Ecological Stoichiometry of Insects
Ecological stoichiometry (ES) is the study of the balance of chemical elements in living systems (Sterner and Elser 2002). Major biomolecules contain different mixtures of elements such as carbon (C), nitrogen (N), phosphorus (P), and calcium (Ca). As a result, important organismal characteristics with particular biochemical demands have distinct elemental signatures. A focus on elements provides simple currencies for integrating across levels of biological organization and diverse types of organisms, and provides a common language facilitating collaboration between biologists and chemists.
The Marsh and Kay labs will conduct collaborative projects on ES focusing on spittlebugs. Spittlebugs feed on plant xylem and reside within a foamy exocrine product (spittle) during development. Because xylem contains little organic material, xylem feeders face a significant challenge to obtain the balance of elements necessary for growth and survival. One spittlebug project will focus on the biochemical basis of exceptional elemental concentrations in arboreal spittlebugs. Preliminary work by the Marsh and Kay labs showed arboreal species contain Ca and P concentrations nearly 2 orders of magnitude higher than levels measured in any other insect; they also have very low concentrations of C and N relative to grassland species. We hypothesize differences between arboreal and grassland species reflect variation in structural investments. Specifically, we propose that arboreal spittlebugs substitute Ca- and P-rich structures for C- and N-rich chitin, the major component of insect exoskeleton. Ca distribution in exoskeleton will be quantified using inductively coupled plasma mass spectrometry (ICP-MS) and fluorescent calcium chelators, while quantitative measurements of purified total chitin will be made using high-performance liquid chromatography (HPLC) analysis of total N-acetylglucosamine after enzymatic digestion (Velasquez and Hammerschmidt 2004) and fluorometry using calcofluor white (a beta-glycan specific dye) (Stagoj et al. 2004). A complementary project will compare the elemental composition and biochemical properties of spittle. Spittle consistency clearly varies among species, but there is little comparative data on its makeup or functional properties. We hypothesize that spittle elemental composition reflects variation in the elemental composition of host-plant xylem, and that differences in spittle composition reflect biochemical differences influencing spittle viscosity and persistence. Identification of the elemental and molecular composition of spittle will involve fractionation by HPLC and subsequent analysis using standard chemical and instrumental techniques.