Intraspecific Directed Deterrence by the Mustard Oil Bomb in a Desert Plant

Abstract

Plant secondary metabolites (SMs) acting as defensive chemicals in reproductive organs such as fruit tissues play roles in both mutualistic and antagonistic interactions between plants and seed dispersers/predators. The directed-deterrence hypothesis states that SMs in ripe fruits deter seed predators but have little or no effect on seed dispersers. Indeed, studies have demonstrated that birds are able to cope with fruit SMs whereas rodents are deterred by them. However, this mechanism was only demonstrated at the class level, i.e., between birds and mammals, based on differences in the vanilloid receptors. Here we present experimental and behavioral data demonstrating the use of the broad-range, class-independent "mustard oil bomb" mechanism in Ochradenus baccatus fruits to force a behavioral change at an ecological timescale, converting rodents from seed predators to seed dispersers. This is achieved by a unique compartmentalization of the mustard oil bomb, causing activation of the system only upon seed and pulp coconsumption, encouraging seed dispersal via seed spitting by rodents. Our findings demonstrate the power of SMs to shift the animal-plant relationship from predation to mutualism and provide support for the directed-deterrence hypothesis at the intraspecific level, in addition to the interspecific level.

Physiological and behavioural effects of fruit toxins on seed-predating versus seed-dispersing congeneric rodents

“…For experiments in which separation of pulp and seeds was needed, fruits were kept at 4°C and pulp was manually separated from the seeds within 2days of collection and then kept separately at -20°C. Stored fruits maintained active myrosinase and GLS content (Samuni-Blank et al, 2012). Ochradenus baccatus fruiting occurred year-round, with high peaks in May andNovember 2008-2012.…”

“…Variability in fruit eating can be a function of multiple parameters, such as age, satiation/hunger, feeding location, seasonality, or availability of other food or water (Levey, 1987;Corlett and Lucas, 1990;Gautier-Hion et al, 1993;Kaplin and Moermond, 1998;Lambert, 1999;Samuni-Blank et al, 2012). In addition, fruits may contain plant secondary compounds (PSCs) to deter seed predators or alter the behaviour of the consumer (Cipollini and Levey, 1997;Samuni-Blank et al, 2012).…”

“…In addition, fruits may contain plant secondary compounds (PSCs) to deter seed predators or alter the behaviour of the consumer (Cipollini and Levey, 1997;Samuni-Blank et al, 2012). In folivorous herbivores, evolutionary pressure has resulted in various behavioural and physiological adaptations for coping with PSCs, thus altering plant-herbivore interactions (McArthur et al, 1991;Dearing et al, 2005;Torregrossa et al, 2011).…”

“…However, the seeds of O. baccatus also represent a valuable nutrient source, as they are rich in protein (~25% of dry mass). To prevent seed predation, the fruits of O. baccatus defend themselves with a unique arrangement of the mustard oil bomb, where GLSs are found in the pulp and the myrosinase enzyme is found in the seeds (Samuni-Blank et al, 2012). Thus, activation of the mustard oil bomb only occurs if consumers crush both seeds and pulp together.…”

Untitled

“…Plants have been found to produce and accumulate secondary metabolites which Ruellia prostrata (Acanthaceae) is a perennial fall into various groups based on their biosynthetic origin. Humans have used these plant secondary metabolites as medicines and flavoring agents 6 .…”