Factors affecting the overwintering of tadpoles in a temperate amphibian

Climate mediates the trade-offs associated with phenotypic plasticity in an amphibian polyphenism

Polyphenisms occur when phenotypic plasticity produces morphologically distinct phenotypes from the same genotype. Plasticity is maintained through fitness trade-offs which are conferred to different phenotypes under specific environmental contexts. Predicting the impacts of contemporary climate change on phenotypic plasticity is critical for climate-sensitive animals like amphibians, but elucidating the selective pressures maintaining polyphenisms requires a framework to control for all mechanistic drivers of plasticity. Using a 32-year dataset documenting the larval and adult histories of 717 Arizona tiger salamanders (Ambystoma mavortium nebulosum), we determined how annual variation in climate and density dependence explained the maintenance of two distinct morphs (terrestrial metamorph vs. aquatic paedomorph) in a high-elevation polyphenism. The effects of climate and conspecific density on morph development were evaluated with piecewise structural equation models (SEM) to tease apart the direct and indirect pathways by which these two mechanisms affect phenotypic plasticity. Climate had a direct effect on morph outcome whereby longer growing seasons favoured metamorphic outcomes. Also, climate had indirect effects on morph outcome as mediated through density-dependent effects, such as long overwintering coldspells corresponding to high cannibal densities and light snowpacks corresponding to high larval densities, both of which promoted paedomorphic outcomes. Both climate and density dependence serve as important proxies for growth and resource limitation, which are important underlying drivers of the phenotypic plasticity in animal polyphenisms. Our findings motivate new studies to determine how contemporary climate change will alter the selective pressures maintaining phenotypic plasticity and polyphenisms.

The genome sequence of the common frog, Rana temporaria Linnaeus 1758

We present a genome assembly from an individual female Rana temporaria (the common frog; Chordata; Amphibia; Anura; Ranidae). The genome sequence is 4.11 gigabases in span. The majority of the assembly is scaffolded into 13 chromosomal pseudomolecules. Gene annotation of this assembly by the NCBI Eukaryotic Genome Annotation Pipeline has identified 23,707 protein coding genes.

Sucralose Affects Thyroid Hormone Signaling in American Bullfrog [Rana (Lithobates) catesbeiana] Tadpoles

Nonnutritive sweeteners used in food and beverage products are widespread, persistent aquatic pollutants. Despite this, their impact on aquatic organisms, particularly vertebrates, is not well-studied. Recent findings in rodents suggest sucralose, a chlorinated disaccharide, alters thyroid hormone (TH) metabolism. Because amphibian tadpole metamorphosis is TH-dependent, we hypothesized sucralose may alter signaling for this postembryonic developmental process. The present study used the American bullfrog, Rana (Lithobates) catesbeiana, as a sensitive, environmentally relevant model for testing TH disruption in the absence and presence of thyroxine (T₄), a hormone that induces metamorphosis. Premetamorphic R. catesbeiana tadpoles were immersed in 1-, 15-, and 32-mg/L sucralose solutions ± 5 nM (3.9 µg/L) thyroxine (T₄) for 48 h. RNA transcripts encoding thyroid hormone receptors alpha and beta (thra and thrb) and TH-induced basic region leucine zipper protein (thibz) were analyzed in four tissues: back skin, liver, olfactory epithelium, and tail fin, using reverse transcription quantitative real-time PCR (RT-qPCR). We found that sucralose altered the expression of fundamental TH-response genes involved in anuran metamorphosis in a tissue- and TH-status dependent manner. As organochlorines induce xenobiotic metabolism, we isolated and characterized three novel R. catesbeiana gene transcripts involved in xenobiotic metabolism: pregnane X receptor (nr1i2), constitutive androstane receptor (nr1i3), and cytochrome p450 3a4 (cyp3a4). We analyzed their expression using RT-qPCR and found evidence of their modulation by sucralose. To our knowledge, these data are the first to show xenobiotic and thyroid-disrupting activities in amphibians and further investigations into cumulative effects of environmental sucralose exposure are warranted.