The unassuming raspberry plays host to an ecological battleground, as a fly, a yeast and a fungus vie for dominance on its surface.
Raspberries produce ethylene gas as they mature, and often get colonised by Botrytis cinerea, the grey mould you find on fruit left too long in the fridge. This poses a problem for the fruit fly Drosophila suzukii, which feeds on raspberries, as its larvae are very sensitive to both the fungus and ethylene.
Now, at the Swedish University of Agricultural Sciences (SUAS) and his colleagues have discovered that the fly can employ a biological weapon to even the odds. It spreads a yeast called Hanseniaspora uvarum with it from fruit to fruit that not only makes the raspberry mature more slowly, and thus produce less larvae-killing ethylene, but also attacks the B. cinerea聽fungus with volatile compounds, hampering its ability to harm the larvae.
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But the yeast is no mere passenger 鈥� it releases chemicals that attract the flies, helping ensure it gets picked up in the first place. 鈥淭he yeast releases hundreds of volatile compounds,鈥� says team member , also at SUAS. 鈥淭hese attract other insects too, but are most effective at summoning D. suzukii.鈥�
H. uvarum also grows more vigorously in the presence of the fly larvae, though exactly why is unclear. 鈥淚t might be the mechanical processing of the pulp by the larvae, which opens up the fruit surface, giving access to a good substrate for the yeast to grow on,鈥� says Becher.
This growth boost gives the yeast the edge to keep the B. cinerea聽at bay, which otherwise outcompetes it on fruits with no larvae. Thus, the fly and the yeast work together to ensure their survival on the raspberry battleground.
鈥淲e often think of ecosystems as being large scale, but individual fruit form miniature ecosystems, hosting a variety of interacting organisms,鈥� says at the University of Maryland. 鈥淵easts seem to be important players in these, and often mediate interactions.鈥�
Ultimately, the yeast may have the edge in this tiny tussle, says the team, at least in northern Europe. There, climate change is causing the fly to expand its range, in turn spreading H. uvarum to attack various previously uninfested species of wild berry-bearing plant, including blackberries and others that are important for local wildlife.
Functional Ecology
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