The role of rx genes in establishment and maintenance of the medaka ciliary marginal zone

Postembryonic growth is a highly coordinated and delicate process, during which shape and function of complex organs have to be maintained. Anamniotes, such as fish and amphibia, grow lifelong. This property is mediated by stem cells, which must ensure full organ functionality along with proper scaling with the increasing body size. In the anamniote retina, postembryonic neurogenesis and growth is mediated by retinal stem cells in the ciliary marginal zone (CMZ). The family of Rx transcription factors is highly conserved among vertebrates and plays a key role in early eye development. In the teleost medaka (Oryzias latipes), the retina-specific homeobox transcription factor 2 (Rx2) has been shown to be a marker for multipotent retinal stem cells. Its closely related paralogue Rx1 has not been studied in detail so far. I hypothesise a role for rx1 and rx2 genes in establishment of the retinal stem cell domain and therefore in mediating postembryonic growth and shape of the eye. I used novel CRISPR/Cas9-mediated gene tagging approaches, developed in the framework of this thesis, to visualise endogenous medaka rx2 and, for the first time, rx1 expression. I carried out clonal loss-of-function studies of medaka rx1 and rx2, indicating a role in balancing stem cell fate decisions. Systemic loss-of-function studies of rx1-/-/rx2-/- double mutants revealed a role in early retinal morphogenesis, as well as in establishment and maintenance of the CMZ by conferring retinal stem cell identity. Furthermore, I established tools necessary to generate a labelled allelic series of rx1 and rx2 mutants. Eventually, I characterised an rx2 conditional allele, which will be instrumental to address late function of rx2 in the CMZ, thereby deciphering whether the retinal stem cell niche is dynamic or rather deterministic. Taken together, my results reveal a function for rx1 and rx2 in establishment and maintenance of the medaka CMZ and indicate a role in regulation of growth and scaling of eye size