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United States Department of Agriculture

Agricultural Research Service

Title: Temporal Variation in Overwintering Gene Expression and Respiration in the Solitary Bee Megachile Rotundata

Authors
item Yocum, George
item Kemp, William
item Bosch, Jordi - UNIV BAR SP&USU, UTAH
item Knoblett, Joyce

Submitted to: Journal of Insect Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: May 18, 2004
Publication Date: July 1, 2005
Citation: Yocum, G.D., Kemp, W.P., Bosch, J., Knoblett, J.N. 2005. Temporal variation in overwintering gene expression and respiration in the solitary bee Megachile rotundata. Journal of Insect Physiology. 51(6):621-629.

Interpretive Summary: Megachile rotundata (Hymenoptera: Megachilidae) is a cavity-nesting, leaf-cutting bee that has been extensively cultured as a superior pollinator of alfalfa (Medicago sativa L.), following the first North American records of its presence. At present, M. rotundata is the pollinator of choice for alfalfa seed production on more than 70,000 ha in western North America, and is the most widely used commercially managed pollinator for nuts and small fruit crops, after the honey bee, Apis mellifera L. M. rotundata send approximately 9 months of their life cycle in diapause. A more through understanding physiological mechanisms regulating diapause and post-diapause development is needed to improve the management these bees. The expression of four genes HSP90, HSP70, HSC70 and actin was examined in field collect bees from Logan, Utah. MrHSP90 and MrHSP70 were highly upregulated in non-diapausing pupae exposed to 40 deg. C for one hour, while MrHSC70 was only slightly induced by heat shock. Levels of MrHSC70 and MrHSP90 showed little change between field collected diapausing prepupae and non-diapausing pupae. In contrast, MrHSP70 was highly upregulated in diapausing prepupae and Mractin was at or below the level of detection in diapausing prepupae. Transferring field reared overwintering prepupae in February to 25 deg. C for 3 days induced an expression pattern of MrHSP70 and Mractin more typical of non-diapausing bees, indicating the likelihood that the transition to postdiapause development had occurred prior to February. However, measurements revealed possible cyclic respiration patterns with low RQ values from January through March, transitioning during April to more continuous respiration with elevated RQ values. This was the first investigation of gene expression in diapausing insects under actual field conditions. The results of this investigation was counter to what was expected based on laboratory studies underscoring the necessity of acknowledging the context in which diapause and overwintering mechanisms have evolved when designing experiments. Thus, our results suggest the possibility that there are at least two subsets of genes expressed in overwintering temperate zone insects. The first subset regulates diapause development, with diapause being defined as the inability of the insect to respond to environmental cues for subsequent development, and the second subset consists of the overwintering genes that enable the insect to survive extreme environmental conditions encountered during winter.

Technical Abstract: Partial clones of Megachile rotundata HSP90, HSP70, HSC70 and actin were developed by RT-PCR. These clones were used to generate probes to screen for the expression of their respective transcripts in heat-shocked pupae and in diapausing prepupae through nondiapausing pupae. Northern blot analysis revealed transcript sizes for MrHSP90, MrHSP70, MrHSC70, and Mractin of 3.6, 2.3, 2.5, and 1.4 kb, respectively. MrHSP90 and MrHSP70 were highly upregulated in non-diapausing pupae exposed to 40 deg. C for one hour, while MrHSC70 was only slightly induced by heat shock. Levels of MrHSC70 and MrHSP90 showed little change between field collected diapausing prepupae and non-diapausing pupae. In contrast, MrHSP70 was highly upregulated in diapausing prepupae and Mractin was at or below the level of detection in diapausing prepupae. Transferring field reared overwintering prepupae in February to 25 deg. C for 3 days induced an expression pattern of MrHSP70 and Mractin more typical of non-diapausing bees, indicating the likelihood that the transition to postdiapause development had occurred prior to February. However, measurements revealed possible cyclic respiration patterns with low RQ values from January through March, transitioning during April to more continuous respiration with elevated RQ values. Thus, our results suggest the possibility that there are at least two subsets of genes expressed in overwintering temperate zone insects. The first subset regulates diapause development, with diapause being defined as the inability of the insect to respond to environmental cues for subsequent development, and the second subset consists of the overwintering genes that enable the insect to survive extreme environmental conditions encountered during winter.

Last Modified: 10/23/2014
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