Evolution of an aquatic courtship pheromone in the genus Taricha

Author

Adam Cummings

Graduation Date

2015

Document Type

Thesis

Program

Other

Program

Thesis (M.S.)--Humboldt State University, Biology, 2015

Committee Chair Name

John Reiss

Committee Chair Affiliation

HSU Faculty or Staff

Keywords

Molecular evolution, SPF, Salamandridae, Chemical communication, Sodefrin precursor-like factor, Humboldt State University -- Theses -- Biology

Abstract

Different loci within a multigene family often experience unique evolutionary pressures; this phenomenon is often observed in proteins involved in reproduction. Chemical communication using secreted proteinaceous pheromones is critical for reproductive success in salamanders. Differential recognition of these signals can create a reproductive barrier between lineages. In the newt genus Taricha, there is no discernable differentiation in courtship behavior among congeners, in spite of multiple regions of sympatry among the species. This suggests that divergence in chemical communication may serve as a reproductive barrier. The goal of this project was to study the diversification of one such pheromone--sodefrin precursor-like factor (SPF)--expressed in male cloacal glands. My objective was to investigate its role in the diversification of Taricha and test the role of conflicting selection modes in shaping that diversification. Males of the four species of Taricha (T. granulosa (n = 4), T. torosa (n = 8), T. rivularis (n = 2), and T. sierrae (n = 7)) were collected from regions of sympatry throughout California. Reverse transcriptase-PCR of abdominal gland extracts using degenerate primers resulted in isolation of 63 unique sequences of SPF. Phylogenetic analyses show that at least two SPF genomic loci (Beta-1 Beta-2) are expressed in Taricha, although patterns of expression vary among the species: T. granulosa and T. sierrae exclusively express Beta-1 while T. torosa and T. rivularis exclusively express Beta-2, except in a known hybridization zone between T. torosa and T. sierrae in the Kaweah River drainage within Kern Co., California. Here, both species present express both loci, presumably due to introgression. Beta-1 and Beta-2 SPF exhibit different patterns of protein evolution: Beta-1 showed evidence of gradual allopatric divergence with relatively few codons experiencing Darwinian selection while Beta-2 transcripts in the known hybridization zone are radically altered via alternate splicing patterns. Splicing patterns and highly concentrated codons under positive selection indicate that a key functional region for SPF binding may be in the second three-finger protein domain. The results provide insights into multi-locus signals in salamanders, specifically and the evolutionary mechanisms that shape courtship pheromones more generally.

https://scholarworks.calstate.edu/concern/theses/w9505290g

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