Genetics. Published Articles Ahead of Print: September 2, 2005, Copyright © 2005
doi:10.1534/genetics.105.048819


A more recent version of this article appeared on December 1, 2005.


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Medaka receptors for somatolactin and growth hormone: phylogenetic paradox among fish growth hormone receptors

1 University of Tokyo
2 National Research Institute of Fisheries Science

* To whom correspondence should be addressed. E-mail: fukasho{at}k.u-tokyo.ac.jp.

Submitted on July 26, 2005
Revised on August 15, 2005
Accepted on 15 August 2005


Abstract

Somatolactin (SL) in fish belongs to the growth hormone/prolactin family. Its ortholog in tetrapods has not been identified and its function(s) remains largely unknown. The SL-deficient mutant of medaka (color interfere; ci) and an SL receptor (SLR) recently identified in salmon provide a fascinating field for investigating SL' function(s) in vivo. Here, we isolated a medaka ortholog of the salmon SLR. The mRNA is transcribed in variable organs. Triglycerides and cholesterol contents in the ci are significantly higher than those in the wild type, providing the first evidence of SL' function in suppressing lipid accumulation to organs. Interestingly, phylogenetic comparisons between the medaka SLR and growth hormone receptor (GHR), which is also isolated in this study, in relation to GHRs of other fish, suggested that all GHRs reported from non-salmonid species are, at least phylogenetically, SLRs. An extra intron inserted in medaka and pufferfish SLRs and flounder and sea bream GHRs also supports their orthologous relationship, but not with tetrapod GHRs. These results may indicate lineage-specific diversification of SLR and GHR functions among fish, or just an inappropriate naming of these receptors. Further functional and comparative reassessments are necessary to address this question.

Key Words: color interfere (ci), growth hormone receptor (GHR), lipid metabolism, medaka (Oryzias latipes), somatolactin receptor (SLR)




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