Figure 1.

Bidirectional selection for learning in blowflies. (A) Food-deprived individual blowflies were subjected to 15 trials of a classical conditioning procedure that paired one of two tarsal chemosensory stimuli (CSs; either water or saline) with a sucrose (US; reward) stimulus applied to the proboscis. Normally, the US produced a robust PER. After a few paired CS-US trials, the CS also began to elicit a conditioned PER. Learning scores were based on the number of CS-induced PERs during the last eight training trials. Eight pairs of the highest or lowest scoring flies were mated together each generation in the bright or dull strains, respectively. The response to selection required several generations to reach an asymptote, suggesting a polygenic basis. After 12 generations, mean scores for the bright and dull strains differed significantly from each other and from that of a free-mated control strain. (Data replotted from ref. 15.) (B) Food-deprived but water-satiated individual blowflies were subjected to a water pretest delivered to the tarsi, followed immediately by tarsal stimulation with sucrose. Flies then were subjected to a tarsal water posttest either 15, 30, 45, or 60 s later. Proboscis extensions to the 15-, 30-, 45-, or 60-s water posttests were given scores of 1, 2, 3, or 4, respectively. Each fly received three trials with each of the four posttest periods. Sensitization scores thus ranged from 0 to 30. Eight pairs of the highest or lowest scoring flies then were mated together in the high or low strains, respectively. The response to selection was nearly complete in one generation, suggesting a one-gene mode of inheritance. After one generation, mean scores for the high and low strains differed significantly from each other and from that of a free-mated control strain. (Data replotted from ref. 16.)