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Regressive Evolution of Pigmentation in the Blind Mexican Cavefish Astyanax mexicanus

Stahl, Bethany A
http://orcid.org/0000-0001-9218-2996
http://rave.ohiolink.edu/etdc/view?acc_num=ucin1439281730

Abstract Details

2015, PhD, University of Cincinnati, Arts and Sciences: Biological Sciences.
The natural world reflects profound biodiversity in every corner of the globe. These phenotypes range from wing spot variation for camouflage in butterflies, to protective armored plates in marine stickleback fish, to extravagant mating displays in peacocks. For many phenotypes, the “selective”?benefit is self-evident. However, some forms of phenotypic evolution?are less obvious, such as regressed or “lost” characters, since it can be difficult to determine the association between a? discarded trait and a selective advantage. Cave-dwelling animals such as the blind Mexican cavefish, Astyanax mexicanus, serve as excellent models to investigate regressive evolution. The surface form is extant, allowing for direct comparisons between river- and cave-dwelling conspecifics. Cavefish likely evolved from an “ancestral” surface-dwelling form, which invaded the caves of Northeastern Mexico. As a consequence of roughly 3 million years in darkness, these remarkable cavefish lost their coloration. Moreover, the recurrent loss of pigmentation in geographically isolated populations renders this system ideal for investigating the broader changes mediating regressive evolution in nature. To investigate these fundamental questions, we utilize an integrative approach to characterize genetic mechanisms contributing to coloration loss in nature. For this, we aimed to describe multiple genetic components – simple traits, complex characters and global changes in gene expression – that may contribute to regressive pigmentation. Although some simple traits (albinism and brown) have been characterized, the roles of cis-regulatory mutations affecting these single locus traits have not been described. We investigated populations of cavefish that harbor “brown” yet depict an intact coding sequence. We discovered many sequence alterations present in the 5’ putative promoter region of the causative locus Mc1r, some of which co-localized to highly conserved non-coding elements that may play a critical role in gene regulation. We next sought to characterize the complex trait of melanophore number variation. We scored pigment cell number variation in a large surface x Pachon cave F2 pedigree, and association mapping identified multiple significant QTL that co-localize to 19 distinct regions of our linkage map. We then co-analyzed QTL based on positional information and available Gene Ontology, along with RNA-seq data, to identify prospective structural and expression alterations. This led of the identification of Tyrp1b and Pmela as genes that mediate complex pigmentation in Astyanax. Finally, we characterized extreme differences in gene expression that accompany colonization into the cave. We compared the transcriptomic profiles of two geographically distinct cavefish populations relative to surface fish. Strikingly, these studies revealed convergence of gene expression in many of the same loci, which may implicate certain genes as crucial for life in caves. Moreover, we performed a pathway analysis and discovered potential upstream regulators, including Otx2 and Mitf, which appear to be similarly affected in these geographically distinct cavefish lineages. This dissertation suggests that regressive evolution of pigmentation results from diverse genetic mechanisms working in concert to give rise to reduced pigmentation phenotypes in cavefish. Broadly, this work provides insight to the mechanisms governing trait evolution in the wild, and further, characterizes genes that may have implications in human degenerative disorders.
Joshua Gross, Ph.D. (Committee Chair)
John Layne, Ph.D. (Committee Member)
Heather Norton, Ph.D. (Committee Member)
Kenneth Petren, Ph.D. (Committee Member)
Stephanie Rollmann, Ph.D. (Committee Member)
168 p.
Biology
evolution; genetics; cavefish; regressive evolution; complex traits; gene expression

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Citations

  • Stahl, B. A. (2015). Regressive Evolution of Pigmentation in the Blind Mexican Cavefish Astyanax mexicanus [Doctoral dissertation, University of Cincinnati]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1439281730

    APA Style (7th edition)

  • Stahl, Bethany. Regressive Evolution of Pigmentation in the Blind Mexican Cavefish Astyanax mexicanus. 2015. University of Cincinnati, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=ucin1439281730.

    MLA Style (8th edition)

  • Stahl, Bethany. "Regressive Evolution of Pigmentation in the Blind Mexican Cavefish Astyanax mexicanus." Doctoral dissertation, University of Cincinnati, 2015. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1439281730

    Chicago Manual of Style (17th edition)

ucin1439281730
277
© 2015, all rights reserved.
This open access ETD is published by University of Cincinnati and OhioLINK.