Mechanism for saltational shifts in pheromone communication systems.

T he 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 malefemale) selection that would strongly resist any shifting off of came expressed As a result, the position of the double bond shifted in the pheromone of females expressing the gene A convincing case is made (6) that the shift to the new pheromone was accomplished by ancestral males via asymmetric tracking of females emitting the new blend Evidence comes from a very few broadly tuned unusual males found in their European corn borer, 0 nubzlalis, (Fig 1) laboratory population that were capable of flying upwind to the new pheromone blend molecules emitted by 0 furnicalzs females as well as to those emitted by their own females It is only fitting that the species' norm (1, this stunning report (6) ft 2) Recently, howcomes from the Roeever, this view has Males are sensitive lofs laboratory Their been questioned (3, and narrowly tuned to the pioneering work had 4), and evidence has female-emitied conspecific ear lier hglPed show accumulated that an how the majority of asvmmetrv exists in sex pheromone blend. moth oheromone blend 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 In this issue of PNAS, Roelofs et a1 (6) now provide compelling evidence that a saltational event occuned in an ancestral Ostrinza moth population and gave rise to a new pheromone and new species, the Asian corn borer, Ostrznza furnicalis A