Persistence of an egg mass polymorphism in Ambystoma maculatum: differential performance under high and low nutrients

Polymorphisms play critical roles in allowing organisms to adapt to novel environments while enabling ecological speciation under divergent selection. Ambystoma maculatum, the spotted salamander, exhibits a unique polymorphism in the structure and appearance of its egg masses with two common morphs, white and clear. Amphibian egg jelly layers mediate interactions between embryos and the environment and are more responsive to ecological pressures of natural selection than other egg coat components. The A. maculatum egg mass polymorphism was hypothesized to be adaptive with regard to varying dissolved nutrient levels in ponds. We conducted two mesocosm experiments, collected field data, and constructed a population projection model to determine how dissolved nutrient levels affect embryonic and larval development and relate to the distribution of the morphs in natural ponds. We found that upon hatching there was an interaction between nutrient level and egg mass morph wherein individuals from white morphs were larger in low nutrient habitats. This interaction persisted throughout the larval stage, and along with the higher abundance of white morphs in ponds with low conductivity, we demonstrate that the white morph is advantageous in low nutrient environments. Our findings provide evidence for the role of environmental heterogeneity in enabling the persistence of a structural egg mass polymorphism, with maintenance occurring across multiple scales and persistence across its range. This indicates that polymorphisms can maximize performance in heterogeneous environments, while persisting over long timescales without leading to sympatric speciation.