Award details

From light to darkness. Metamorphic dynamics of the eel eye

Reference	BB/D009820/1
Principal Investigator / Supervisor	Professor Glen Jeffery
Co-Investigators / Co-Supervisors	Professor James Bowmaker, Professor David Hunt
Institution	University College London
Department	Institute of Ophthalmology
Funding type	Research
Value (£)	212,911
Status	Completed
Type	Research Grant
Start date	01/08/2006
End date	30/11/2009
Duration	40 months

Abstract

Before reaching sexual maturity, eels migrate from fresh water to the deep ocean. During this period they undergo a dramatic metamorphosis that includes major changes to their visual system. In the outer retina there is a marked shift in the rod/cone ratio, away from a cone rich retina towards a heavily rod dominated retina. This is a unique adaptation to a change in the photic environment, from relatively bright, broad spectrum daylight in fresh water to dim monochromatic blue light of the deep sea. Although details of the changes in the visual pigments of the rods are well documented, there is almost no information on changes in the cone populations or in the remodelling that must occur in the inner retina. We will examine these changes in animals in the laboratory by inducing metamorphosis with hormones. The changes in the outer retina occur as a consequence of two mechanisms, rod addition across the entire retina and cone loss. However, it is not know if all cones types are lost or whether any eventually remain. We will trace the change in the rod/cone ratio anatomically, physiologically and genetically. Anatomically, cell death will be monitored by counting pyknotic nuclei and by using TUNEL labelling. Cells that have entered the cell cycle will be identified with Ki67 and BrdU labelling, where the fate of cells generated at specific times can be determined, will be used to confirm that this results in the production of additional rods. The functional consequence of cone loss and rod addition will be monitored by recording both the scotopic and photopic ERGs. The spatio-temporal patterns of cone loss and rod addition will be charted. Specific cone types will be identified using microspectrophotometry and in situ hybridization. We shall identify and isolate the cone opsin genes and follow the changes in their levels of expression during metamorphosis. Such a dramatic change in the outer retina will require significant remodelling of the inner retina. For example, unusually for teleosts, eels have a single class of horizontal cell that receives both rod and cone inputs which must be substantially modified. Such retinal plasticity will be analysed anatomically in relation to the changes in the rod/cone ratio. The genetic regulation of the eel's metamorphosis has not been studied. We will explore the role of candidate genes that may regulate such changes. The Otx2 homeobox gene is a key regulator of photoreceptor differentiation in the mammalian retina, where it has been shown to steer retinal progenitor cells towards a photoreceptor pathway of development. The pattern of expression of Otx2 will be followed in the eel retina to establish the basic pattern of expression. Induction of expression during maturation will imply that the remodelling of the eel retina involves the differentiation of photoreceptors from progenitor cells. In this way we will be able to link together the expression of key genes that result in the remodelling of the retina during a transition in this animals behaviour.

Summary

Eels migrate from relatively shallow fresh water to the sea. When this migration takes place their visual system changes from one that is designed to see the world in colour to one that is more sensitive but can only see in back and white. This change is more suitable to their new environment where there is little light and almost no colour. Eels probably do this by changing the type of cells that process light information, with some cells that are colour sensitive being lost and others that only respond in darker environments being added. In no other adult animal is there such a dramatic change in its visual system. However, no one has ever looked at exactly which cells are lost, which are added, and how this happens By inducing this change with hormones in animals in the laboratory, we will be able to discover which types of cell are lost, which types are added, and the time over which this occurs. The addition and loss of these light capturing cells means that the whole visual system must have to change in a unique manner, but such changes in response to different environments have never been investigated. Genes regulate these changes in the eels visual system as it moves from rivers to the sea. We will explore the role of genes in this process. In this way we will be able to link together genes that are responsible for changing the eel's visual system with changes in the organisation of the retina and the animal's behaviour.

Committee	Closed Committee - Animal Sciences (AS)
Research Topics	Neuroscience and Behaviour
Research Priority	X – Research Priority information not available
Research Initiative	X - not in an Initiative
Funding Scheme	X – not Funded via a specific Funding Scheme

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