The direct response of Drosophila melanogaster to selection on knockdown temperature

We selected on knockdown temperature, the upper temperature at which insect s lose the ability to cling to an inclined surface, in replicate population s of Drosophila melanogaster for 32 generations (46 generations of rearing) . Knockdown temperature (T-kd) was initially bimodally distributed in both control and selected lines, and a similar pattern was found in several popu lations surveyed from two other continents. Within 20 generations of select ion, the Up-selected lines (top 25% each generation) had lost the lower mod e and the Low-selected lines (selected to fall out at approximate to 37 deg rees C) had largely lost the upper mode. The realized heritability of Tkd c omputed over the first 10 selection episodes was approximate to 0.12 in the Up-selected and approximate to 0.19 in the Low-selected lines. Realized he ritability rose dramatically in the Low-selected lines over the first 20 ge nerations of selection. The two modes, plus this rise in heritability, sugg est that knockdown temperature is the product of one or two genes of large effect. The global polymorphism for knockdown temperature, coupled with the ease of selective removal of either mode, suggests that genetic variation for knockdown temperature may be maintained by natural selection.