Dermal Application of Courtship Pheromones Does Not Influence Receptivity in Female Red-Legged Salamanders (Plethodon shermani)

Pheromonal communication in urodelan amphibians

Pheromonal communication is an ancient and pervasive sensory modality in urodelan amphibians. One family of salamander pheromones (the sodefrin precursor-like factor (SPF) family) originated 300 million years ago, at the origin of amphibians. Although salamanders are often thought of as relatively simple animals especially when compared to mammals, the pheromonal systems are varied and complex with nuanced effects on behavior. Here, we review the function and evolution of pheromonal signals involved in male-female reproductive interactions. After describing common themes of salamander pheromonal communication, we describe what is known about the rich diversity of pheromonal communication in each salamander family. Several pheromones have been described, ranging from simple, invariant molecules to complex, variable blends of pheromones. While some pheromones elicit overt behavioral responses, others have more nuanced effects. Pheromonal signals have diversified within salamander lineages and have experienced rapid evolution. Once receptors have been matched to pheromonal ligands, rapid advance can be made to better understand the olfactory detection and processing of salamander pheromones. In particular, a large number of salamander species deliver pheromones across the skin of females, perhaps reflecting a novel mode of pheromonal communication. At the end of our review, we list some of the many intriguing unanswered questions. We hope that this review will inspire a new generation of scientists to pursue work in this rewarding field.

Co-option and evolution of non-olfactory proteinaceous pheromones in a terrestrial lungless salamander

Gene co-option is a major force in the evolution of novel biological functions. In plethodontid salamanders, males deliver proteinaceous courtship pheromones to the female olfactory system or transdermally to the bloodstream. Molecular studies identified three families of highly duplicated, rapidly evolving pheromones (PRF, PMF, and SPF). Analyses for Plethodon salamanders revealed pheromone mixtures of primarily PRF and PMF. The current study demonstrates that in Desmognathus ocoee--a plesiomorphic species with transdermal delivery--SPF is the major pheromone component representing >30% of total protein. Chromatographic profiles of D. ocoee pheromones were consistent from May through October. LC/MS-MS analysis suggested uniform SPF isoform expression between individual male D. ocoee. A gene ancestry for SPF with the Three-Finger Protein superfamily was supported by intron-exon boundaries, but not by the disulfide bonding pattern. Further analysis of the pheromone mixture revealed paralogs to peptide hormones that contained mutations in receptor binding regions, such that these novel molecules may alter female physiology by acting as hormone agonists/antagonists. Cumulatively, gene co-option, duplication, and neofunctionalization have permitted recruitment of additional gene families for pheromone activity. Such independent co-option events may be playing a key role in salamander speciation by altering male traits that influence reproductive success.

Chemosignals, hormones, and amphibian reproduction

This article is part of a Special Issue "Chemosignals and Reproduction". Amphibians are often thought of as relatively simple animals especially when compared to mammals. Yet the chemosignaling systems used by amphibians are varied and complex. Amphibian chemosignals are particularly important in reproduction, in both aquatic and terrestrial environments. Chemosignaling is most evident in salamanders and newts, but increasing evidence indicates that chemical communication facilitates reproduction in frogs and toads as well. Reproductive hormones shape the production, dissemination, detection, and responsiveness to chemosignals. A large variety of chemosignals have been identified, ranging from simple, invariant chemosignals to complex, variable blends of chemosignals. Although some chemosignals elicit straightforward responses, others have relatively subtle effects. Review of amphibian chemosignaling reveals a number of issues to be resolved, including: 1) the significance of the complex, individually variable blends of courtship chemosignals found in some salamanders, 2) the behavioral and/or physiological functions of chemosignals found in anuran "breeding glands", 3) the ligands for amphibian V2Rs, especially V2Rs expressed in the main olfactory epithelium, and 4) the mechanism whereby transdermal delivery of chemosignals influences behavior. To date, only a handful of the more than 7000 species of amphibians has been examined. Further study of amphibians should provide additional insight to the role of chemosignals in reproduction.

Proteomic analyses of courtship pheromones in the redback salamander, Plethodon cinereus

The evolutionary success of plethodontid salamanders for ~100 MY is due partly to the use of courtship pheromones that regulate female receptivity. In ~90 % of plethodontid species, males deliver pheromones by "scratching" a female's dorsum, where pheromones diffuse transdermally into the bloodstream. However, in a single clade, representing ~10 % of Plethodon spp., males apply pheromones to the female's nares for olfactory delivery. Molecular studies have identified three major pheromone families: Plethodontid Receptivity Factor (PRF), Plethodontid Modulating Factor (PMF), and Sodefrin Precursor-like Factor (SPF). SPF and PMF genes are relatively ancient and found in all plethodontid species; however, PRF is found exclusively in the genus Plethodon - which includes species with transdermal, olfactory, and intermediate delivery behaviors. While previous proteomic analyses suggested PRF and PMF are dominant in slapping species and SPF is dominant in non-Plethodon scratching species, it was unclear how protein expression of different pheromone components may vary across delivery modes within Plethodon. Therefore, the aim of this study was to proteomically characterize the pheromones of a key scratching species in this evolutionary transition, Plethodon cinereus. Using mass spectrometry-based techniques, our data support the functional replacement of SPF by PRF in Plethodon spp. and an increase in PMF gene duplication events in both lineage-dependent and delivery-dependent manners. Novel glycosylation was observed on P. cinereus PRFs, which may modulate the metabolism and/or mechanism of action for PRF in scratching species. Cumulatively, these molecular data suggest that the replacement of pheromone components (e.g., SPF by PRF) preceded the evolutionary transition of the functional complex from transdermal to olfactory delivery.