Date of Completion

Spring 5-1-2011

Thesis Advisor(s)

Eric Schultz

Honors Major

Biological Sciences


Evolution | Integrative Biology


The objective of this study is to determine whether there are evolutionary trade-offs among populations isolated to different salinities in nature with regards to mitochondria rich (MR) cell production. We expected differences among populations in MR cell abundances due to the energy requirements needed to maintain and produce these cells. Eggs from three populations of threespine stickleback (Gasterosteus aculeatus) were hatched and reared for 10 weeks and then the fish were put into one of two salinity challenges: either 0.4 ppt freshwater or 35 ppt saltwater for 13 days. The populations consisted of one isolated to freshwater, one isolated to saltwater, and one that is anadromous. Mitochondria rich cells were characterized by the categorization scheme of Lee et al. (1996) as either deep hole (ion secreting cells), shallow basin (potential transitional cell type), or wavy convex (ion absorptive cells). Scanning electron microscopy was used to determine cell densities. There were strong salinity effects for deep hole cells as they were absent in freshwater and present in salt, and wavy convex cells which were nearly absent in saltwater but abundant in fresh, however there were no differences in cell densities of any cell type between populations. This indicates that either the populations never lost their ability to osmoregulate in different salinities or that measuring cell abundances is not an effective way to measure osmoregulatory ability. Analyses were also conducted on the along filament position of MR cells, and we found that cell densities were significantly lower at the base and the tip of the filament. We also found that in freshwater, MR cells are more abundant toward the distal half of the filament and in saltwater, MR cells are more abundant toward the proximal half.