Patterns of bacterial diversity in embryonic capsules of the spotted salamander Ambystoma maculatum: an expanding view of a symbiosis

ABSTRACT The unicellular green alga, Oophila amblystomatis, populates egg capsules of the spotted salamander Ambystoma maculatum. This nutrient-exchange mutualism is widely perceived as a bipartite interaction, but the presence and contributing effects of bacteria to this symbiosis are unknown. We used standard cultivation techniques and amplicon sequencing of the V4/V5 region of 16S rRNA gene to identify and compare diversity of bacterial taxa in embryonic capsules with that in the aquatic breeding habitat. Our sampling regime allowed us to investigate diversity among individual capsules of an egg mass and between two ponds and sampling years. Capsules contain much lower diversity of bacteria than pond water, and spatial and temporal variation in intracapsular and pond bacterial diversity was observed. Despite this variation, sequences corresponding to species in the orders Burkholderiales and Oligoflexales were either prevalent or abundant, or both. Isolates most commonly recovered from capsules were closely related to species in the genus Herbaspirillum (Burkholderiaceae); other isolates were pseudomonads, but in all cases are closely related to known vascular plant-associated species. We conclude that, despite observed variation, there are bacterial taxa whose presence is held in common spatially and temporally among capsules and that the symbiosis between O. amblystomatis and A. maculatum may involve these taxa.

Balancing selection and drift in a polymorphic salamander metapopulation

Understanding how genetic variation is maintained in a metapopulation is a longstanding problem in evolutionary biology. Historical resurveys of polymorphisms have offered efficient insights about evolutionary mechanisms, but are often conducted on single, large populations, neglecting the more comprehensive view afforded by considering all populations in a metapopulation. Here, we resurveyed a metapopulation of spotted salamanders ( Ambystoma maculatum ) to understand the evolutionary drivers of frequency variation in an egg mass colour polymorphism. We found that this metapopulation was demographically, phenotypically and environmentally stable over the last three decades. However, further analysis revealed evidence for two modes of evolution in this metapopulation—genetic drift and balancing selection. Although we cannot identify the balancing mechanism from these data, our findings present a clear view of contemporary evolution in colour morph frequency and demonstrate the importance of metapopulation-scale studies for capturing a broad range of evolutionary dynamics.

Habitat patchiness drives spatial structure in morphological trait variation and co-variation in spotted salamanders (Ambystoma maculatum)

The influence of intraspecific trait variation in species interactions makes trait-based approaches critical to understanding eco-evolutionary processes. Given that species occupy habitats that are patchily distributed in space, advancement in trait-based ecology hinges on understanding how trait variation is spatially structured across the landscape. We sampled larval spotted salamanders (Ambystoma maculatum) across spatially discrete ponds to quantify spatial structure in morphology. Spatial structure explained 7-35% of total observed variation in the length and shape of salamander larvae, depending on the body segment measured (i.e., head, body, tail). Salamander tail morphology was more variable and exhibited more spatial structure than head or body morphology. Salamander mass was also highly variable, and was strongly correlated with total length. Analysis of allometry revealed that the slopes of mass-length relationships were similar across space, but the intercepts differed spatially. Preliminary evidence hints that newly constructed ponds are drivers of spatial differences in allometric intercepts. Pond construction may therefore bolster diversity in trait co-variation, and in so doing instil more adaptive potential of salamanders under current and future environmental change.

Keeping salamanders off the streets: Evaluating one of the first US amphibian road tunnels 30 years later

Culverts are often installed under busy roads to help a variety of animals, from small frogs to bears, safely cross roads that bisect their habitats. One of the first roadway culvert systems designed specifically for amphibian use in the United States was installed along Henry Street in Amherst, Massachusetts in 1987 to protect spotted salamanders (Ambystoma maculatum). These salamanders cross Henry Street during their annual migration to their breeding pools. In recent years, anecdotal evidence from volunteers monitoring the site suggested that salamanders were no longer using the tunnels. To evaluate this concern we conducted salamander counts in 2016, 2017, and 2018 to quantify tunnel use. In 2016, only 11% of observed salamanders used the tunnels– a substantial decrease from 68% in 1988, one year after their installation, when the tunnels were last evaluated. Subsequently, we implemented two tunnel modifications in an effort to increase tunnel usage above the established 2016 baseline. Unfortunately, neither retrofit was successful. Previous studies have demonstrated that salamanders prefer minimum tunnel apertures of >0.4 m, so it is likely that the 0.2 m apertures here are inadequate. This may create differential light and humidity inside and outside the tunnels that is recognized by the salamanders. While many studies have evaluated amphibian tunnel use in lab and field settings, ours is one of the first studies to have examined tunnel usage data long after initial installation. These long-term data are critical for evaluating what factors are necessary for maintaining tunnels over decades-long time scales.