Evolution of a crustacean chemical communication channel: Behavioral and ecological genetic evidence for a habitat-modified, race-specific pheromone.

An earlier study of population genetics of an estuarine amphipod provided evidence from genomic DNA analysis for a habitat-specific race of amphipods within the speciesEogammarus confervicolus. In some estuaries of the northeast Pacific, this race of amphipods exists sympatrically with other members of the species. Here we present evidence for a race-specific pheromone that appears to be the consequence of differential metabolism of the algae (Fucus distichus andPelvetia fastigiata) characteristic of the habitat occupied by this race. The race-specific pheromone identified in this study is a subtle modification of an already existing communication system: females of the habitat-specific race produce the pheromone characteristic of the species as a whole but have an ability not shared by other females of the species to modify this pheromone when raised on the algal substrate characteristic of their habitat. Only males of this race make a distinction between the more specific pheromone and the species pheromone. The formation of hybrids (conceived and raised on the algal substrate) between members of the habitat-specific race and the other members of the species disrupted the ability to produce and distinguish the race-specific pheromone; hybrids still produced a pheromone, but it was indistinguishable from that produced by the species as a whole. Behavioral assays and the results of reciprocal, interpopulation crosses indicated pheromone response in males had evolved with production; males however, did not have to be raised on the algal substrate to respond to the alternate pheromone. No evidence for maternal effects or sex linkage were detected in the results of the crosses; more specific indications of the genetics underlying pheromone production were not evident.