Title: Thermal History Influences Diapause Development in the Solitary Bee Megachile Rotundata Authors
Submitted to: Journal of Insect Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 14, 2006
Publication Date: November 17, 2006
Citation: Yocum, G.D., Kemp, W.P., Bosch, J., Knoblett, J.N. 2006. Thermal history influences diapause development in the solitary bee Megachile rotundata. Journal of Insect Physiology. 52(11-12):1113-1120. Interpretive Summary: The capacity to predict or manipulate the timing of the termination of this refractory period (diapause termination) has significant implications for agriculture. For example, predicting pest emergence dates and forecasting crop damage levels require knowledge of the diapause termination date. This varies regionally and yearly based on environmental conditions. Similarly, synchronizing the emergence of managed pollinating solitary bees with the peak bloom of target crops is central for developing these pollinators in North America. Unlike A. mellifera, most solitary bee species used as pollinators enter diapause under exposure to ambient conditions and are overwintered under artificial conditions. Thus, improving our understanding of diapause and wintering in solitary bees will not only yield benefits including healthier, more vigorous pollinator populations, but may also lead to the ability to manipulate diapause termination and manage post diapause development under field as well as artificial conditions, management that could greatly expand the use of solitary bees as greenhouse pollinators throughout North America. One possible means of achieving this goal would be the identification of molecular markers related to diapause and post diapause development. To better understand diapause termination in M. rotundata in order to predict termination date and post diapause development under field conditions, and to be able to manipulate the termination date to match the bees’ emergence with the peak bloom of target crops, we initiated a series of investigations. In the first study, we examined gene expression and respiration patterns in field maintained bees. This study demonstrated that there is no sudden transition between diapause and post diapause development. Because of the complexity of multiple variables in field experiments, we next asked the question, “Can a simple constant low temperature treatment be used to accurately model gene expression and respiration patterns of field maintained insects?” The results of current investigation indicate that the level of gene expression for selected genes in diapausing and post-diapause bees is highly influenced by their thermal history. Based on our observations of the prolonged elevated levels of HSP70 expression and differences in expression patterns of HSP90, HSC70 and actin as compared to field collected bees we conclude the following: 1) A constant low-temperature regime is not an accurate model for gene expression and respiration patterns of field-collected insects; 2) Examining diapause development at several levels (e.g., organismal and molecular) and under different environmental regimes will allow us to tease apart the different components of diapause development.
Technical Abstract: Respiration rate, time to pupation and the expression patterns of selected genes were examined during the diapause to post diapause transition in the alfalfa leafcutting bee, Megachile rotundata held at constant 4°C in winter storage. Respiration quotients were at or below 0.7 from December to May and then increased to over 0.8 in June and July. The time required for prepupae to reach the pupal stage following transfer to 29°C decreased from 23 days in December to 10 days in July. HSP70 was expressed at a consistently high level in all the diapausing prepupae stored at 4°C. In contrast, we demonstrated previously that HSP70 expression in diapausing prepupae maintained under field conditions began decreasing in March and was expressed at trace levels in the June samples. Transferring prepupae stored at 4°C to 25°C at monthly intervals from December to July induced a significant decrease in HSP70. Levels of HSC70 showed no changes during the transition to post-diapause development in prepupae maintained at 4°C. Transferring the prepupae to 25°C during the April to June time interval elicited an increase in HSC70 expression. HSP90 was expressed at a consistent level in prepupae stored at 4°C but decreased to very low levels after being transferred to 25°C in December - February prepupae: no decrease was noted in the April – July prepupae. Actin was expressed at trace levels in the diapausing prepupae maintained at 4°C and increased slightly in the post-diapausing pupae. Transferring prepupae stored at 4°C to 25° at monthly intervals from December to July induced an increase in actin expression. These results indicate that the level of gene expression for selected genes in diapausing and post-diapause bees is highly influenced by their thermal history.