Comparison of dlGALRs deduced amino acid sequences with that
Comparison of dlGALRs deduced amino 4μ8C sequences with that of human GALRs demonstrate that the GALR1 ortholog sequences have diverged less than those of GALR2. Since often duplicate genes undergo divergent evolution through sub-functionalization, loss or gain of new functions (Prince and Pickett, 2002), it is surprising that both gene products of the two fish GALR1 paralogues have maintained the key features that characterize the mammalian GALR1, including the key amino acids for its selective binding to the galanin peptide (Kask et al., 1998, Church et al., 2002). In contrast, the dlGALR2 duplicate sequences not only have less conservation with their mammalian ortholog as well as differ in several key features. Indeed, mutagenesis and analysis of three-dimensional models for the hGALR2 have identified amino acids that are responsible for its ligand binding activity and have underlined the importance of the N-terminus in this process (Lundström et al., 2007, Jurkowski et al., 2013). In this respect, dlGALR2b is the most structurally similar to hGALR2, as it has highly conserved N- and C-termini and conservation of all the key amino acids for ligand binding. In contrast, dlGALR2a has a shorter N-terminus, more similar to what is seen in hGALR3 and a shorter C-terminus like human and sea bass GALR1 receptors. It also shows sequence polymorphisms, although they correspond to conserved substitutions in amino acids that have not been identified as important. These differences suggest dlGALR2a may have acquired different functions from those of dlGALR2b in sea bass. Nevertheless, ligand activation and functional studies will be necessary to determine the specificities between these novel receptors in sea bass. A recent report identified GALR1 and GALR2 genes from fish to mammals (Liu et al., 2010). Our search for human GALR2 orthologs also allowed the identification of previously unsuspected duplicated GALR2 in other species. Indeed, phylogenetic analysis and gene synteny maps of this novel gene duplication confirmed that it is present not only in fish (e.g. sea bass, zebrafish and spotted gar) but also in some tetrapods (e.g. Xenopus and chicken). In addition, we have searched for GALR3 orthologs but failed to find it in fish genomes. However, unlike previous assumptions (Liu et al., 2010), the GALR3 gene is not exclusive of eutherian mammals and it could be confirmed also in amphibian, bird and marsupial genomes. Thus, phylogenetic analysis of fish and tetrapod GALRs resulted in the separation of two major clades formed by GALR1 and GALR2/GALR3 genes, suggesting that the two clades emerged from a common ancestral receptor and underwent distinct evolutionary trajectories. The GALR1 gene duplication appears to have resulted from the teleost specific whole genome duplication event (TGD) as it was only detected in several species of teleost fish (Fig. 9). Within the GALR2/GALR3 clade, GALR3 gene is absent in genomes from the Neopterygii lineage, including teleosts, suggesting that either fish lost this gene during evolution or it was acquired latter by the Tetrapoda lineage (Fig. 9). Conversely, the GALR2 gene duplication appears to have arisen prior to the separation of Sarcopterygii and Actinopterygii, but was lost in Mammalia (Fig. 9). Gene expression analysis suggest the brain as the main site of GALRs transcription in agreement to what has been described for mammals (reviewed in Mensah et al., 2010, Webling et al., 2012). Interestingly, we have detected a sexual dimorphism in the expression of dlGALR1a and dlGALR1b in adult ovary and testis, as these receptors are expressed in testes but have very low expression in the ovaries, as previously reported for rat GALR1 (Sullivan et al., 1997). These results are different from what has been reported for zebrafish where only one duplicate GALR1 gene was expressed in the ovaries and testis (zfGALR1b) and its expression was not dimorphic (Li et al., 2013). Nevertheless, it should be stressed that there is great variability in tissue distribution of the different GALR subtypes in vertebrates and significant differences in GALR1 expression in peripheral tissues are also found between human and mice (Wang et al., 1997b).