|Ph.D Student||Samuni-Blank Michal|
|Subject||The Ecophysiological Response to Secondary Metabolites in|
Fruits: Seed Predators versus Seed Dispersers
|Department||Department of Biology||Supervisors||Professor Emeritus Zeev Arad|
|Professor Ido Izhaki|
|Full Thesis text|
Plant secondary metabolites (PSMs) 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 PSMs in ripe fruits deter seed predators but have little or no effect on seed dispersers. Indeed, studies have demonstrated class-dependent deterrence where frugivorous birds were able to cope with fruit secondary metabolites while granivorous rodents were deterred by them. However, this mechanism was only demonstrated at the class level, i.e., between birds and mammals.
Here I demonstrate 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.
In a series of field observations, controlled feeding trials and biochemical analyses I investigated the ecophysiological and behavioral aspects of rodents feeding on Ochradenus baccatus fleshy fruits.
In this research I found that the fruits of O. baccatus have a unique compartmentalization of PSMs. The fruit pulp has high concentrations of glucosinolates (GLSs) that are hydrolyzed into active toxic, foul tasting compounds upon contact with the enzyme myrosinase released from the seeds that are crushed during consumption.
Experienced individuals of Acomys cahirinus from the field as well as naïve individuals from a captive colony circumvented the activation of the GLSs by orally expelling intact, vital seeds. In contrast, A. cahirinus given fruits containing seeds with deactivated myrosinase ate most seeds. Accordingly, the body mass and dry matter intake of A. cahirinus were significantly higher when feeding on diets that did not contain the GLSs hydrolyzed products. Thus, the mustard oil bomb deters these animals from crushing the seeds during fruit consumption but not from dispersing them, resulting in the first reported evidence of PSMs promoting seed dispersal by mammals.
Acomys minous on the other hand, circumvent the activation of the GLSs by making a hole in the pulp and consuming only the seeds. In contrast, A. russatus activates GLSs by consuming the whole fruit. I propose that A. russatus possesses physiological adaptations to cope with the toxic compounds generated from the GLSs-myrosinase system whereas A. cahirinus and A. minous exhibit behavioral adaptations to avoid the activation of these toxins.
These findings demonstrate the power of PSMs in altering animal behavior and plant-animal interaction and the extreme ecological/evolutionary flexibility of this plant-animal symbiosis to shift from predation to mutualism.