Why fruits go to the dark side

The colours of fleshy fruits are usually attributed to attract seed dispersers to the plant. A cursory look at the gaudy colours of fleshy fruits on offer in a local fruit stall gives the impression that plants use primarily bright colours to attract fruit consumer. This impression is misleading; many small fruits ‘go to the dark side’ and become dark purple or black when ripe. Intermingled in foliage, these colours, which are produced by anthocyanins, can be fairly inconspicuous and are thus not easily reconciled with a signalling function to attract seed dispersers. In this review I therefore discuss complementary hypotheses on the function and evolution of fruit colouration. First, I focus on the evidence that fruit colours indeed function as signals to attract seed dispersers. I then show that anthocyanins, the most prevalent fruit pigments, are important dietary antioxidants that can be selected by blackcaps (Sylvia atricapilla) which are important avian seed dispersers of many European plants. Moreover, the consumption of anthocyanins increases the likelihood that blackcaps mount an immune response during immune challenges. As a next step, I review evidence that anthocyanins accumulate in fruit skin in response to abiotic factors, in particular high illumination coupled with low temperature favour the increase of anthocyanins. Finally, I show that anthocyanins can also be selected for by fruit antagonists, consumers that do not disperse seeds. In particular, high contents of anthocyanins strongly reduce fungal growth in fruit tissue. Taken together, there are various selective pressures which likely influence fruit colour evolution. Currently, the relative importance of each of these selective agents is unknown. There is consequently a need to develop a more encompassing framework on fruit colour evolution.

► Plant pigments fulfil multiple physiological and ecological functions.
► Anthocyanins function in fruits to attract animals visually and to protect chemically the fruits against fungi.
► An encompassing perspective on the various selective pressures acting on plant pigments is needed to understand fruit colour evolution.