Functionality of the spotted salamander egg mass polymorphism

The spotted salamander, Ambystoma maculatum, exhibits a unique polymorphism in the structure and appearance of its egg masses. This polymorphism is genetically determined and is due to the substitution of one protein in the outer jelly layer of egg masses. Based on the observation that the proportion of white egg masses in ponds across its range correlated with dissolved nutrient levels, we tested whether this polymorphism is advantageous to egg masses in ponds with different nutrient levels using two mesocosm experiments and by collecting field data from natural ponds at the University of Mississippi Field Station (UMFS). Larvae from white egg masses were larger upon hatching in low nutrient conditions than those from clear egg masses, and these differences persisted throughout the larval period. Higher proportions of white egg masses were found in ponds with low conductivity (an indicator of dissolved nutrient levels) at UMFS. These results support the idea that polymorphisms can be adaptations to, and are maintained by, environmental heterogeneity.