A characterization of adult retinal neurogenesis in the Pacific hagfish (Eptatretus stoutii) to elucidate the evolutionary origins of the vertebrate retina
Creators: Bradshaw, Sarah
Publisher: name: University of Alberta Library
Year: 2023

<p>The vertebrate retina is a vital sensory structure that has a murky evolutionary origin. Living vertebrates possess a strikingly complex retina and eye with highly conserved development and physiology. In contrast, the photoreceptive organs of the closest non-vertebrate relatives are comparatively simple clusters of pigment cells and photoreceptors. No extant vertebrates possess characters that are suggestive of a ‘transitional state’ that could help guide interpretations of how the eye or retina may have initially formed. However, the early-branching jawless vertebrates (Cyclostomata - hagfishes and lamprey) are in an ideal phylogenetic position to shed light upon the origin of the vertebrate eye. <br>
The eyes of cyclostomes are vastly understudied compared to the eyes of jawed vertebrates (Gnathostomata). The available literature has revealed that both hagfish and lamprey eyes contain unique features not present in other vertebrates. Hagfish eyes are particularly notable as the eye (and retina) is small and rudimentary in form. The hagfish eye lacks pigment and a lens. The retinal layers are also more disorganized than in other vertebrates. These observations have led to the interpretation that the hagfish eye could represent the ancestral vertebrate eye condition. Recent morphological and molecular studies, including data in this thesis, now suggest that many of these features in the hagfish eye are due to secondary loss rather than retention of ancestral traits. However, further investigation of the cyclostome retina will be crucial for inferring the state of the ancestral vertebrate (proto-vertebrate) eye. <br>
Although several studies have investigated the morphology of cyclostome eyes, data on development of the eye and retina are extremely limited. This is particularly true for hagfish, whose embryos are notoriously difficult to acquire. Neurodevelopmental data could provide new insights on eye evolution where morphological data from extant organisms or fossil data are lacking. Here, I provide genomic and RNA sequencing data that suggest many genes critical for gnathostome retinal development are also expressed in the hagfish eye. In addition, recent work suggests adult Pacific hagfish (Eptatretus stoutii) have continued retinal growth past embryonic development. Therefore, in this thesis I have begun to characterize the mechanisms driving retinal development in the Pacific hagfish (E. stoutii) by taking advantage of a putative proliferative zone in the adult retina. <br>
To achieve this, I applied a brief pulse of EdU to several hagfish to label proliferating cells, if any, in the retina. I also utilized bioinformatics and in situ hybridization to assess if homologs of gnathostome retinal genes also drive retinogenesis in this jawless vertebrate. I observed EdU positive cells within the retinal periphery of the hagfish (a region reminiscent of the ciliary marginal zone of gnathostomes) and within the central retina. I also found evidence that hagfish possess homologs of several key genes required for retinal neurogenesis in other vertebrates and that these genes are expressed within the hagfish eye. Finally, through in situ hybridization I demonstrated that two of these genes, OtxA and Rx (retinal homeobox), are expressed in the hagfish retina (including at the proliferative retinal periphery). This work has revealed candidate genes and mechanisms that may be involved in hagfish retinogenesis. This establishes a starting point for future studies to dissect the pathways of retinal neurogenesis in jawless vertebrates. Further efforts comparing retinal development between cyclostomes and gnathostomes are warranted, as this work has also uncovered evidence for deeply conserved mechanisms for retinogenesis within the vertebrate lineage.</p>