HATLE, JD; BORST, DW; JULIANO, SA; Illinois State Univ: Plasticity of control of reproductive tactics in lubber grasshoppers

Reproductive tactics are correlated with fitness. Reproductive plasticity is predicted to affect fitness, and it is likely that endocrine mechanisms regulate this plasticity. We have studied how the reproductive physiology of the lubber grasshopper if affected by feeding treatments producing identifiable plastic and canalized phases. During the first half of oogenesis, grasshoppers respond (i.e., are plastic) to changes in food availability. Grasshoppers that are initially well fed and then are starved for 48 h have reduced rates of vitellogenin (Vg; egg-yolk protein) production, yet they show no change in Vg-mRNA levels. This implies the reduction in Vg production is controlled translationally. Likewise, grasshoppers that are initially fed a limiting diet but are then switched to a well-fed diet respond with an increase in hemolymph levels of Vg and juvenile hormone (JH; a regulator of oocyte development). In contrast, during the second half of oogenesis, grasshoppers fail to respond (i.e., are canalized) to changes in food availability. Grasshoppers that are initially well fed and then are switched to a limiting diet show no change in Vg levels, JH levels, or time to oviposition. This canalized phase always included the maximum levels of both Vg and JH. We posit these maxima are physiological landmarks of oogenesis. We hypothesize that the transition from plastic to canalized is controlled by the accumulation of a threshold level of hemolymph protein that is subsequently used to fuel oogenesis. Studies in which the feeding rates of grasshoppers are manipulated provide results consistent with this model. In summary, our studies suggest that reproductive plasticity is regulated as several levels, but that reproduction includes long periods of physiological unresponsiveness.