Previous biochemical evidence suggests that a cytochrome P450 specific to male antennae of the pale-brown chafer, Phyllopertha diversa, has evolved as a pheromone-degrading enzyme. By using a bioinformatics approach, we have now cloned three P450 cDNAs: CYP4AW1, CYP4AW2, and CYP6AT1. RT-PCR indicated that CYP4AW2 is expressed in all tissues examined, that CYP6AT1 is antennae-rich, and that CYP4AW1 is antennae-specific. Both tissue specificity and electrophysiological studies strongly support that CYP4AW1 in P. diversa is a pheromone-degrading enzyme involved in pheromone inactivation. Highly sensitive, pheromone-specific olfactory receptor neurons in male antennae were completely desensitized by direct application of metyrapone into the sensillar lymph. When tested in the same or different individuals, the metyrapone treatment had no effect on olfactory receptor neurons tuned to the plant volatile (Z)-3-hexenyl acetate, which might be inactivated by an esterase. Metyrapone treatment did not affect pheromone reception in the Japanese beetle, Popillia japonica, in the scarab beetle, Anomala octiescostata, or in the Oriental beetle, Exomala orientalis. Metyrapone-induced anosmia was restricted to the pheromone detectors in P. diversa, which became insensitive to physiological concentrations of pheromones for a few minutes. As opposed to previous trials, the specificity of the inhibitor and pheromone system led to unambiguous evidence for the role of pheromone-degrading enzymes in the fast inactivation of pheromones.
Insects rely heavily on their chemical communication skills to perform fundamental behaviors. Females, for example, advertise their readiness to mate and recruit males for reproduction by releasing sex pheromones. To avoid ''chemical conspicuousness,'' they release minute amounts of the species-specific chemical signals. To detect low levels of such unique signals in a noisy environment, males have evolved sensory systems with remarkable sensitivity that are highly tuned to these species-specific pheromones. Females of the pale-brown chaffer Phyllopertha diversa (Coleoptera or Scarabaeidae), for example, produce an alkaloid pheromone, 1,3-dimethyl-2,4-(1H,3H)-quinazolinedione (DMQ) (Fig. 1) (1), which is specifically detected by highly sensitive olfactory receptor neurons in male antennae (2, 3).While taking an odorant-oriented flight toward a pheromoneemitting female, a male encounters intermittent chemical signals comprising short bursts of high flux separated by periods during which the flux is zero (4, 5). Sustainable flight and orientation requires that the sensory system be reset on a millisecond timescale while navigating through the ''clean'' space between filaments. Two dichotomous hypotheses have been suggested for the fast inactivation of chemical signals. Based on the estimation that in the presence of an antennae-specific pheromonedegrading enzyme (PDE) (6) the pheromone has a half-life of 15 ms, it has been suggested that pheromone signals are deactivated by an enzymatic process (7). On th...