Mammalian neurons rapidly undergo excitotoxic cell death during anoxia, while neurons from the anoxia-tolerant painted turtle can survive without oxygen for hours without apparent damage. An anoxia-mediated decrease in AMPA receptor currents are an important part of the turtle’s natural defence however the mechanism underlying it is unknown. Here I investigate a mechanism that involves activation of a mitochondrial KATP channel that subsequently signals a decrease in AMPAR currents. Whole-cell AMPAR currents were stable during normoxia, but anoxia or pharmacological activation of mKATP channels resulted in a 50% decrease in AMPAR currents. Conversely, mKATP antagonists blocked the anoxia-mediated decrease. Mitochondrial KCa channel modulators responded similarly. Blocking the Ca2+-uniporter also reduced normoxic AMPAR currents by 40%, and including BAPTA in the recording abolished the anoxia or agonist-mediated decrease. Therefore, the mKATP channel is involved in the anoxia-mediated down-regulation of AMPAR activity and is a common mechanism to reduce glutamatergic excitability.

Mammalian neurons rapidly undergo excitotoxic cell death during anoxia, while neurons from the anoxia-tolerant painted turtle can survive without oxygen for hours without apparent damage. An anoxia-mediated decrease in AMPA receptor currents are an important part of the turtle’s natural defence however the mechanism underlying it is unknown. Here I investigate a mechanism that involves activation of a mitochondrial KATP channel that subsequently signals a decrease in AMPAR currents. Whole-cell AMPAR currents were stable during normoxia, but anoxia or pharmacological activation of mKATP channels resulted in a 50% decrease in AMPAR currents. Conversely, mKATP antagonists blocked the anoxia-mediated decrease. Mitochondrial KCa channel modulators responded similarly. Blocking the Ca2+-uniporter also reduced normoxic AMPAR currents by 40%, and including BAPTA in the recording abolished the anoxia or agonist-mediated decrease. Therefore, the mKATP channel is involved in the anoxia-mediated down-regulation of AMPAR activity and is a common mechanism to reduce glutamatergic excitability.

Neonatal painted turtles (Chrysemys picta) spend their first winter inside the shallow, subterranean nest cavity where they completed embryogenesis. Consequently, hatchlings at high latitudes may be exposed to ice and cold during the winter. This study was undertaken to determine how long hatchlings withstand freezing at temperatures slightly below 0°C because tolerance for freezing has been proposed to be the key to survival by overwintering animals. A thermocouple was glued to the carapace of each hatchling. The animal was dipped in water to provide a site of nucleation of ice and was then placed into a glass jar that was partially immersed in a circulating bath at -2°C. Carapace temperature was monitored throughout the procedure. When a freezing exotherm was detected, timing of the freezing event began. Animals were maintained in a frozen state for 12-48 h prior to being warmed to room temperature. Of the 39 hatchlings, 22 did not survive, and mortality increased as the duration of freezing increased. Logistic regression indicates that no turtle would have survived in a frozen state for more than 54 h. These results indicate that hatchlings can survive only brief exposure to freezing of the body fluids. Thus, hatchlings cannot tolerate freezing during prolonged periods of cold.

Sex ratio theory suggests that the strength of intersexual selection will increase as a population more male-biased; reflecting increased selectivity in mate choice. Populations of pond turtle have varying adult sex ratios, in painted turtles (Chrysemys picta), reported sex ratios range from female biased (1:3) to male biased (3:1). I investigated the effect of sex ratio on male reproductive success (quantified as “fertilization success”) in painted turtles. I examined the mating system of painted turtles in a female-biased population using microsatellite paternity analysis, relating variation in male fertilization success to male phenotype and offspring survival, employed ex situ behavioural observation to clarify the mechanism behind the variation in male fertilization success and used agent-based modeling to simulate the effects of changing sex ratio, population density and proportions of male phenotype on male fertilization success. Small males contributed sperm to a greater number of clutches than did larger males, but were not more likely to reproduce in a season than larger males. There was no offspring fitness advantage related to male body size and no relationship between male claw length and fertilization success. Large male painted turtles courted at a higher frequency than small males. I found no relationship between male courtship behavior and claw length. Females showed no preference for males of any phenotype. Agent-based simulations were based on the distribution of best fit from the observed data; an amalgam of two Poisson distributions, each with its own probability of success and proportional representation in the final distribution. Increased female sex ratio bias, increased population density and increased proportions of “more successful” males all increased the mean and variance of male fertilization success, based on increased encounter rate among turtles. Small and large male painted turtles enjoy different fertilization success. It is uncertain whether this difference is based on active female choice, cryptic female choice, sperm competition or a combination of factors. Sex ratio simulations predict the opposite result as that predicted by sex ratio theory. These contrary results should be compared to simulations manipulating choosiness and field data from painted turtle populations to clarify mechanisms influencing male reproductive success.

Mémoire numérisé par la Division de la gestion de documents et des archives de l'Université de Montréal.

Mémoire numérisé par la Division de la gestion de documents et des archives de l'Université de Montréal

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