OLSON, JR*; TILLMAN, EG; HATLE, JD; JULIANO, SA: A threshold for initiating the canalized phase of reproduction in the lubber grasshopper

The reproductive cycle of the grasshopper Romalea microptera is believed to consist of two phases: a plastic (flexible) phase during which feeding rate affects reproductive timing and allotment, and a later, canalized (inflexible) phase, in which feeding rate has no effect on reproductive outcome. The duration and cues for initiation of this phase are currently unknown. We hypothesize that the initiation of this canalized phase of oogenesis begins when the level of hemolymph protein (derived from feeding) exceeds a fixed threshold, and is therefore dependent on the amount of food eaten. A mathematical description of the timing of these phases of reproduction predicts that time to oviposition should be linearly related to the inverse of the daily food ration, with the slope of the regression estimating the threshold (in units of mass of food) and the intercept estimating duration of the canalized phase. We tested this hypothesis by assigning newly eclosed females to different daily rations of Romaine lettuce. We tested linear, quadratic, and cubic regressions of age at oviposition versus the inverse daily food ration to test the prediction of linearity. Linear regression was significant, with r²= 0.642. Quadratic and cubic parameters were not significant (P>0.05 in both cases). Our results yield estimates of the duration of the canalized phase of 16.5 days and a threshold of 5.6 g (dry mass) of lettuce. Our data are thus consistent with our hypothesis of a simple, fixed threshold for food eaten that is the determinant of the initiation of the canalized phase of reproduction. In a related project we use these estimated regression parameters to derive estimates of the level of hemolymph protein associated with the threshold (see Tillman et al). Supported by NSF grant DBA-9978810 to SAJ.