Mechanism for saltational shifts in pheromone communication systems

The specificity and sensitivity involved in moth sex pheromone communication are remarkable and renowned. Males are exquisitely sensitive and narrowly tuned to the female-emitted conspecific sex pheromone blend, which usually consists of two or three components (1, 2). Slight changes in the ratios of these components can shut off the upwind flight attraction response of males of one species and turn on the response of males from a different species. For years, it had been assumed that blends are under a very high degree of stabilizing (symmetrical male–female) selection that would strongly resist any shifting off of the species' norm (1, 2). Recently, however, this view has been questioned (3, 4), and evidence has accumulated that an asymmetry exists in the strength of selection pressures on female senders and male receivers. The asymmetry, with lower pressure exerted on females' signaling systems, should allow for much higher variance in the females' blends than had been assumed (3, 4). The correspondingly higher selection pressure on males' response systems also should select for some subset of the male population to follow major variations in females' blends over generations by adjusting their olfactory response spectrum through asymmetric tracking (4, 5). Until now (6), there had been no solid evidence from one pair of species to support this process, nor was there evidence for how a departure from the normal female blend might arise to initiate asymmetric tracking. Males are exquisitely sensitive and narrowly tuned to the female-emitted conspecific sex pheromone blend.