Evolution of Vertebrate Vision by Means of Whole Genome Duplications Zebrafish as a Model for Gene Specialisation
Abstract: The signalling cascade of rods and cones use different but related protein components. Rods and cones, emerged in the common ancestor of vertebrates around 500 million years ago around when two whole genome duplications took place, named 1R and 2R. These generated a large number of additional genes that could evolve new or more specialised functions. A third event, 3R, occurred in the ancestor of teleost fish. This thesis describes extensive phylogenetic and comparative synteny analyses of the opsins, transducin and phosphodiesterase (PDE6) of this cascade by including data from a wide selection of vertebrates. The expression of the zebrafish genes was also investigated. The results show that genes for these proteins duplicated in 1R and 2R as well as some in 3R.Expression analyses of the zebrafish genes revealed additional specialisations for the 3R gene duplicates. The transducin beta subunit genes, gnb1a and gnb1b, show co-localisation in rods but are expressed at different levels. Gnb3a and gnb3b show different expression in the adult retina with low expression of gnb3a and expression of gnb3b in cones of the dorso-medial retina. The transducin gamma subunit genes gngt2a and gngt2b are expressed in the ventral and dorso-medial retina respectively. The both of PDE6 gamma subunit genes, pde6ga and pde6gb are both expressed in rods but pde6ga shows rhythmic changes of expression with low daytime levels. Pde6ha and pde6hb are expressed in cones however pde6ha show high daytime expression. All investigated transducin and PDE6 subunit genes, but gnb1b, were also expressed in the adult pineal complex or at some point during development.These results provide compelling evidence that the 1R and 2R genome duplications facilitated the evolution of rods and cones by generating gene duplicates that could evolve distinct expression and function. This supports existence of colour vision before the origin of vertebrates, elaboration of this in the early vertebrate ancestor, along with origin of the black-and-white dim-light vision of rods. Furthermore, the different expression patterns observed in the zebrafish retina for teleost 3R duplicates demonstrate multiple additional specialisations.
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