Ammon Perkes and Byron Adams, Biology
Introduction
The Growth Rate Hypothesis (GRH) proposes that since high-growth protein synthesis in organisms requires phosphorous-rich rRNA, growth is correlated with and dependent on phosphorous availability to said organisms. The limitation imposed by phosphorus availability thus becomes a selective factor that drives evolution. (Elser et al, 2002). This project seeks to test the GRH in vitro using Plectus murrayi a species of Antarctic nematodes. Previous research in the McMurdo Dry Valleys in Antarctica found a correlation between life history in P. murrayi, including growth, and phosphorous availability. (Adhikari et al, in prep). We seek to replicate this difference in vitro, in order to test the emerging GRH and better understand how nutrient limitation affects evolution and ecology in natural systems.
Methodology
Nematodes were previously collected from soil samples from the McMurdo Dry Valleys in Antarctica. Individuals from the more Phosphorous-limited soil were separated into two groups and cultured on different media with high or low Phosphorous levels. After many generations, plates were selected for Trizol® RNA extraction and reverse transcription in order to perform gene count with qPCR.
Results
Culture of nematodes was successful, and stable populations were established on both media. These populations were maintained over the course of 2 years, including 16 generations. Those with excess phosphorous showed greater vitality and resilience over time. The first round of RNA extraction was unsuccessful, yielding no DNA, and thus qPCR could not be performed. Additional culturing is required in order to make a larger population from which to extract RNA.
Discussion
The cultures had been created and maintained by a previous contributor, but had been left for over a year without being cultured. It was difficult to establish stable populations from these old plates, but due to the extreme resilience of P. murrayi, stable populations were established. Due to the difficulty of establishing sufficient populations for RNA extractions, the project could not be pursued to its completion before the author left for graduate school. Observations from culturing confirmed the fact that phosphorous concentration is vital to the growth and reproduction of P. murrayi, however it is impossible to know, just from observation, if the different media have resulted in different gene expression and life history, or if nematodes merely thrive on higher phosphorous concentrations, with no change in life history. We should expect changes in gene count following extended exposure to increased selection in one group and not in the other but confirming the existence of differences in gene count will require analysis through qPCR.
Conclusion
Sadly, at this point, this study has yielded few results, but this research is on going, and successful RNA extractions and initial results are likely just weeks away. P. murrayi is an excellent model species with which to test the GRH due to it’s ease of culture, short generation, native resilience to stress, and facility of observation and extraction. Although I personally was unable to pursue this project to its completion, I expect that results will be forthcoming.
Scholarly Sources
Adhikari, B.N. Simmons, B. L. Ball, B.A. Wall, D.H. Adams, B.J. 2013. Links between genes and elements: evolutionary stoichiometry of phosphorus content in Antarctic nematodes. (in prep)
Elser J, Frost P, Kyle M, et al. 2002 Effects of light and nutrients on plankton stoichiometry and biomass in a P-limited lake. Hydrobiologia, 101-112