Morphological and transcriptomic analyses reveal three discrete primary stages of postembryonic development in the common fire salamander, Salamandra salamandra

Abstract

The postembryonic development of amphibians has been characterized as divided into three predominant periods, hereafter named primary developmental stages: premetamorphosis (PreM), prometamorphosis (ProM), metamorphic climax (Meta), and completion of metamorphosis (PostM), largely based on examination of anuran development. Here, we categorized the postembryonic development of larvae of a poisonous fire salamander (Salamandra salamandra) by integrating morphology and gene expression (transcriptomic) data. Morphological analysis revealed three distinct clusters suggestive of PreM, ProM, and Meta, which were confirmed in parallel by microarray-derived gene expression analysis. In total, 3,510 probes targeted transcripts differentially expressed between the clusters we identified. Genes upregulated in PreM related to organogenesis, and those upregulated in Meta underlie structural proteins and related to development of anatomical structures and pigmentation. Biosynthesis pathways of pigments (pteridines and melanin) were upregulated during late ProM and Meta. Gas chromatographic analysis of alkaloids indicated the onset of steroidal alkaloid biosynthesis at ProM. When comparing gene expression in the fire salamander to that in other amphibians-three anurans, Xenopus laevis, X. tropicalis, and Michrohyla fissipes, and one caudate, Ambystoma mexicanum- we identified genes with conserved expression patterns involved in basic metamorphic processes such as skin restructuring and tail fin resorption. Our results support that primary stages of postembryonic development in caudates are homologous to those of anurans, and offer a baseline for the study of the evolution of developmental modes.

The conspicuous postmetamorphic coloration of fire salamanders, but not their toxicity, is affected by larval background albedo

“…To date, the metabolic pathways that allow some amphibians to synthesize or sequester alkaloids are poorly studied. In the specific case of the fire salamander (see Lüddecke et al, ), we know that their steroidal alkaloids are self‐synthesized using cholesterol as one of the precursor molecules (Habermehl & Haaf, ), and that their synthesis begins at prometamorphosis (Sanchez, Küpfer et al,). This has led to the idea that thyroid hormones might play a role in salamander alkaloid synthesis, as thyroid hormones are known to regulate the metabolism of cholesterol (Tata, ).…”

“…Each extract was evaporated to 100 μl in total, and 1 μL of this concentrated extract was used for gas chromatography. The peaks of samandarine, samandarone, and O-acetylsamandarine were identified based on their retention times, as reported in Sanchez, Küpfer et al (2018). Because samandarine and samandarone had very similar retention times, their peaks overlapped in the chromatograms and we, therefore, did not further differentiate between them.…”

“…To date, | 31 the metabolic pathways that allow some amphibians to synthesize or sequester alkaloids are poorly studied. In the specific case of the fire salamander (see Lüddecke et al, 2018), we know that their steroidal alkaloids are self-synthesized using cholesterol as one of the precursor molecules (Habermehl & Haaf, 1968), and that their synthesis begins at prometamorphosis (Sanchez, Küpfer et al,2018).…”

More yellow more toxic? Sex rather than alkaloid content is correlated with yellow coloration in the fire salamander

“…() found no quantitative correlation between an individual's conspicuousness (chromatic and achromatic contrasts) and the alkaloid content in post‐metamorphic fire salamanders. A recent developmental study, however, showed that the synthesis of alkaloids starts with the establishment of the typical conspicuous coloration during metamorphosis (stage ProM), thus indicating an unambiguous qualitative correlation between colour and alkaloid production (Sanchez et al ., ).…”

“…However, the main component is represented by steroidal alkaloids of the samandarine type (hereafter referred to as SAMs). Nine members of this class of alkaloids were previously described; the three SAMs samandarine, samandarone and O-acetylsamandarine are considered to be the main alkaloids in salamander skin poison (Vences et al, 2014;Sanchez et al, 2018b). Although SAMs might provide an antimicrobial defence (Habermehl & Preusser, 1969, 1970Preusser et al, 1975), they are mostly known for their physiology-disrupting capacities in vertebrates; acting as convulsive neurotoxins that can cause lethal intoxication even in large mammals such as dogs (Erjavec, Lukanc & Zel, 2017).…”