A cumulative feeding threshold required for vitellogenesis can be obviated with juvenile hormone treatment in lubber grasshoppers

Abstract

Developmental thresholds can ensure that an adequate condition has been attained to proceed through major transitions (e.g. initiation of reproduction, metamorphosis). Nutrition is critical to attaining most thresholds, because it is needed for both growth and storage. Attaining a threshold typically stimulates the release of hormones that commit the animal to the developmental transition, yet the relationships between the nutrition needed for developmental thresholds and these endocrine signals are poorly understood. Lubber grasshoppers require a cumulative feeding threshold to initiate vitellogenesis and potentially commit to oviposition. We tested the relative roles of the nutritional threshold and the major gonadotropin (juvenile hormone; JH) in initiating vitellogenesis and committing to oviposition. The source of JH was removed from all females, and then JH analog was applied after different amounts of feeding. Threshold feeding was not required to initiate vitellogenesis, suggesting that sub-threshold grasshoppers are competent to respond to JH. Further, sub-threshold grasshoppers went on to oviposit earlier than supra-threshold grasshoppers treated with JH at the same time. Hence, threshold feeding is required only to cause the production and release of JH. At the same time, we also found that individuals that were restored with JH late in life tended to favor current reproduction, at the expense of future reproduction. Both time to oviposition and vitellogenin profiles were consistent with this developmental allocation. Taken together, our results suggest that lubber grasshoppers adjust reproductive tactics primarily in response to nutrition (which only serves to release JH) and secondarily in response to age.