Figure 3.

Antagonistic selection. The original genetic correlation (Bradshaw et al., 2003) was negative, that is, lines selected for long critical photoperiods resulted in a low response to NH and lines for short critical photoperiod resulted in a high response to NH. The object of antagonistic selection is to break up or reverse this negative correlation by selecting for short critical photoperiods with a low response to NH (selection down) and long critical photoperiods with a high response to NH (selection up). Selection is imposed for 10 generations (five cycles); the first two cycles of up- and down-selection are shown to illustrate the procedures. Selection Down: In the first generation of selection (G1), diapausing larvae are exposed to an L:D=13:11 cycle. Non-pupating larvae are discarded ( × ); pupating larvae are those responding to this day length, are used to calculate effective population size (N_e) for G1, and are used as parents for the next generation. This regimen selects for individuals with shorter critical photoperiods. In G2, diapausing larvae are exposed to L:D=10:24 and L:D=10:26 (T=34 and 36 h, respectively). Pupating larvae are discarded; non-pupating larvae are pooled and reared on long days; their resulting pupae are used to calculate N_e for G2; and their resulting adults are used to found the next generation. This regimen selects for a low response to NH. We use T=34 and 36 h because they bracket maximum response to NH (Figure 2), thereby maximizing selection for the height but not position of the peak. Together, G1 and G2 represent one cycle of antagonistic selection. G3 and G4 illustrate a second cycle of antagonistic selection, but with an increased intensity of selection: to be included in the subsequent generations, larvae must pupate under even shorter L:D cycles and continue to ignore otherwise development-stimulating T=34 and 36 h. Selection up: In G1, diapausing larvae are exposed to L:D=10:24 and L:D=10:26. Non-pupating larvae are discarded ( × ); pupating larvae are those responding to these T=34 and 36 h. Pupae are combined, are used to calculate effective population size (N_e) for G1, and are used as parents for the next generation. This regimen selects for individuals with a high response to NH. In G2, diapausing larvae are exposed to L:D=13:11. Pupating larvae are discarded; non-pupating larvae are pooled and placed on long days; their resulting pupae are used to calculate N_e for G2; and, their resulting adults are used to found the next generation. This regimen selects for a longer critical photoperiod. G3 and G4 illustrate a second cycle of antagonistic selection, but with an increased intensity of selection: to be included in subsequent generations, larvae must not pupate under even longer L:D cycles and continue to pupate in response to even more divergent T cycles.