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Molecular cloning of urea transporters from the kidneys of baleen and toothed whales

Division of Biological Sciences, Graduate School of Science, Hokkaido University, Sapporo, Hokkaido 060-0810, Japan.
Comparative Biochemistry and Physiology Part B Biochemistry and Molecular Biology (Impact Factor: 1.55). 03/2008; 149(2):227-35. DOI: 10.1016/j.cbpb.2006.11.033
Source: PubMed

ABSTRACT

Urea transport in the kidney is important for the production of concentrated urine. This process is mediated by urea transporters (UTs) encoded by two genes, UT-A (Slc14a2) and UT-B (Slc14a1). Our previous study demonstrated that cetaceans produce highly concentrated urine than terrestrial mammals, and that baleen whales showed higher concentrations of urinary urea than sperm whales. Therefore, we hypothesized that cetaceans have unique actions of UTs to maintain fluid homeostasis in marine habitat. Kidney samples of common minke (Balaenoptera acutorostrata), sei (B. borealis), Bryde's (B. brydei) and sperm whales (Physeter macrocephalus) were obtained to determine the nucleotide sequences of mRNAs encoding UT. The sequences of 2.5-kb cDNAs encode 397-amino acid proteins, which are 90-94% identical to the mammalian UT-A2s. Two putative glycosylation sites are conserved between the whales and the terrestrial mammals, whereas consensus sites for protein kinases are not completely conserved; only a single protein kinase A consensus site was identified in the whale UT-A2s. Two protein kinase C consensus sites are present in the baleen whale UT-A2s, however, a single protein kinase C consensus site was identified in the sperm whale UT-A2. These different phosphorylation sites of whale UT-A2s may result in the high concentrations of urinary urea in whales, by reflecting their urea permeability.

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    • "In their transition from a terrestrial to an exclusively marine environment, cetaceans have undergone drastic morphological and physiological changes, involving adaptation in physiological functions such as vision, respiration, thermoregulation, osmoregulation, etc. (Thewissen 1998; Hoelzel 2002). Such strong adaptations can be expected to leave a clear mark in the genome, as has been found in cetacean homeobox genes which control limb development in vertebrates (Wang et al. 2009), in lung surfactant proteins which prevent lung collapse (Foot et al. 2007), urea transporters controlling kidney water retention (Birukawa et al. 2008), and Prestin from cochlear hair cells thought to allow sensitivity to high frequency sounds (Li et al. 2010; Liu et al. 2010). Evidence for adaptation has also been found on a population level within cetacean species, namely in the peptide binding region of the DQb1 locus involved in antigen recognition, suggesting adaptation to different pathogenic environments (Vassilakos et al. 2009). "
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    ABSTRACT: Cetaceans represent an evolutionary lineage marked by drastic morphological and physiological changes during their adaptation to an exclusively marine existence. In addition, several cetacean species exhibit geographical ranges that encompass different marine environments, with genetic breaks being sometimes consistent with environmental breaks. As such, genes that underwent adaptation during the land-sea transition can also be potential candidates for adaptation to different oceanic environments. In this study, we analysed 3 milk protein genes (β-casein, κ-casein, and α-lactalbumin) and 2 immunity related genes (MHC DQβ1 and γ-fibrinogen) for selection based on available phylogenetic datasets of both mammals and cetaceans, and used the results from this analysis to assess adaptation to different environments on a population level in the European common dolphin (Delphinus delphis). We found that evidence for positive selection could be detected in all genes in the phylogenetic analyses, with β-casein showing a further increase in selective pressure in the cetacean lineage. At the population level, both the immune system locus DQβ1 and β-casein genes showed patterns of variation consistent with divergent selection, and in each case the same populations showed differentiation. One of these populations was also differentiated at neutral markers, while the other was not. We discuss possible inference, and the potential for the further development of these ideas using genomic technologies.
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