The elements of sex: Using ionomics to understand the ecology and genetics of sexual dimorphism
Interspecific variation in elemental content is a central axiom in the framework of ecological stoichiometry. While this view has been immensely useful to understand ecological phenomenon, we know little about the evolutionary mechanisms that underlie diversification in elemental content. Information on the evolutionary sources of intraspecific variation in elemental content is needed. Arguably, the greatest source of intraspecific variation is sexual dimorphism. Often, sexual variation in elemental composition is discussed in the context of trait demand (e.g., antlers requiring more P). However, by focusing on a single element, this approach does not consider potentially important tradeoffs in the utilization and content of other elements that maybe required for the expression and maintenance of exaggerated, sexually dimorphic traits. Utilizing tools in inductively coupled mass spectrometry, we quantified the entire elemental profile (i.e., the ionome) of male (brothers) and female (sisters) Hyalella amphipods that exhibit striking sexual dimorphism in common gardens.
Excluding hydrogen (H) and oxygen (O), that were not considered in this study because molarities of these elements are not representative of the functional ionome since all analyses were done on a dry mass basis, 20 elements with known biological functions were detected in Hyalella. PCA analyses on the z-transformed molar concentrations revealed a significant difference in elemental concentrations between male and female Hyalella. The first principal components axis provided significant separation between males and females explaining ~67% of the total variation in elemental concentration. PCA 2 and 3 were not significant, explaining less than ~5% of the observed variation. Overall, the ionomes of males were more variable among full-sib families than females. Furthermore, results indicate that some elemental traits exhibited greater genetic variation than others. In summary, we found that: i) sexual dimorphism manifests at the ionomic level, ii) specific elements vary more than others, indicating that selection on specific elements within the ionome varies, and iii) the genetic variation in elemental content is sex-specific, potentially indicating that ionomic evolution occurs at sex-specific rates. The environmental factors influencing sex-specific ionomic composition remain to be explored. By treating the contents of the 20 elements that represent the Hyalella ionome as the quantitative traits, we can better understand the evolution of organismal stoichiometry.