THE COST OF HORMESIS

Abstract

Exposure to low levels of ionizing radiation is known to trigger an adaptive response that includes immune stimulation and the up-regulation of long-lasting protective effects including improved detection and repair of DNA damage, enhanced growth, and longevity. While the hormetic aspects of the adaptive response clearly increase fitness in the presence of environmental stresses, they must decrease fitness under normal environmental conditions because the responsible biological mechanisms are not maintained in an upregulated state but rather require some form of sensitizing exposure. It has been suggested that stimulation of the adaptive response could be metabolically costly however no direct measurement of the metabolic cost of radiation stimulated growth has been previously attempted. This thesis assessed whether radiation-stimulated growth in the lake whitefish (Coregonus clupeaformis) is accompanied by a trade-off in metabolic efficiency, or by a sustained increase in energetic demands. Exposure to a fractionated regime of ionizing radiation from a 137Cs source was found to significantly stimulate growth in lake whitefish embryos compared to controls with a corresponding increase in yolk consumption. However, measurements of metabolic efficiency using a time series of unpreserved dry weights showed that the stimulated growth was unaccompanied by a reduction in metabolic efficiency. An exponentially transformed mean temperature was used as a quantitative baseline to assess the vulnerability of developing whitefish embryos to phenological mismatch. Hatch timing was found to deviate from baseline predictions under asymmetrically variable thermal regimes. The deviations were attributed to the embryos growing to a more advanced stage of development at low temperatures. The term heterograde is proposed to describe the thermal dependency of hatching stage which may have evolved as a mechanism to synchronize the hatching of viable larvae to the increase of zooplankton density that accompanies spring break-up. A predictive model incorporating heterograde hatching yielded a significant improvement in predictive accuracy over previous models. Energetic efficiency is of particular importance to the lake whitefish as it has one of the longest natural incubation periods of any freshwater fish, at some of the lowest incubation temperatures, and with eggs only 10% the size of most non-coregonine salmonids. The evolution of mechanisms to synchronize hatching to the break-up of winter ice cover highlights the vulnerability of the lake whitefish to phenological mismatch whether through accelerated development that causes embryos to hatch asynchronously with the break-up of winter ice cover, or through a sustained increase in energetic demands that increases their vulnerability to starvation. The absence of any measurable trade-off in metabolic efficiency points to the latter as a plausible general cost of hormesis.