Genetic regulation of photoreceptor specification in the tetrachromat zebrafish retina

• Author / Creator

  Oel, Adam P.

• Vertebrate vision is mediated through the absorption of light by the rod and the cone photoreceptors. Early jawed vertebrates initially had four cone subtypes, which sampled different regions of the visual spectrum to allow colour vision, and a single rod type which enabled vision in dim light. Many lineages of vertebrates retain this original complement of photoreceptors, but genetic mechanisms of photoreceptor specification have been chiefly explored in mammals, which have lost two cone subtypes. Our understanding of the genetic regulation of photoreceptor diversity in the archetypical vertebrate eye, and how this regulation has evolved in different lineages, is therefore incomplete. I used the zebrafish, a diurnal vertebrate that produces the four original cone subtypes and one rod subtype, to investigate the genetic specification of photoreceptor diversity in tetrachromats, and to identify possible genetic mechanisms exploited by various vertebrate lineages to recalibrate their photoreceptor diversity in response to novel evolutionary pressures.
  The zebrafish UV cone is homologous to the mouse S cone, while the zebrafish blue cone has no mouse homolog. It was previously demonstrated that the T-box transcription factor gene tbx2b modulates UV cone and rod abundances in larval zebrafish, but regulators of tbx2b were unknown. Furthermore, regulation of the blue cone type was wholly unknown. On the basis of genetic interaction in early retinogenesis, my colleagues and I examined the genetic interaction of the BMP ligand gene gdf6a and tbx2b on larval photoreceptor diversity. We found that gdf6a modulates the actions of tbx2b in governing UV cone and rod abundance, and on its own modulates blue cone abundance. Together, this represents the beginning of the first genetic pathway regulating tetrachromat cone diversity, including of the ancient blue cones not found in mammalian retinas.
  Molecular evidence suggests that rod genes evolved out of pre-existing cone genes, and the current consensus is that rods probably evolved from cones. Previous work profiling the gene expression of developing mouse rods had suggested a transient cone signature. This led to the hypothesis that vertebrate rods routinely develop from cone- fated precursors. My colleagues and I tested this hypothesis by comparing mouse and
  ii
  zebrafish photoreceptor development. We found that a majority of mouse rods transiently but robustly expressed signature cone genes, but some small proportion of mouse rods did not. I found that zebrafish larvae did not have rods with cone gene expression history. In light of additional evidence that early mammals increased the abundance of rod photoreceptors for a nocturnal lifestyle, we proposed that mammals diverted normally highly abundant cone-destined cells to the rod phenotype. We further proposed that early mammals may have exploited regulatory or activity changes in the transcription factor Nrl as a mechanism.
  The body of mouse photoreceptor specification literature features Nrl as a central node in that transcription factor network, and it was not known whether nrl had conserved activity in zebrafish or any species beyond mouse or mammals. I therefore examined the role of nrl in zebrafish photoreceptor specification. I found nrl to be necessary and sufficient for the rod phenotype in larvae, indicating conservation of nrl function between fish and mammals. Unlike in mammals, I further found that nrl was not required for adult rod production. This is the first evidence for an nrl-independent rod developmental pathway. Finally, I tested the rod-inductive ability of a panel of nrl homologs from several vertebrate taxa, and found that even basally-branching vertebrates have an nrl homolog which can promote the rod phenotype. This suggests that a role for nrl in rod specification may be ancestral in vertebrates.
  Cumulatively, the work in this thesis identified the first genetic regulator of the tetrachromat blue cone, established a novel regulatory pathway governing UV cone and rod abundance, and identified a deep conservation in rod developmental genetics between mammals and zebrafish larvae. It also identified a possible novel rod developmental pathway, which may be a genetic mechanism by which other vertebrate lineages modulate photoreceptor diversity. Finally, it positioned zebrafish to serve as the basis for future direct comparisons of photoreceptor specification genetics between tetrachromats and mammals.

• Subjects / Keywords

  • nocturnal bottleneck
  • genetic regulation
  • evolution
  • nrl
  • tbx2b
  • photoreceptor
  • gdf6a
  • retina
  • zebrafish

• Graduation date

  Fall 2018

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R3FJ29V5H

• License

  Permission is hereby granted to the University of Alberta Libraries to reproduce single copies of this thesis and to lend or sell such copies for private, scholarly or scientific research purposes only. Where the thesis is converted to, or otherwise made available in digital form, the University of Alberta will advise potential users of the thesis of these terms. The author reserves all other publication and other rights in association with the copyright in the thesis and, except as herein before provided, neither the thesis nor any substantial portion thereof may be printed or otherwise reproduced in any material form whatsoever without the author's prior written permission.