The cover image for our March issue features a white-spectacled bulbul (Pycnonotus xanthopygos) feeding on the fruits of the desert plant Ochradenus baccatus near Eilat, Israel. This image relates to the research article: The survival contest of endozoochory: Conflicting interests in a frugivorous avian–plant mutualism, by Beny Trabelcy, Ido Izhaki, and Yoram Gerchman. Here the author Yoram Gerchman tells us the story behind the image.
Ochradenus baccatus fruit pulp is rich in glucosinolates, secondary metabolites common in crucifers’ plants (Brassicaceae) and other plants of the Brassicales order. These metabolites are hydrolyzed by a specialized enzyme, Myrosinase, in a process known as “The Mustard Oil Bomb” that results in the spicy taste of mustard, horseradish, wasabi and such.
Fleshy fruits, such as those of the O. baccatus, are used to attract animals to eat the fruits (frugivores), defecate the seeds (a process known as Endozoochory), and thus facilitate seed dispersal. Given that fruit pulp is commonly poor in protein, while seeds are rich in such, a conflict of interest between the plant and the frugivore arises, as the latter could find digestion of the seeds beneficial.
Our research shows complex interactions between these birds and plants. On one hand, the birds are important distributors of O. baccatus seeds, and separation of the seeds and the pulp through the bird’s digestive system is essential for seed germination. On the other hand, the birds digest and thus destroy up to 80% of the consumed seeds. We also show that the plant utilizes the glucosinolates-Myrosinase system (glucosinolates in the pulp and enzyme in the seeds) to lessen seed predation by the bulbuls. When the birds crush the seeds in their digestive system, the Myrosinase enzyme breaks down the pulp’s glucosinolates, releasing toxic products and resulting in lower digestibility by the birds.