The developmental environment and metabolism in the house mouse (Mus musculus)
Type of Degreethesis
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The environment under which an individual develops plays an important role in determining its physiological phenotype. Maternal diet is one of several variables that can have lasting impacts on offspring phenotype. It’s been suggested that these changes can be set during early development can last through adulthood, ultimately affecting an individual’s ability to function maximally in their adult environment. The environmental matching hypothesis predicts that an individual’s fitness is highest under conditions in which the developmental and adult environments are matched. Accordingly, when those environments are mismatched individuals will have reduced fitness. This response may be important for shaping an individual’s life history strategy. However, there have been few empirical tests of this hypothesis with respect to diet and fitness, particularly in wild populations. In this study I investigated the environmental matching hypothesis in a natural population of house mice (Mus musculus). I experimentally manipulated dietary protein levels and monitored individuals into adulthood. We placed the parental (F0 generation) on either a high (H, 20% protein) or low (L, 10% protein) and the offspring of these mice were then kept on a similar diet (HH, LL) or given the alternate diet (HL, LH). I also measured several reproductive variables including age at first reproduction, reproductive status, and reproductive effort. In addition, I measured resting metabolic rate (RMR) at 30 days (weaning) and 75 days. I also measured circulating levels of the metabolic hormone IGF-1 and IGF-1 gene expression at 30 days and at approximately 1 year. Under the environmental matching hypothesis I predicted 1) individuals matched with their developmental environment to have greater reproductive success than individuals that are mismatched and 2) the developmental environment can alter aspects of an individual’s physiology that persist into adulthood. Dietary treatment had a significant effect on age at first reproduction, with mismatched individuals reproducing significantly earlier than matched individuals. This is in contrast to predictions made by the environmental matching hypothesis. We found no differences between treatment and reproductive status or reproductive effort. However, there was a trend that suggested LH individuals had greater reproductive effort than other treatment groups. There was no difference in RMR between treatment groups at 30 days. It was not until 75 days that metabolic adjustments became apparent. At 75 days, matched individuals exhibited similar RMR while there were significant differences in RMR between mismatched groups. Concentrations of IGF-1 at 30 days did not differ between treatment groups. However, LH individuals had significantly lower concentrations of IGF-1 at adulthood. There were no differences in IGF-1 expression among treatment groups at 30 days or adulthood, although a trend suggested that adults had greater expression levels than juveniles. Interestingly, when we compared IGF-1 levels to reproductive effort, individuals with lower levels of IGF-1 had greater reproductive effort. This result may be due to relative age of animals at the time of sample collection. The results of this study suggest that reproductive success in the house mouse is not determined by matching the metabolic phenotype to the maternal diet.