Plasticity and canalization in the control of reproduction in the lubber grasshopper

The ability to change reproductive tactics during adult development in response to environmental variation is predicted to enhance fitness. Many organisms show phenotypic plasticity early in non-embryonic development, but later exhibit phases of developmental inflexibility (=canalization). Therefore, we studied reproduction-related hormones and proteins and their relationships to plasticity in the Eastern lubber grasshopper. Diet-switching experiments demonstrated plasticity early in the egg production cycle, but a switch to canalization late in the cycle. We measured developmental titers of 4 hemolymph compounds from single individuals from adult molt until first oviposition. These 4 compounds were the egg-yolk precursor protein vitellogenin, juvenile hormone (the central regulator of insect reproduction), major hemolymph proteins, and ecdysteroids (the arthropod molting hormone that ultimately is stored in the egg). Using diet manipulations, we investigated how these developmental titers relate to the switch from plastic to canalized egg production. All 4 hemolymph compounds reached their peak levels during the canalized phase, about 12 day before oviposition. Diet switches after these peak levels did not affect the timing to oviposition. Therefore, these peak titers were physiological events that occurred after the individual committed to laying. We compared these patterns in reproduction to the development toward adult molt, another major life-history event in insects. We observed an extended canalized phase before the adult molt. This canalized phase always included a peak of ecdysteroids. The similar patterns in the physiology of these life-history events suggested that common limitations may exist in major developmental processes of insects that are directed by hormones.