Submitted to: Journal of Insect Physiology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 5/1/2014
Publication Date: 7/1/2014
Publication URL: http://handle.nal.usda.gov/10113/58943
Citation: Owings, A.A., Yocum, G.D., Rinehart, J.P., Kemp, W.P., Greenlee, K.J. 2014. Changes in respiratory structure and function during post-diapause development in the alfalfa leafcutting bee, Megachile rotundata. Journal of Insect Physiology. 66:20-27. Interpretive Summary: The alfalfa leafcutting bee is a solitary, cavity-nesting bee that develops from the egg to the adult stage in cells constructed from leaf pieces within a cavity. Since much of the life of this bee occurs within the cavity, developing bees may be regularly exposed to air that is low in oxygen. The purpose of this study was to test the hypothesis that these bees are very tolerant of low oxygen conditions. This was accomplished by measuring the lowest amount of oxygen they require to be metabolically active (also known as the critical oxygen pressure) and by measuring tracheae within developing bees. We found that the average critical oxygen pressure for this species was 4% oxygen, with a range of 0% to 10%, which is similar to that of other insects. As the age of the insect increased, so did the critical oxygen pressure values, indicating that older insects were less tolerant of low oxygen levels. Using synchrotron x-ray imaging, we determined that the diameter of the tracheae inside the insect increased as the animal aged, although the amount of increase varied by type of tracheae that was measured. While this study is critical to a basic understanding of low oxygen tolerance in this species, additional studies such as 3D modeling will be required for a complete assessment.
Technical Abstract: Megachile rotundata, the alfalfa leafcutting bee, is a solitary, cavity-nesting bee. M. rotundata develop from eggs laid inside brood cells constructed from leaf pieces and placed in series in an existing cavity. Due to the cavity nesting behavior of M. rotundata, developing bees may experience hypoxic conditions. The brood cell itself and the position of cell inside the cavity may impact the rates of oxygen diffusion creating hypoxic conditions for developing animals. We hypothesized that bees would be adapted to living in hypoxia and predicted that they would be highly tolerant of hypoxic conditions. To test the hypothesis, we measured critical PO2 (Pc) in pupal M. rotundata of varying ages. Pc, defined as the atmospheric O2 level below which metabolic rate cannot be sustained, information about an animal’s respiratory capacity. Using flow through respirometry, we measured CO2 emission rates of developing bees exposed to 21, 10, 6, 5, 4, 3, 2, 1 and 0 % O2 and statistically determined the PO2 at which CO2 emission dropped. Mean Pc was 4% O2and ranged from 0% to 10%, similar to that of other insects. Pc was positively correlated with age, indicating that as pupae aged, they were less tolerant of hypoxia. To determine if there were developmental changes in tracheal structures that account for the increase in Pc, we used synchrotron x-ray imaging and measured the diameter of several tracheae in the head and abdomen of developing bees. Analyses of tracheal diameter showed that tracheae increased in size as animals aged, but that the magnitude of the increase varied depending on which trachea was measured. Understanding the structural basis for the change in Pc will require 3D modeling of whole animal tracheal systems.