Hypoxia in the marine environment is a growing environmental concern, and can have profound impacts on organisms. This dissertation seeks to understand the physiologically induced changes in gene expression, the relationship between gene expression and metabolism, and how these parameters vary among populations, in response to hypoxic stress. By comparing evolved intraspecific variation in gene expression and physiological parameters among populations from multiple regions in the Gulf of Mexico we seek to determine the physiologically induced changes that are essential to hypoxic survival.
First, whole body metabolism, measured as oxygen uptake, was profiled across seven decreasing oxygen concentrations. Metabolism and the critical oxygen tension (PO2crit) were compared between populations from across the Gulf of Mexico. This study demonstrated a significant interaction of body mass with the hypoxic response. Additionally, populations only differed in their metabolism at the lowest oxygen concentration, 1.8 kPa. PO2crit did not differ between populations, but was body mass dependant.
Next, the effects of hypoxia on gene expression were examined. These studies examined the effects of hypoxia on gene expression over time and at different hypoxic doses, utilizing a 384 gene microarray. In the first studies individuals were subjected to 0, 4, 8, 12, 24, 48, or 96 hours of hypoxia. Different genes had different times for peak gene expression, with most changes occurring after 96 hours of exposure. However, only 14 genes had significant changes in gene expression. To determine the effect of differing hypoxic dose, individuals were exposed to normoxia, 7.8 kPa O2 (moderate hypoxia), or 1.8 kPa (severe hypoxia) for 4 or 48 hours. Sixty-nine genes had significant changes in gene expression for either dose or time. To elucidate the relationship between effect of time and dose, genes were examined for dose response within each time. The maximum number of changes occurred at 1.8 kPa after 48 hours of exposure. Interestingly different sets of genes had changes in gene expression at either 7.8 or 1.8 kPa.
Finally, to ascertain the difference among populations, for thousands of genes, individuals from six populations of Fundulus grandis were exposed to hypoxia (1.8 kPa) for 4 or 96 hours. Hypoxia had a significant effect on the expression of 609 genes, while population affected the expression of 355 genes. Genes with significant differences in expression among populations reflect geographic separation. For the 59 genes with significant differences in expression for both hypoxia response and population, shared hypoxic histories appears to be more important than simply the neutral patterns expected with geographic distance. The majority of significant changes for the 609 hypoxia responsive genes take place after 96 hours of hypoxia exposure.
This research demonstrates that F. grandis cope with hypoxia through changes in metabolism and gene expression. Overall, the response to hypoxia is dependent on an individual's size (body mass), the ambient oxygen concentration, and the duration of hypoxia exposure. Additionally, there appear to be some differences between populations with differing exposure history to hypoxia in the Gulf of Mexico.