Thermal Stress and the Heat Shock Response in Embryonic and Young of the Year Juvenile Lake Whitefish (Coregonus Clupeaformis)

Abstract

I investigated the effects of thermal stress associated with changing environments and industrial thermal pollution by characterizing the kinetics of the heat shock response (HSR) throughout Lake Whitefish (Coregonus clupeaformis) development. Lake Whitefish are a cold water species that spawn in the late fall to early winter. Their embryos develop at 0.5 – 6 °C, usually under the cover of ice. The HSR is a universal response to thermal and other types of stressors that offer protection at the cellular level. This response is characterized by the synthesis of a group of highly conserved proteins called the heat shock proteins (Hsps). Here I isolated five different hsp cDNAs from Lake Whitefish and quantified changes in mRNA transcript levels in response to varying heat stress conditions in embryos and young of the year (YOY) juveniles. Lake Whitefish were subjected to three different heat shock temperatures (3 °C, 6 °C or 9 °C above control) and six different heat shock durations (0.25, 0.50, 1, 2, 3, and 4 hr) followed by a 2 hr recovery period prior to sampling. In addition, the duration of the HSR was examined by varying the post-heat shock recovery time period prior to sampling. In this recovery experiment, Lake Whitefish were permitted to recover for 1, 2, 4, 8, 12, 16, 24, 36, and 48 hr following a 2 hr heat shock. My data suggest that Lake Whitefish embryos may be resilient to short bouts of heat shock. In embryos, hsp70 mRNA levels were elevated following a 2 hr, 9 °C heat shock. However, levels of the hsp70 did not increase in response to any of the 3 °C or 6 °C heat shocks, irrespective of heat shock duration. It is also significant that embryos did not upregulate mRNA levels of the normally inducible hsp90α or hsp47, in response to any of the heat shock exposures. These embryo data are different from those in 60 days post-hatch YOY juveniles. In YOY juveniles, all three inducible hsps were upregulated in response to both a 6 °C and 9 °C heat shock, suggesting that YOY juveniles will more readily initiate a HSR and that this response is more robust in that it involves multiple hsps. In embryos, once triggered, the HSR was relatively long lasting with hsp70 mRNA levels remaining elevated 48 hr post-heat shock. In contrast, in YOY juveniles the HSR was shorter lived with hsp70 levels beginning to decrease as early as 4 - 8 hr post-heat shock. In summary, my data indicates that Lake Whitefish can initiate a HSR during embryogenesis. In comparison to YOY juveniles, embryos are more resistant to heat stress as they only initiated a HSR at relatively high heat shock temperatures and this response was limited to increases in hsp70. However, once initiated the HSR was relatively long lasting. Collectively, these data will help us better understand the potential impact of thermal stress associated with changing environments and industrial thermal pollution on development of this and other coldwater fish species.