RUI: Smelling in water and air: Evolution of olfaction in plethodontid salamanders

This project focuses on how the olfactory system of amphibians is modified to function in water and in air. The emergence of vertebrate animals from water onto land was one of the major transitions of vertebrate evolution. For many living amphibians, this historical transition is mirrored by a current developmental transition, as they metamorphose from an aquatic larva to a terrestrial juvenile.

Like other sensory systems, the olfactory system must be able to adjust and function in both environments across this developmental transition. This project breaks new ground by examining olfactory morphology and gene expression patterns across eight related salamander species with a wide range of life histories, ranging from typical larvae with metamorphosis, to direct development with no free-living larval stage, to secondarily re-evolved larvae.

The study will answer the following questions: (1) Are patterns of olfactory organization consistent across species with the same developmental mode, regardless of evolutionary history? (2) Are water-sensing olfactory components lost in terrestrial, direct-developing species? (3) Do species that have re-evolved a larval period regain water-sensing olfactory components? This work will take place at Humboldt State University, an Hispanic Serving Institution located on the rural northern coast of California.

It will directly support 15-20 undergraduate and two graduate students in hands-on research, enhancing their skills and thereby furthering their careers and job prospects. The research will be integrated into classes, benefitting approximately 300 students per year. A public outreach component will involve lectures to local community members, increasing scientific literacy on evolution and development and promoting appreciation for this understudied group of vertebrates.

This project will examine how the evolution of alternative developmental modes impacts olfactory system structure and function in the most speciose family of salamanders, the plethodontid or lungless salamanders. Olfactory morphology and gene expression patterns of eight plethodontid species and one outgroup species that span a wide range of developmental modes will be examined.

Techniques used will include traditional histology to compare the basic structure of the olfactory organs, transmission and scanning electron microscopy to establish morphological classes of olfactory receptor neurons and supporting cells, microCT scanning and 3-D reconstruction to provide a visual image of overall olfactory organ anatomy, neuronal tracing to establish pathways of olfactory receptor neuron connection to the brain, and protein and mRNA localization of olfactory marker genes. This work will for the first time elucidate patterns of olfactory evolution with respect to olfaction in water and air, in larvae and adults, within an explicit phylogenetic framework.

By doing so, it will allow a better understanding of how the olfactory system evolves in living amphibians, and of changes associated with the evolutionary emergence of tetrapods onto land. A total of 15-20 undergraduates and two graduate students will participate directly in the project, and the research will be incorporated into five courses that enroll approximately 300 students.

The research will also be presented to the public at the institution’s natural history museum and at a science lecture series at a local coffeehouse.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.