Enzymatic adaptations of herbivorous insects and mites to phytochemicals

Abstract

A variety of oxidases, reductases, esterases, epoxide hydrolases, and group transferases in herbivorous insects and mites detoxify and facilitate the excretion of toxic phytochemicals (allelochemicals). Current theory indicates that the cytochrome P-450-dependent mixed-function oxidases (MFOs) are by far the most important enzymes because they have many attributes that are essential for an effective detoxification system. Data presented here on the midgut microsomal MFO activity of larvae of the gypsy moth,Lymantria dispar, are discussed in the light of previous work and support the theory. In the gypsy moth, the MFO levels exhibit a parallel trend with changes in specific feeding rates, and changes in the specific activity of the enzyme appear to be regulated ontogenetically and by inductive effect of chemicals in the diet. The specific activity of the MFOs rises more sharply on leaves of a highly preferred type-1 plant, the pin oak, than on an artificial wheat germ diet; the increase from mid-second instar to mid-fifth is 4.5- and 1.8-fold, respectively. The relationship of food consumption rate to increase in body mass (W) was slightly in excess of a 1∶1 ratio for both pin oak and the artificial diet, indicating that the feeding rate surpasses the increase in W (a rare phenomenon in insects). Moreover, the surface-to-volume ratios are fairly constant for combined data of gut lumen and epithelium in second to fifth instars, because the volume occupied by the epithelial cells is much larger than in older ones. Thus, it is concluded that greater specific activity of the MFO is necessary with larval advancement to higher instars in order that they may process dietary allelochemicals with an efficiency comparable to younger larvae. Additional data suggest that MFO level increases reflect further adaptation to: (1) normal, seasonal changes in plants' allelochemical composition and concentration; (2) increase in allelochemical concentration in response to leaf damage; and (3) the risk faced by dispersing larvae of encountering a greater amount and variety of allelochemicals on suboptimal/ less suitable plants. Evidence also has emerged recently for MFO-catalyzed metabolism/deactivation of numerous plant allelochemicals, including compounds that induce the enzyme. MFOs are further adapted for participation in the biogenesis of substances physiologically important to insects. Moreover, the catalytic center of the MFO system, cytochrome P-450, occurs in multiple forms; the significance of this important feature is discussed.