Novel vertebrate features identified in the rudimentary eye of the Pacific hagfish (Eptatretus stoutii)

Abstract

A disparate range in eye complexity exists between vertebrates and their sister group, tunicates. In tunicates, “eyes” take the shape of simple shadow-detecting photoreceptive patches, while vertebrates possess one of the most intricate and complex eyes seen in the animal kingdom. We do not yet know how the complex vertebrate eye could have arisen from a highly simplified eye structure in the shared ancestor with tunicates. With no extant transitionary forms between tunicates and vertebrates, we must use the earliest diverging vertebrates to better understand the evolutionary history of vertebrate eyes. Hagfish are one of two surviving jawless fish lineages, and so represent one of the earliest examples of vertebrate eyes. Despite being vertebrates, hagfish lack many of the conserved vertebrate eye features that are conserved from lamprey to humans. Their eye is unpigmented, obscured by a transparent epidermis, and lacks a three-layered retina, lens, and ocular musculature. This makes hagfish a unique case in vertebrate evolution. The cause of this simplicity has been explained by several different potential histories. One school of thought is that the rudimentary features can be thought of as ancestral, meaning that the hagfish eye represents an early stage in vertebrate eye evolution that existed prior to the evolution of the typical vertebrate camera-style eye. However, this hypothesis indirectly suggests that lamprey evolved their eye in parallel with jawed vertebrates, which is in opposition to a number of molecular phylogenetic studies. Our competing hypothesis conversely suggests that these rudimentary features are a result of regression from an ancestrally complex eye. Very little molecular data exist to characterize the eyes of hagfish, and so we set out to test our hypothesis by defining the molecular characters of the retina of the Pacific hagfish (Epatatretus stouii). In order to do this, we produced a hagfish eye transcriptome from which we pulled a iii number of novel transcripts including homologs of retinoid cycling genes (RPE65, LRAT, and others) and interneuron markers (PKC-a, calbindin, Pax6 and others). By in situ hybridization, we were able to localize the expression of several of these novel transcripts to regions of the hagfish retina homologous with those in other vertebrates. Identification of these novel transcripts revealed the presence of a greater diversity of cell types within the retina than previously characterized, including the first evidence of interneurons and a supporting functional retinal epithelium. The presence of these features in conjunction with the presence of these features in lamprey and jawed vertebrates, suggests that hagfish eyes share a highly conserved eye with other vertebrates. Our work to identify novel features of the hagfish retina has opened the door to a number of future studies and provided substantial evidence to support the degenerative eye hypothesis.