bouquet there were two special "fragrance notes", namely 2, 3-butanediol acetate and acetoin acetate, according to Stensmyr. These molecules are in fact not present in the blooms of flowering plants and are characteristic of vinegar, in particular balsamic vinegar, and wine, that is to say two fermentation products produced by yeast. These two and four other compounds, which are also produced by yeast fermentation, also showed the most stable and strong signals in the electroantennogram.

Optical Imaging of activated glomeruli of Drosophila melanogaster (center). The colored image (right) shows the stimulation when the calla smell smells (red areas, false color display). Left: Fruit fly Drosophila melanogaster. Antonia Strutz

Two receptors, a scam

In neurophysiological studies fruit flies were then exposed to various natural fragrance bouquets, such as putrid peaches or bananas, as well as Lambrusco (red wine) and Aceto Balsamico (vinegar). The respective Elektroantennogrammen strikingly resembled the records with flies that were exposed to the smell of black Calla - almost identical to the Calla scent red wine and vinegar behaved, so the targeted by Hefeg rung products won.

The flies can not distinguish the arum from the rotten fruit - they are cheated by the plant because it only imitates the smell of the yeast, but does not even offer yeast as food STKL. For their involuntary assistance as pollinators, the insects are not even rewarded, but remain hungry in the flower until it reopens after 24 hours.

"Yeast detector"

Thanks to a calcium-sensitive dye, Sachse and Strutz were able to follow the special yeast fragrance stimuli down to the brain of the flies. With this procedure, called functional imaging, they showed that eleven different fragrance receptors responded. Because there were different Drosophila species that invaded the Black Calla, it was obvious that among these receptors could also be evolutionarily early specimens, which is indeed the case.

"The sequence of two scent receptors, Or42b and Or92a, is very conserved - so they can be described as a" yeast detector, "says Bill Hansson. And this result gives reason to fear that Black Calla has been cheating for millions of years.

(idw - Max Planck Institute for Chemical Ecology, 08.10.2010 - DLO)