The impact of maternal protein intake and litter size on organ and stress axis development in the house mouse (Mus musculus)

Abstract

The phenotype of an individual is determined by interactions between its genotype and its surrounding environment. Through these interactions, an individual can express a greater range of characteristics than dictated by its genotype alone. There is strong evidence that the early-life environment can have a lasting impact on an individual’s organs and physiological characteristics. The quality and quantity of food that an individual is exposed to via its mother can both impact the development of form and function of organs and be used to ‘predict’ its future environment and thus, determine the optimal endpoint for development in what is known as the a predictive adaptive response (PAR). This response is predicted to play a vital role in shaping an individual’s life history strategy. These changes in phenotype may even be transferred across generations. However, few studies have evaluated the relative importance of the quality of an individual’s mothers diet verses the quantity of food that an individual had available during development (determined by the size of the litter it was born into) on its subsequent phenotype. With my thesis I attempted to close this gap by evaluating the relative impacts of early-life food quality and quantity broadly, on the size of organs (chapter 1) and specifically, on HPA axis development (chapter 2) throughout the life on an animal. I investigated the effects of maternal diet quantity and litter size on an individual’s organ and stress axis development in semi-natural populations of house mice (Mus musculus). Parents were assigned a high (20%) or low (10%) protein diet. After birth, the size of the litter than each pup (F1 generation) was born into was monitored and after weaning, each pup was assigned to a protein diet that matched (HH, LL) or did not match (HL, LH) that of it parents to evaluate possible predictive adaptive response. For chapter 1, I measured absolute and relative organ mass of (heart, liver, spleen, kidney, abdominal fat, and testis) of the mice at three stages of life for the F1 generation (4 weeks, 8 weeks and 1 year). I also collected organ mass data for the F2 generation at 4 weeks to evaluated transgenerational effects. I found results following a possible PAR in F1 males at 8 weeks. The matched diet groups had significantly higher abdominal fat compared to the mismatched groups, potentially indicating an advantage for early breeding. Countering a PAR, the liver and kidney of both sexes and the spleen of males as well as the abdominal fat for females were mainly influenced by an individual’s diet after weaning. Individuals consuming a high protein diet in adulthood displayed higher kidney and liver mass but lower spleen and abdominal fat mass. Diet quantity had a positive effect on development, with mice born into larger litters displaying higher body masses both at weaning and in females at 1 year of age. Being born into a small litter may have stimulated the development of a “grow now pay later phenotype” that stimulated rapid mass deposition when food was abundant. For chapter 2, I measured liver and hippocampal glucocorticoid receptor (GR) levels as well as hair glucocorticoids (GC) levels. I collected data on GR and GC level for F1 mice at 4 weeks and 1 year. Though maternal diet quantity and quality show significant effects on offspring development in laboratory studies, stress axis GR development wasn’t affected in wild derived mice maintain under conditions that mimic the wild. However, hair GC levels were higher in the HH group than other treatment groups. This may reflect the costs of reproduction in these females that appeared to have high reproductive performance.