Adaptive Divergence in the Thyroid Hormone Signaling Pathway in the Stickleback Radiation

Jun Kitano, Tohoku University
Sean C. Lema, University of North Carolina - Wilmington
J. Adam Luckenbach, Northwest Fisheries Science Center
Seiichi Mori, Gifu-keizai University
Yui Kawagishi, Tohoku University
Makoto Kusakabe, University of Washington - Seattle Campus
Penny Swanson, Northwest Fisheries Science Center
Catherine L. Peichel, Fred Hutchinson Cancer Research Center

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Copyright © 2010 Elsevier. The definitive version is available at http://dx.doi.org/10.1016/j.cub.2010.10.050.

NOTE: At the time of publication, the author Sean C. Lema was not yet affiliated with Cal Poly.

Abstract

During adaptive radiations, animals colonize diverse environments, which requires adaptation in multiple phenotypic traits. Because hormones mediate the dynamic regulation of suites of phenotypic traits, evolutionary changes in hormonal signaling pathways might contribute to adaptation to new environments. Here we report changes in the thyroid hormone signaling pathway in stream-resident ecotypes of threespine stickleback fish (Gasterosteus aculeatus), which have repeatedly evolved from ancestral marine ecotypes. Stream-resident fish exhibit a lower plasma concentration of thyroid hormone and a lower metabolic rate, which is likely adaptive for permanent residency in small streams. The thyroid-stimulating hormone-β2 (TSHβ2) gene exhibited significantly lower mRNA expression in pituitary glands of stream-resident sticklebacks relative to marine sticklebacks. Some of the difference in TSHβ2 transcript levels can be explained by cis-regulatory differences at the TSHβ2 gene locus. Consistent with these expression differences, a strong signature of divergent natural selection was found at the TSHβ2 genomic locus. By contrast, there were no differences between the marine and stream-resident ecotypes in mRNA levels or genomic sequence in the paralogous TSHβ1 gene. Our data indicate that evolutionary changes in hormonal signaling have played an important role in the postglacial adaptive radiation of sticklebacks.