TILLMAN, EG*; OLSON, JR; HATLE, JD; JULIANO, SA: Estimating a threshold of hemolymph proteins necessary for reproduction of lubber grasshoppers

The reproductive cycle of Romalea microptera is divided into two phases: an early plastic phase during which feeding rate affects timing of reproduction, and a later canalized phase during which feeding does not affect timing. We hypothesized that the shift from plastic to canalized phases occurs when stored resources from adult feeding exceed a fixed threshold. This hypothesis predicts a linear relationship between time to oviposition and inverse of feeding rate, and we test that prediction in a separate paper (Olson et al). Here, we used measurements of hemolymph proteins and regression results from Olson et al. to test whether hemolymph proteins are the storage product critical for this transition. Further, we estimated the titer of protein needed for canalization. We reared seven groups of females on different daily food rations, and every 3 d collected a 5ul hemolymph sample from each animal. We measured total hemolymph protein for each sample from each grasshopper. Then, we estimated hemolymph protein titers associated with the time of transition from plastic to canalized reproduction using two estimates from Olson et al.: 1) duration of the canalized phase; 2) cumulative quantity of food needed to initiate canalization. This apparent protein threshold did not differ among treatment groups (P=0.53 & P=0.47), and the two calculation methods did not differ in estimated protein thresholds. Times at which protein thresholds were attained did differ among food treatments (P<0.001), as predicted. These data are consistent with the model of a fixed threshold of hemolymph protein as a determinant of the shift from plastic to canalized development, with the threshold independent of age and feeding rate. Supported by NSF grant DBA-9978810 to SAJ.