Chemical cues and the molecular basis of olfactory chemoreception in caudate amphibians

Graduation date: 2010Chemoreception is one of the dominant sensory modalities for many species of salamanders (reviewed in Chapter 2). At least seven of the ten currently recognized salamander families are known to respond to some sort of chemical cue. These responses are as varied as delaying hatching, seeking refuge, or initiating aggressive behaviors. However, a major limitation to our understanding of many salamander chemoreception systems is that the specific chemical eliciting these responses has not\ud been isolated. However, in one family of salamanders, the Plethodontidae, several pheromones have been isolated. Plethodontid salamander males deliver protein pheromones to females during stereotyped courtship interactions. In these salamanders, a male will deliver pheromones if the female is not immediately receptive to mating. Female receptivity to a particular male may profoundly affect male mating success, and thus acts as an agent of sexual selection and sexual isolation.\ud In the majority of plethodontid species, a male delivers pheromones, produced in a specialized mental gland, to a female’s dorsal skin (transdermal delivery). A deviation from this ancestral mode of delivery occurred in one clade that evolved a\ud delivery method that directly stimulates the female's accessory olfactory system (olfactory delivery). In Chapter 3, I report that female Plethodon shermani (a species with olfactory delivery) do not respond behaviorally to pheromone delivery via the ancestral mode (transdermal delivery). In Chapter 4, I addressed the question of whether the pheromone composition is different in males of species that use transdermal verses\ud olfactory delivery. Evolutionary shifts in pheromone composition were determined by expressed sequence tag (EST) analysis of pheromone-gland RNA from three distantly related plethodontid species. Two of the species use the transdermal delivery mode,\ud Desmognathus ocoee and Eurycea guttolineata, and have some proteins in common, including two previously identified pheromone components, Plethodontid Modulating Factor and Sodefrin Precursor-like Factor. However, these species also express other\ud unique components that may act as pheromones by changing female physiology. Another pheromone protein, Plethodontid Receptivity Factor (PRF), is the dominant RNA transcript in my focal species with olfactory delivery, P. shermani. This protein is related to four-helix bundle cytokines and so it may stimulate cytokine receptors. In Chapter 5, isolation of a common cytokine receptor from P. shermani females reveals that this receptor is expressed in the female olfactory system and may interact with\ud PRF. However, since multiple proteins are delivered during courtship, I also investigated the classical families of receptors in vertebrate olfactory organs.\ud Chemical cues are generally detected by two olfactory sensory organs, the main olfactory and accessory olfactory (vomeronasal) epithelia. I used polymerase chain reaction to survey for molecular components of the signaling cascade used to mediate neuronal stimulation in response to chemical signals in P. shermani (Chapter 6). Salamander sensory tissue expressed homologues to (1) mammalian odorant and pheromone receptors, (2) olfactory-specific ion channels, and (3) three different\ud families of G proteins. The molecular conservation between amphibian and mammalian olfactory systems suggests that reception of salamander courtship pheromones (and other social signals) is mediated by pheromone receptors expressed in the accessory olfactory epithelium