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The first two whole genome duplication events pictured by a single chromosome duplicating. Crossed over boxes represent gene duplicate losses and blue arrows indicate whole genome duplication events. Illustration by David Lagman. Whole genome duplications is today most common in plants where many species is polyploid i.e. have several copies of each chromosome (Van de Peer et al. 2009). In vertebrates, however, it is not as clear due to that most of today's vertebrates are diploid i.e. have two copies of each chromosome (Van de Peer et al. 2009) often the result of a regression towards a diploid state after duplication. This is believed to happen because diversification of gene copies and their chromsomes over time makes them less similar to the other in the pair (Wolfe 2001). Examples of extant tetraploid vertebrates are the African clawed frog Xenopus laevis and common carp Cyprinus carpio (Larhammar and Risinger 1994).

The first two whole genome duplication events pictured by a single chromosome duplicating. Crossed over boxes represent gene duplicate losses and blue arrows indicate whole genome duplication events. Illustration by David Lagman. Whole genome duplications is today most common in plants where many species is polyploid i.e. have several copies of each chromosome (Van de Peer et al. 2009). In vertebrates, however, it is not as clear due to that most of today's vertebrates are diploid i.e. have two copies of each chromosome (Van de Peer et al. 2009) often the result of a regression towards a diploid state after duplication. This is believed to happen because diversification of gene copies and their chromsomes over time makes them less similar to the other in the pair (Wolfe 2001). Examples of extant tetraploid vertebrates are the African clawed frog Xenopus laevis and common carp Cyprinus carpio (Larhammar and Risinger 1994).

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Figure 1: General structure of the vertebrate eye and neural retina. A)...
Table 1 : Primer pairs used and annealing temperature used in the...
Figure 2: A schematic picture of the phototransduction cascade and its...
Figure 3: Suggested time points of the major whole genome duplications...
+9 Figure 4: The first two whole genome duplication events pictured by a...
Evolution of transducin alpha, beta and gamma subunit gene families
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The genomes of modern vertebrates have expanded due to two rounds of genome duplication (2R) in the ancestor of the vertebrates, followed by a third genome duplication in the teleost fish ancestor (3R). Gene duplications can give rise to new functions (neo-functionalizations), similar and more specialized functions than the original copy (sub-funct...
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... have been shown to mostly be in non-coding DNA. The genomes of human and fugu pufferfish, which differs greatly in overall size, contain about the same amount of coding genes (Ohno 1999). The 2R hypothesis proposes that invertebrate genes shall have four counterparts in vertebrates (i.e. orthologs), if no subsequent gene losses have occurred (Fig. 4)( Panopoulou and Poustka 2005). This is not the case, revealed as more and more genomes are sequenced. When the human genome project was finished the human genome was estimated to have around 25 000 genes which is not much more than the estimated 20 000 genes of invertebrates and it implies a huge loss of duplicated genes in vertebrates ...
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... which belong to the agnathans, diverged early in vertebrate evolution, which makes it of special interest to study which GNAT they use in their photoreceptors. The phylogenetic analyses of GNAT subunits place the putative lamprey orthologs at the base of the vertebrate branch of the GNAT1 subunit ( figure 4). This would suggest that both GNAT subunits present in lampreys are GNAT1 orthologs and that a GNAT2 ortholog may have been lost. ...
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... data suggest that long photoreceptors are cones and short photoreceptors are rod photoreceptors with several cone-like features (Govardovskii and Lychakov 1984;Muradov et al. 2008). The view that short photoreceptors are rods is supported by the expression of GNATS, which clusters with the rod transducin GNAT1 in the phylogenetic tree ( figure 4), and that they express rhodopsin ( Muradov et al. 2008). However long photoreceptors express long wavelength sensitive opsins (LWS) and appear in other ways to be cones, but also express GNATL, which diverges basally on the GNAT1 branch (figure 4) ( Muradov et al. 2008). ...