MBE Advance Access originally published online on March 10, 2004
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Mol. Biol. Evol. 21(6):1032-1041. 2004
DOI: 10.1093/molbev/msh093
© 2004 by the Society for Molecular Biology and Evolution. ISSN: 0737-4038
Stabilizing Selection on Behavior and Morphology Masks Positive Selection on the Signal in a Salamander Pheromone Signaling Complex




* Department of Zoology, Oregon State University, Corvallis
Department of Biochemistry, School of Medicine, University of Louisville, Louisville, Kentucky
School of Biology, Georgia Institute of Technology, Atlanta
Department of Biology, Utah State University, Logan
|| Department of Zoology, North Carolina State University, Raleigh
E-mail: wattsri{at}science.oregonstate.edu.
Natural selection maintains the integration and coordination of sets of phenotypic characters that collectively perform a task. In functional complexes in which characters span molecular to behavioral levels of organization, we might then expect similar modes of selection to produce similar patterns in evolutionary divergence at each level. To test this expectation, we diagnosed selection at behavioral, morphological, and molecular levels for courtship pheromone signaling by plethodontid salamanders. At the levels of morphology and behavior tens of millions of years of stasis (stabilizing selection) occur on each side of a transition from vaccination to olfactory delivery modes. As a proxy for the molecular level, we used plethodontid receptivity factor (PRF), a protein that is an active component of the pheromone. We cloned PRF from 12 Plethodon spp. spanning the delivery transition and obtained multiple alleles from each individual surveyed. Analyses of 61 alleles for PRF identified elevated nonsynonymous over synonymous substitution rates along lineages in a molecular phylogeny, and at 8% of sites in the protein, indicating that positive (directional) selection has acted on this vertebrate pheromone gene. Structural models showed PRF is in a family of cytokines characterized by a four
-helix bundle. Positive selection in PRF was associated with receptor binding sites that are under purifying selection in other cytokines of that family. The evolutionary dynamics of the plethodontid pheromone delivery complex consists of stabilizing selection on morphological and behavioral aspects of signal delivery but positive selection on the signal mediated by receptors. Thus, different selection modes prevail at different levels in this reproductive functional complex. Evolutionary studies of integrated sets of characters therefore require separate analyses of selective action at each level.
Key Words: positive selection pheromone plethodontid receptivity factor cytokine pheromone delivery
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