Article Review: Egg Mass Polymorphism in Ambystoma maculatum

This article interested me because it brought together ideas from my Ecology class as well as information from our class in regards to a local salamander species. The article discussed an egg mass polymorphism that is found in Ambystoma maculatum. The ecologists were mainly interested in studying selection for alternative phenotypes via polymorphisms as they can lead to new adaptations that can help species better survive and occupy multiple niches. As we have learned in class, the spotted salamander produces jelly-layered eggs that allow for the entry of conspecific sperm and mediate interactions with the surrounding environment while keeping the larvae moist. A. maculatum produces either clear or white eggs (the polymorphism at the focus of the study). These morphs differ in consistency and protein make-up (presence or absence of a specific protein). The questions that the study aimed to answer were: Does hatchling size and time to hatching vary between egg mass morphs in high and low nutrient environments? If so, do differences in larval size persist throughout the larval period? Does the proportion of white egg masses in ponds correlate to pond conductivity, which can be an indicator of dissolved nutrient levels? How do performance differences between morphs influence population dynamics under different pond conditions? The researchers hypothesized that the egg mass polymorphism was adaptive with regard to varying dissolved nutrient levels in ponds. 
The researchers conducted mesocosm experiments (a bridge between a controlled lab experiment and a field experiment), collected field data, and created a population projection model to test their questions. In the embryonic development experimental setup, they deposited the egg masses in fishless ponds at the University of Mississippi Field Station. They collected 12 clear egg masses and 11 white ones after one night of breeding and placed them in individual cages in separate pools (either a high nutrient pool, or low nutrient pool). For the larval development experiment, they established wading pools of different nutrient levels. The measured/counted temperature, pH, DO, conductivity, and egg mass morphs. Then, the ecologists analyzed mean hatching date with clutch size, mean hatchling total length per clutch, and pool condition. 
They found that larvae took longer to hatch in white egg masses and high nutrient pools. They also found that hatchlings from white egg masses were larger than those from clear masses in low nutrient pools. Additionally, high nutrient pools had higher conductivity. They found that larval survival was higher in those hatched from white egg masses, but larvae were smaller in pools with more surviving offspring. Body size was greater in high nutrient pools- larvae from white masses where large in low nutrient pools, but larvae from clear masses were larger in high nutrient pools. 
Overall, the conclusions can be summed up as the following- the polymorphism in  A. maculatum is advantageous in different nutrient levels at breeding sites. The researchers highlight the importance of the role of environmental heterogeneity in the maintenance of this egg mass polymorphism. They also suggest that the polymorphism is adaptive to nutrient levels and affected by predation (species interactions). The polymorphism is not strong enough to lead to speciation, but it does have an effect on fitness. 
Why is this article important? For starters, I was completely unaware of the fact that a local species we have studied all semester even had this egg mass polymorphism that has a distinct effect on fitness. Secondly, it is important to see that amphibians can be the focus of ecological studies as it pertains to their relevance and role in our ecosystems. The study shows that specific environmental conditions can lead to major differentiations, making the study of herps even more interesting and complex. 

Pintar, M. R. and Resetarits, W. J. (2017), Persistence of an egg mass polymorphism in Ambystoma maculatum: differential performance under high and low nutrients. Ecology, 98: 1349–1360. doi:10.1002/ecy.1789

http://onlinelibrary.wiley.com/doi/10.1002/ecy.1789/full