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Thermal acclimation and adaptation in the common killfish, Fundulus heteroclitus : thermal reaction norms and underlying mechanisms Fangue, Nann A.

Abstract

I have quantified the effects of temperature on the performance of killifish (Fundulus heteroclitus) from the northern and southern ends of the species' distribution along the East coast of North America, and examined potential mechanisms that could be involved in shaping these traits. I demonstrated that upper and lower thermal tolerance limits were higher in southern fish by ~1.5°C, consistent with adaptation to warmer habitat temperatures (Chapter Two). Thermal tolerance limits were, however, plastic in both populations, changing by >10°C with temperature acclimation. In thermal preference experiments, the thermally tolerant southern killifish chose lower temperatures than the less tolerant northern fish (29.0°C versus 30.6°C; Chapter Three), consistent with countergradient variation. Swimming performance (U[sub crit]) trials on thermally acclimated and acutely challenged killifish revealed a broad zone (~7-34°C) over which performance varied little with temperature in both populations (Chapter Four). Taken together, these data showed that although there were differences in thermal tolerance limits between populations, there was little evidence of local thermal adaptation in other measured traits. As a candidate mechanism underlying differences in thermal tolerance, I examined the role of heat shock proteins (Hsps) and found significant differences between populations in hsp mRNA expression and across multiple isoforms (Chapter Two), but these differences were not obviously consistent with the thermal tolerance differences between killifish populations. I also explored the adjustment of aerobic capacity at the level of the mitochondria (Chapter Five) as a potential mechanism defining an organism's thermal niche, as proposed in the hypothesis of oxygen limited thermal tolerance. In response to cold acclimation, both killifish populations increased mitochondrial oxidative capacity and content, with northern fish increasing both to a greater degree. With warm acclimation, however, killifish mitochondrial function and content were very similar between populations. These data suggest the possibility of evolutionary modulation at the level of the mitochondria to enhance aerobic performance in the cold. Overall, I have shown that there are differences between northern and southern killifish at the physiological, biochemical, and molecular levels. These data highlight the importance of considering multiple mechanisms that could underlie differences in whole-organism performance when investigating local thermal adaptation.

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