Evolution of vertebrate vision
A new study published in BMC Evolutionary Biology by the team lead by Xesús Abalo and Dan Larhammar, Department of Neuroscience at Uppsala University and SciLifeLab, sheds light on the evolutionary origin fast vertebrate vision and the specialisations in zebrafish to adapt to changing lighting conditions
Light perception is crucial for the survival of all major animal groups, including our own: the vertebrates. Evolution has favoured selection the camera eye that arose in the vertebrate ancestor over 500 million years ago. Light perception takes place in the cone and rod photoreceptor cells of the retina, a structure at the back of the eye, through a set of proteins denominated phototransduction cascade proteins. The light information registered by these cells is partially processed in the retina and subsequently forwarded to the brain for further processing and integration with other sensory systems, eventually leading to outputs such as endocrine regulation of behaviours.
Twenty years ago, the first studies of the light receptor proteins (opsins) in birds indicated that colour vision, mediated by the cones, arose before the dim light monochromatic vision provided by rods. This hypothesis was recently confirmed by the team of Abalo and Larhammar in a detailed study on the visual opsin gene family analysing a broad range of vertebrate species (Lagman and Ocampo Daza et al., 2013).
In the current study, the same group presents a detailed analysis on the evolution of the main effector of the phototransduction cascade, the PDE6 enzyme. They report that the genes encoding the different subunits of PDE6 in cones and rods arose from ancestral genes that duplicated in the early vertebrate genome doublings, and further expanded in teleosts due to the extra genome duplication that took place in this lineage. Interestingly, they also identified another ancient vertebrate gene copy, which they named PDE6I, and has been lost in amniotes.
To characterize functionally the PDE6 family members, they used zebrafish (Danio rerio) as model to analyse the specific specialisations in its visual system, since zebrafish have three extra gene duplicates that arose after it diverged from the lineage leading to humans. The zebrafish displays the same distinction as humans between cone and rod PDE6 versions. In addition to this, the gene duplicates in zebrafish display strikingly different expression during the day-night cycle, presumably to allow efficient regulation of photoreceptor cells under dramatically different light conditions.
Altogether, the data presented in this study reinforces the evolutionary importance of the two rounds of whole genome duplication that occurred in the vertebrate ancestor and sheds light on the differential behaviour of gene duplicates that arose in these events. The extensive difference between day and night for the gene duplicates opens doors to functional studies of behaviours regulated by light in different animal groups.