Elemental Stoichiometry as a Driver of Life History Evolution: An Experimental Test of the Growth Rate Hypothesis

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Elemental Stoichiometry as a Driver of Life History Evolution:  An Experimental Test of the Growth Rate Hypothesis

Authors

Xue, Xia(Summer)¹, Adihikari, Bishwo N.², Perkes, Ammon^1,4, Martin, Mac^1,5, Wall, Diana H.³ and Adams, Byron J.¹

1.     Department of Biology and Evolutionary Ecology Laboratories, Brigham Young University, Provo, UT, USA.

2.     USDA-ARS, Tucson, Arizona, USA

3.     Department of Biology and Natural Resource Ecology Laboratory, Colorado State University, Fort Collins, CO, USA

4.     Current address:  Department of Biology, University of Pennsylvania, Philadelphia, PA, USA

5.     Current address: Arizona College of Osteopathic Medicine, Midwestern University, Glendale, AZ, USA

All organisms are composed of elements. During development, their somatic elemental ratios reflect underlying biochemical allocations that are produced to meet the demands of development. Elemental stoichiometry is a powerful integrator of biological and geochemical evolution, provides a useful framework for understanding sources and controls of nutrient availability, and has been widely applied in the study of different ecosystems, including the McMurdo Dry Valleys. Prior in situ research on natural populations of the McMurdo Dry Valley soil nematode Plectus murrayi revealed a link between cellular phosphorus (P) and organismal development as postulated by the growth rate hypothesis (GRH). This hypothesis infers that high biomass P-content reflects an increased allocation to P-rich ribosomal RNA is needed to meet the protein synthesis demands of increased development. In accordance with the GRH, we hypothesize that in a P-limited environment, animals will grow more slowly but achieve a larger body size at maturity.  We also predict that in a P-deficient environment we will find lower cellular RNA concentrations and that natural selection will reduce the number of copies of RNA genes in the genome, and subsequently lower rates of overall gene expression. To test the GRH in P. murrayi under laboratory conditions, we manipulated the amount of available P to see if we could replicate in the laboratory the pattern previously identified in the field, and to see if we could identify some of the specific mechanisms connecting elemental constraints and ontogeny. Because even under the best conditions field and laboratory-reared populations of P. murrayi are relatively slow growing, we replicated our experiments with the more rapidly growing nematode, Caenorhabditis elegans.  Our results for C. elegans are consistent with resource availability and the GRH. We found that the number of copies of the 18S ribosomal DNA tandem array in C. elegans cultured in a P-limited environment is 13 times less than populations reared in a P-enriched environment. Under similar conditions, P. murrayi also evolved a decrease in rDNA gene copy number, although not as dramatic (0.24 fold reduction). Additionally, the adult body size of both C. elegans and P. murrayi reared in excess P was significantly smaller than those reared in P-limited conditions.  Our findings underscore the important relationship between the evolution of life history traits and genome organization, as well as the role of elemental stoichiometry in shaping the organization of trophic interactions and, ultimately, ecosystem structure and functioning, in the McMurdo Dry Valley soil ecosystems.